Division of Agricultural Sciences
UNIVERSITY OF CALIFORNIA
CONTROLLING
FIELD RODENTS
IN CALIFORNIA
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TRACY I. STORER
CALIFORNIA AGRICULTURAL
Experiment Station
Extension Service
CIRCULAR 434
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Controlling Field
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THE DAMAGE DONE BY
FIELD RODENTS . . .
. . . to California crops amounts to several million
dollars a year. Moreover, some rodents carry bubonic
plague, tularemia, and other serious diseases that may
be transmitted to man, either directly or by fleas, ticks,
or mites.
EFFECTIVE CONTROL involves more than merely set-
ting a trap or scattering some poisoned bait. Good
results can be expected only by understanding the
habits of the rodent to be controlled, by putting this
knowledge to use, and then keeping at control through-
out the year so that the number of rodents never be-
comes large.
THIS CIRCULAR replaces the former Extension Circular
138. It is intended to aid in the control of field rodents
on farms and in home gardens. It describes the ani-
mals, tells where and how they live, and what they
eat — all information basic to the control program. It
describes the control methods, including the use of
poison baits and poisonous gases, trapping, shooting,
exclusion, and encouragement of natural enemies. It
gives instructions for applying these controls, and
formulas for the poisons recommended.
Section 1 deals with rodent damage and means of
control, Section 2 with ground squirrels and tree squir-
rels, Section 3 with pocket gophers and moles, and
Section 4 with meadow mice, kangaroo rats, muskrats,
and rabbits.
THE AUTHOR:
Tracy I. Storer is Professor of Zoology, University of
California, Davis.
JULY, 1953
Rodents in California . . .
TRACY I. STORER
I. Rodent Damage . . Means of Control
This section discusses over-all methods
of controlling rodents and answers the
question "Why is rodent control necessary?"
Economic reasons
Field rodents (the ground squirrels,
tree squirrels, pocket gophers, field mice,
kangaroo rats, muskrats, and rabbits)
are all seeking their needs for successful
existence — to obtain enough food, to find
adequate shelter, and to escape their
enemies. Whenever man's farms and gar-
dens offer food or shelter for rodents,
they will become his competitors. They
affect his business, pleasure, and health.
The losses caused by rodents are diffi-
cult to estimate, but the total amount of
damage in California probably amounts
to several million dollars annually. Where
there is no control, ground squirrels may
cause losses to cereal crops amounting to
10 or 15 per cent. They materially reduce
the forage in pastures and on range lands.
Pocket gophers may seriously injure or
kill individual orchard trees and can be
an expensive nuisance in alfalfa, truck
crops, and home gardens. Ground squir-
rels, pocket gophers, and muskrats may
damage ditchbanks and levees by burrow-
ing. Jack rabbits and, less often, cotton-
tails may gnaw bark on trees and vines
and thus decrease production or even kill
the plants. Rabbits may seriously reduce
the production of truck and field crops
and home gardens. Control measures by
farmers and gardeners on their own lands
and by government officials on public
lands serve to reduce the total damage.
Rodent control in California, public
and private, has cost fully one million
dollars annually in some recent years, and
several million acres have been treated.
A few of California's native rodents are
actually beneficial, and many are neutral
so far as man's interests are concerned.
Some, such as muskrats and cottontail
rabbits, are useful as fur bearers or game
animals to trappers or sportsmen, but can
be harmful to the farmer. Certain rodents
are believed to benefit the soil by "culti-
vation," and in other ways.
Public health
Some rodents carry diseases that may
be transmitted to man. Plague was first
detected in California in 1900 among
rats in San Francisco. From rats it spread
to the California ground squirrel and
other rodents, and by 1946 plague had
been demonstrated at one time or another
in 35 counties. In humans the disease is
called bubonic plague. In rodents it is
called sylvatic plague. It is transmitted
chiefly by fleas. Rats and ground squirrels
are the animals most often affected, but
it sometimes occurs in other rodents.
[3]
When transmitted to man from squirrels
it may take on a more deadly form known
as pneumonic plague. This type may be
transmitted directly from one person to
another by coughing (droplet infection).
Two small epidemics of pneumonic
plague have occurred in California, each
with a mortality of more than 90 per cent.
A native disease, tularemia, was first
discovered in the California ground
squirrel. It occurs also in rabbits and,
less often, among other rodents in many
parts of California and elsewhere. Man
may contract the disease while skinning
infected animals, or, rarely, by eating
improperly cooked rabbit flesh. The dis-
ease is also transmitted by several insects,
notably deer flies (for which reason it is
called "deer-fly fever") , and by ticks. The
reservoir for tularemia, however, is in
wild rodents.
Rocky Mountain spotted fever is a seri-
ous and often fatal tick-borne disease
present in Plumas, Lassen, Modoc, and
eastern Siskiyou counties, and in states
of the arid western interior. It is trans-
mitted to man by the bite of infected
ticks inhabiting various mammals, in-
cluding rabbits.
Another disease, relapsing fever, is
present around Lake Tahoe, Big Bear
Lake in the San Bernardino Mountains,
and other mountain regions. Carried by
small chipmunks, it may be transmitted
to man by the bite of certain ticks. A
number of cases have occurred among
humans in recent years.
Because squirrels and other rodents may
be carriers of disease, capturing them as
pets and handling live or dead specimens
should be avoided.
General information about diseases in
wild rodents is available in a book by
Hull (1947) . See page 8 for list of refer-
ences. The California Department of
Public Health and U. S. Public Health
Service issue information circulars and
technical reports on some rodent-borne
diseases. If a rancher or land owner sees
sick or dying field rodents or other evi-
dence of disease he should report the fact
at once to his county agricultural com-
missioner, health officer, or the Depart-
ment of Public Health.
Several government agencies aid in
control of field rodents. The U. S. Fish
and Wildlife Service supervises rodent
control on federally owned lands and, to-
gether with the California Department of
Agriculture, advises and helps to direct
operations in the counties. The U. S.
Public Health Service studies rodent-
borne diseases and conducts campaigns
against rodents in and about seaports.
The California Department of Public
Health makes surveys of rodents to deter-
mine the presence of diseases that may be
transmitted to human beings. When in-
fections are found, reports are sent to the
State Department of Agriculture. That
agency then carries on control operations
when necessary. The county agricultural
commissioners are responsible for rodent
control in the counties. But except where
special danger to public health is con-
cerned, the responsibility for rodent con-
trol on privately owned lands rests with
the individual.
There is no one easy way to control all
kinds of rodents, but certain methods
that have been tested by repeated use will
keep most species in check.
To decide what control measures to
use, one must first know what rodent is
causing the trouble. Knowledge of the
habits and especially of the food prefer-
ences, seasonal or year round, of each
kind of rodent is essential for successful
control. Brief descriptions of the habits
of the principal injurious field rodents
are given in this circular, but anyone who
attempts to control rodents must study
the animal in his area and check upon
the results of each effort at control. The
seasons when rodents breed and hiber-
nate will determine when control meas-
ures will do the most and the least good.
With burrowing rodents, the type and
extent of the burrows will indicate
[4]
whether poisonous gases are practical or
how and where to place traps or poison
baits. To aid in choosing and applying
control measures, later sections describe
the methods to use against each important
kind of field rodent in California.*
The general methods of control are
briefly described in this section. Some
important precautions are also given.
More detailed directions will be found in
the sections on particular rodents. There
are six general means of control: (1) poi-
son baits, (2) poison gases, (3) trap-
ping, (4) shooting, (5) exclusion, and
(6) encouragement of natural enemies.
Poison baits
One of the commonest means of con-
trol is by poison baits. Food that the
rodent likes — grains, greens, pieces of
vegetables or fruits — is poisoned and
scattered broadcast or placed in burrows
or other protected spots. Baits should not
be scattered on the ground if they will be
dangerous to livestock, beneficial wild
life, or human beings.
Control work with poison, even under
official agencies, has been criticized be-
cause it may kill other animals besides
rodents. Hardly any control operation
(except selective shooting) is without
some possible danger to other forms of
wild life. But careful use of the more con-
servative methods will keep this danger
low.
The county agricultural commissioners
prepare and sell certain kinds of poison
bait, and sometimes have clean oat groats
for making ground-squirrel bait. Several
counties have one or more special rodent
inspectors to direct control measures or
actually apply them. But in most counties
the landowner or tenant pays for ma-
terials and either provides or pays for the
labor.
If the county agricultural commis-
sioner cannot supply the bait needed, he
* Control of house rats and mice and of wood-
rats and white-footed mice is described in Cali-
fornia Agricultural Extension Circular 410.
can give the names of reliable commer-
cial brands. Or the baits can be mixed at
home, particularly if large quantities are
needed. Formulas are given in Sections 2,
3, and 4.
Both the poison and the bait should be
adapted to the kind of rodent to be con-
trolled.
Strychnine is the chief poison used
for pocket gophers, jack rabbits, and
some lesser rodents, and has been long
used for ground squirrels.
Zinc phosphide has proved useful in
controlling ground squirrels and meadow
mice.
In earlier years both phosphorus and
arsenic compounds were used in rodent
control, but they have been replaced by
other poisons and their use is not recom-
mended.
Commercial preparations of various
types are sold for rodent control. Besides
the poisons mentioned here, such as
strychnine and zinc phosphide, there are
compounded poisons or prepared baits
sold under brand or trademarked names.
Any poison or poisoned bait to be offered
for sale in California must first be sub-
mitted to the State Department of Agri-
culture. Only those which are satisfactory
are licensed for sale in California ( Agric.
Code Sees. 1061-1079) . Poisons and poi-
soned baits sold in interstate commerce
must also meet the requirements of the
Federal Insecticide, Fungicide, and Ro-
denticide Act of June 25, 1947. The name
and the percentages of the active in-
gredients, with proper warnings concern-
ing the poison used, must be printed on
the label of each product. Some com-
panies and storekeepers make up ap-
proved "government" ground-squirrel
poison and other formulas; for small
applications such preparations are eco-
nomical.
Certain other poisons have proved use-
ful under special conditions, but either
are not suited to field rodents or are too
dangerous for use by untrained oper-
ators.
[5]
Antu (alpha-naphthylthiourea) , a new
material, is useful for Norway rats but is
not sufficiently poisonous to other rodents
for use in control.
Warfarin, a poison of special value in
control of domestic rats and mice, can be
used in some cases for ground squirrels
about residences, farm headquarters, and
summer camps.
Two other rodent poisons are not avail-
able for use by the general public because
of the relatively greater hazards they in-
volve. Baits poisoned with these chemi-
cals bear no indications of their poison-
ous nature in appearance, taste, or odor.
Both are poisonous to rodents, other wild
animals, domestic livestock, and man.
These are thallium sulfate and Compound
1080.
Thallium sulfate (thallous sulfate,
T1 2 S0 4 ), a "heavy metal" poison ob-
tained from smelters, has been used by
government agencies for controlling both
rats and field rodents. Thallium-poisoned
grain several times came into the hands
of private individuals who innocently
used the grain for human food. As a re-
sult, several persons became ill, and some
died. Statements that continued use of
thallium-coated grain would lead to the
sterilization of lands for plant growth
have proved to be unwarranted.
Compound 1080 (sodium fluoroace-
tate) , a wartime discovery, has been used
since 1945 by military and government
agencies for rodent control. It is as poi-
sonous for all kinds of animals as any
other material used. Wherever it is used,
there is great danger that dogs and cats
may eat rodents killed by 1080 and be
poisoned. This material, therefore, should
be employed only by properly trained
persons and only in certain kinds of
places.
Sale or possession of these two poisons
for rodent control in California is re-
stricted by law to federal, state, county,
and municipal officers and employees and
to licensed structural pest-control opera-
tors (California Agricultural Code, 1951, ,
Sec. 1066.5-1066.6).
There is no certain antidote for either
of these poisons. Thallium is a slow-acting
poison, but 1030 works so rapidly that it
has been impossible to save experimental
animals even with prompt attention and
under the best laboratory conditions.
Poisonous gases
Several poisonous gases have served
to control ground squirrels and, less
often, gophers; the most generally used
is carbon disulfide. The method of ap-
plication varies with the gas. These gases
can be bought in cans or drums from
dealers in garden supplies.
Sulfur dioxide has been widely used
in the past for killing ground squirrels
and is still employed by some persons in
special machines (see page 18).
Carbon disulfide, although inflam-
mable and explosive, is now used in large
quantities for rodent control in Califor-
nia. It may be pumped into the burrows,
or waste balls may be soaked in fluid
carbon disulfide and thrown into the bur-
rows.
Hydrocyanic acid gas was tried for
squirrel control but did not prove satis-
factory in California. This gas is used,
with good results, for rat control. It is
generated by placing calcum cyanide in
flake or dust form in the burrows.
Methyl bromide, first employed
against insects, has been used by the State
Department of Agriculture for follow-up
operations against ground squirrels (see
Berry, 1938). Since this gas kills
fleas and other insects in the burrows, it
has a decided advantage in plague-control
work. It is very poisonous, and at least
one human death has occurred through
its improper use in insect control. Special
applicators are necessary to distribute the
gas, and it costs too much (3 cents or
more per burrow) for routine work. It
is bought as a fluid under pressure in v
heavy steel cylinders, or in 1-pound cans
for smaller operations.
[6]
Trapping, shooting, and scale it is expensive, and it cannot be ap-
exclusion plied to all kinds of rodents or in all
Trapping is effective for control of places,
pocket gophers and moles, mice in houses,
and wood rats, and is often used for other Encouragement of
species.
natural enemies
Shooting will control small numbers All the common rodents originally had
of ground squirrels and rabbits. natural enemies that helped to check their
Exclusion, where practicable, is the increase. Many of these enemies have
best method because it may bring lasting been reduced in numbers and some even
results. It can be used to protect small eliminated by man, and yet they are an
plots or gardens against ground squirrels, asset to agriculture. Some of them, such
pocket gophers, and rabbits. On a large as coyotes, also prey on domestic animals
GENERAL PRECAUTIONS WITH POISONS
All of the substances used for poisoning field rodents are dan-
gerous to human beings and to domestic animals. They should
therefore be handled, stored, and labeled with great care.
The following precautions should never be forgotten :
1. Label the containers for poison and poisoned baits "POISON."
2. Keep all such containers locked up out of the reach of children, irre-
sponsible adults, pets, and livestock.
3. Store carbon disulfide in tightly stoppered cans or drums in a cool dry
place, out of doors or in a separate building, away from all fire, matches, and
sparks. It is a dangerous explosive.
4. Mix poison baits, particularly zinc phosphide baits, out of doors or in a
well-ventilated building, where there will be less hazard to the operator.
5. Do not breathe the dust when sifting dry strychnine or other dry poison
over baits.
6. Wear gloves when mixing or distributing poison baits, particularly if they
contain zinc phosphide.
7. Wash hands carefully after mixing or handling baits, even though gloves
have been worn.
3. Wash utensils after mixing baits, and do not use them for any other pur-
pose.
9. When using poison gases, always handle the materials out of doors; do
not breathe the fumes; stand up-wind when using or placing the material in
burrows.
The minimum lethal dosage of strychnine for a human being is thought to
be about 0.5 grain, or 35 milligrams. Thus about 250 kernels of strychnine-
coated barley would be enough to kill a man. About 5 ounces of zinc phosphide
bait may be lethal for a man. Carbon disulfide is lethal in a concentration of
about 1 part in 1,000 parts of air with 30 minutes exposure.
Antidotes for these poisons are uncertain in their action and are different
for the various poisons mentioned. // accidental poisoning occurs, a physician
should be called at once.
[7]
and birds, and their economic value as
checks on rodents has to be balanced
against the harm they do. Certain natural
enemies, however, such as the red-tailed
hawk, the barn owl, and the gopher snake,
are so useful that only a very shortsighted
person would destroy them; yet many
farmers kill every hawk and snake they
see, without regard for its usefulness.
Whenever possible, farmers should pro-
tect the owls, hawks, snakes, and other
animals that prey on rodents.
Undesirable methods
of control
There is a popular idea that certain
rodents may be killed by disease germs
(virus) spread on baits. Bacterial cul-
tures have been sold for ths purpose, but
the State Department of Public Health
has made such sale illegal in California.
These cultures not only often fail to re-
duce the number of rodents, but also
sometimes carry infection to man. When
these baits are spread among a popula-
tion of rodents, some of the exposed ani-
mals die, but a certain number always
survive, become carriers of the infection,
and sometimes, by their droppings, con-
taminate supplies of food intended for
human use. The use of such bacteria
(which belong to the paratyphoid group,
Salmonella) to control rats about certain
institutions has several times resulted in
outbreaks of food poisoning among the
human inmates. Efforts at control of
rodents by spreading disease among them
are impractical and dangerous to human
beings.
For more information . . .
Additional materal about rodents is available in the following reference works:
Hull, T. G.
1947. Diseases transmitted from animals to man. 3d ed. xvii + 571 p. Illus. C. C. Thomas, Spring-
field, 111.
Ingles, L. G.
1947. Mammals of California. Stanford University Press, Stanford University, Calif, xix + 258 p.,
42 pis., 56 figs.
Martin, A. C, H. S. Zim, and A. L. Nelson.
1951. American wildlife and plants. McGraw-Hill Book Co., New York, ix + 500 p. Illus.
Storer, T. I.
1952. Controlling rats and mice. California Agricultural Experiment Service Circular 410: 1-36.
17 figs.
[8]
II. Ground and Tree Squirrels
California ground squirrels have long been important
harmful rodents in California; they destroy crops,
damage irrigation structures, and carry disease.
The commonest ground squirrels in
this state are the large, long-tailed "dig-
ger," or California ground squirrels
(Citellus beecheyi and its subspecies;
fig. 1) that inhabit most of the state
except the desert portions; the smaller,
short-tailed Oregon ground squirrel
(Citellus oreganus; fig. 7) that lives in
the northeastern plateau counties; and
the golden-mantled ground squirrel
Fig. 1. California ground squirrels; head-and-body length 9Vi to 1 1 inches, tail 6 to 8 inches.
Above, Beechey ground squirrel; below, Douglas ground squirrel.
[9]
(Callospermophilus chrysodeirus or Cit-
ellus lateralis) of the higher mountains.
The latter two species are discussed in
this section, page 20. Several smaller
species live in arid regions, both west
and east of the Sierra Nevada, but usually
are of minor economic importance. For
detailed accounts of species see Fitch
(1948), Grinnell and Dixon (1918), and
Howell (1938).
Seven varieties or subspecies of the
California ground squirrel occur within
the state, each in a separate area. The
most widespread of these are (1) the
dark-colored Douglas ground squirrel
(Citellus beecheyi douglasii) , with much
black between the shoulders, which oc-
curs northward from San Francisco Bay
throughout the regions west and north of
the Sacramento and Feather rivers; (2)
the brownish Beechey ground squirrel
(C. beecheyi beecheyi), which occupies
coastal California from the Golden Gate
and Carquinez Strait south nearly to San
Diego; and (3) the gray-toned Fisher
ground squirrel (C. beecheyi fisheri) ,
which inhabits the greater part of central
California from the Feather and Sacra-
mento rivers south to the southern end
of the San Joaquin Valley. Other varieties
occur in the Owens Valley, the Tahoe
region, and extreme south parts of Cali-
fornia.
These are the largest ground squirrels
in California; the head and body are
about 9Vi> to 11 inches long and the tail
from 6 to 8 inches long. The differences
between the varieties are not important in
economic relations, except that the Doug-
las ground squirrel seems to be somewhat
easier to control.
Every ground squirrel has two thin,
internal cheek pouches, opening just in-
side the lips, one on either side of the
mouth, which are used to carry food. This
characteristic is important in control be-
cause ground squirrels may be killed by
absorbing poison baits through the lining
of the pouches, as discussed on page 13.
Ground squirrels were the most im-
portant rodent pests of agriculture in
California for nearly 150 years, but they
have been reduced by use of 1080 poi-
son by the county agricultural commis-
sioners.
Habits y
Ground squirrels are ground-living;
they find most of their food on the sur-
face of flat country, hillsides, or embank-
ments. Some kinds can climb trees,
however, and may reduce yields from
fruit or nut trees. Some live in open
forest, scattered chaparral, or rock piles,
but seldom in heavy growths of forest or \
brush. They are fair-weather animals,
active by day throughout the warmer
season of the year and on warm days in
the winter months.
Burrows. All species of ground squir- *
rels dig burrows, which they use for
safety retreats, for shelter during very
hot or rainy weather and during hiber-
nation, for occasional storage of food,
and for rearing their young. Burrows are
made in flat lands, in hillsides or among
rocks, and also in ditch, road, and rail-
road embankments. The entrances to
squirrel burrows are always open.
The burrows (figs. 2, 3) of California
ground squirrels average about 4 inches
in diameter, and individual burrows are
5 to 30 feet or more in length. Most tun-
nels are within 2% to 4 feet of the ground
surface, but at Davis one was found 6%
feet deep and in Fresno County one was
dug into 28 feet below ground level in a
chalk pit. It is important to know the
approximate volume of burrows when
gas is used for control. The estimated
volume of certain representative burrows
ranged from 1 to 18 cubic feet. Some are
simple short tunnels, but others have
many branches. Often there are two or
more openings. Some are "colonial bur-
rows" occupied by several squirrels. The
most complicated system yet found had ^_
33 openings, a total of 741 feet of tun-
nels, and a volume of about 100 cubic
feet. It contained 6 females and 5 males.
[10
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Fig. 3. Beechey ground squirrel at entrance to a well-used burrow from which trails lead out.
(Photo by California Forest and Range Experiment Station.)
Feeding habits. During the rainy
months, November to March or April,
ground squirrels feed chiefly on green
herbage. Seeds lying on the ground sur-
face are hulled and eaten as found. Later,
when the new seed crops begin to ripen,
the squirrels gather seeds without hulling
and put them in their cheek pouches, to
carry off and hide in shallow caches exca-
vated in the ground surface, or in crevices
between rocks. Some are carried into the
burrows for later use. Seeds of both wild
and cultivated plants, particularly grains,
are taken in quantity. In orchard dis-
tricts these squirrels climb trees for
almonds, walnuts, and the soft pits of
growing peaches and apricots. Sometimes
they may also forage in truck patches.
The feeding habits of ground squirrels
cause greatest damage to grain and pas-
turage. In fields they dig up sprouting
grain, and later pull down the ripening
heads. After harvest they may eat and
carry off quantities of grain from shocks
and stacks. On pasture lands they may
take a good deal of grass and herbage
that could otherwise be used by domestic
stock. The green forage eaten daily by the
average squirrel has been estimated at
2% ounces (70 grams) ; then 450 squir-
rels would eat as much as one steer. Both
on isolated ranches and in well-developed
areas, they often seriously deplete grain,
nut, and fruit crops.
Hibernation and estivation. All
California ground squirrels living at high
altitudes and some of the population,
mostly adults, at lower elevations hole up
for a part of each year. Before this period
of inactivity, each animal acquires a sur-
plus of fat. After going below ground the
squirrel plugs part of the tunnel just
above the nest with earth to as much as
3 feet in length, and curls up in its nest
below the tunnel plug. The burrow en-
trance remains open. While the squirrel is
holed up, the rates of heartbeat and
respiration are greatly reduced, and the
body temperature drops nearly to that of
the burrow.
The time at which any one squirrel or
those in a given locality hole up varies.
In the lowlands some enter estivation
early in summer, whereas in the high
mountains others do not begin hiber-
nation until late in autumn. Emergence
occurs in winter or early spring. Estiva-
tion ("summer sleep") begins as early as
mid-May in the hills east of Livermore,
and by late June in the hills of eastern
[12
Kern County, whereas it does not com-
mence until early August in Siskiyou
County.
This habit of a prolonged sleep below
ground explains why old breeding adults
suddenly appear in spring, after all squir-
rels active in a field during autumn had
been killed and there seemed no chance
for migration from the surrounding
fields.
The exact extent of estivation among
our ground squirrels is unknown because
it is very difficult to follow an individual
squirrel through its activities for any
length of time. Nevertheless, it is extremely
important to carry on intensive control in
the late winter, spring, and early summer,
when all squirrels are active. During hot
summer weather and in the fall and early
winter, some of the squirrels may be
underground and out of reach. And there
is also some doubt whether dormant ani-
mals are fatally injured by fumigating
their burrows with carbon disulfide.
Breeding. The California ground
squirrel in central California breeds
mainly during the first half of the year,
but some young are produced later in the
season. In the interior valleys, females
carrying young are most numerous in
February and March. In Los Angeles
County regular breeding activity was
found to have begun by December, and
in March 70 per cent of some 2,000
females examined there in 1925 were
pregnant (Storer, 1930). The breeding
season is somewhat later along the coast
and in the mountains.
Ground squirrels probably produce
only one litter a year. Of the average
litter, possibly about 5 or 6 squirrels
survive long enough to appear above-
ground. Where squirrels had been far
reduced by control for two or more
years and food for those remaining was
greater, an average litter of nearly 10 has
been noted (Jacobsen, 1923) .
The young grow rapidly and are seen
in greatest numbers from late April until
June, when they may scatter out to new
territory or move to unoccupied old
burrows. Control operations, therefore,
should be especially intensive in late win-
ter and early spring.
The rate of reproduction in ground
squirrels is such that unless 90 per cent
are eliminated in a given year there will
be no general reduction in numbers.
Theoretically, it would require eight to
nine years of control at this rate to rid a
given piece of land of squirrels entirely.
And this would occur only if there were
no reinvasion by migration from adjacent
areas. While most squirrels move about
but little, some have been known to mi-
grate from 1 to 5 miles into new areas.
These facts show that persistent and in-
tensive work is needed to keep down the
squirrel population. Furthermore, co-
operative efforts by all farmers in a region
are essential; otherwise cleared areas will
be invaded by squirrels from lands where
control is not practiced or is ineffective.
Control methods
Various baits and several kinds of poi-
son have been used for squirrel control.
Whole barley and strychnine alkaloid
(formula 1) have been most widely used
by private landowners. In recent years,
zinc phosphide (formulas 2 and 3) on
oats or barley has found favor.
Strychnine-coated barley. This
poisoned grain is used in the part of the
dry season when squirrels are "pouch-
ing" — that is, gathering seeds and grain
in their cheek pouches to store or to carry
to some favorite eating place. The barley
is coated with the less soluble strychnine
alkaloid, together with substances thought
to disguise the taste. Kernels are picked
up by the squirrels and placed in their
pouches. There the strychnine coating is
absorbed through the delicate lining of
the pouches, killing the animals. At other
times of year, the squirrels take scattered
seeds as found, eating each kernel after
quickly removing the hull, and the strych-
nine coating usually does not have a
chance to affect them. Sometimes squir-
[13]
rels will refuse to take strychnine-coated the person doing control work. It will
grain. Where grain or other crops are vary according to locality and year. The
maturing, the animals may turn to these activities of the squirrels should be ob-
and avoid poisoned bait. served. Prebaiting part of an area with
The exact time to use strychnine grain unpoisoned grain is a simple test which
for best results must be determined by should first be made to determine whether
FORMULA 1
For California Ground Squirrel
Barley (clean whole grain) 16 quarts
Strychnine (powdered alkaloid) 1 ounce
Bicarbonate of soda (baking soda) 1 ounce
Thin starch paste 3/4 pint
Heavy corn syrup 1/4 pint
Glycerine 1 tablespoon
Saccharin 1/10 ounce
Mix the strychnine, baking soda, and saccharin together dry. Prepare the starch
paste by stirring 1 heaping tablespoon of dry gloss starch in a little cold water until
smooth, pour into % pint of hot water, boil and stir until clear. Add the dry strych-
nine, soda, and saccharin, then the corn syrup and glycerine; stir thoroughly. Pour
the hot mixture over the grain, turning and stirring until each kernel is coated. Spread
the coated grain out in a thin layer until the coating is thoroughly dried. Then store
in a can or sack, properly labeled as POISON, until used.
FORMULA 2
For California Ground Squirrel
Oat groats (hull removed), lightly rolled 100 pounds
Zinc phosphide, powdered 10 ounces
Petrolatum, or petroleum jelly (vaseline) 40 ounces
Melt the petrolatum and stir in the zinc phosphide until evenly mixed. Pour this
mixture, a little at a time, over the grain, stirring continuously and vigorously until
all kernels are evenly coated with the blackish zinc phosphide.
A small batch (10 to 20 pounds) may be made in a large bucket or pan with a
wooden paddle or large spoon. Large quantities are best prepared in a steel or wooden
drum (or clean cement mixer) with inside baffle plates and mounted at an angle on a
shaft so that it can be turned easily. The drum should be revolved long enough to
ensure even coating of all grain.
Workers should wear gloves and should avoid breathing fumes from the poison
mixture.
FORMULA 3
For California Ground Squirrel
Whole barley or whole oats 100 pounds
Zinc phosphide, powdered 14-16 ounces
Petrolatum, or petroleum jelly 32-40 ounces
Mix as for formula 2. More poison and petrolatum are needed to coat whole grains.
Under wet conditions use the larger amounts of zinc phosphide and petrolatum.
See page 7 — General Precautions with Poisons
[14]
the squirrels will take and "pouch" the
bait used.
The poisoned grain should be scattered
by hand or with a spoon on hard bare
ground on or near the cleared surface of
squirrel runways. It is less likely to be
found and eaten if dropped in tall grass
or on the soft earth around burrows.
About 1 tablespoon should be scattered
at each spot to cover 1 to 2 square feet;
one quart provides 30 to 35 baits.
In farmyards or pastures where live-
stock are concentrated, the poisoned
grain should be placed inside the squirrel
burrows.
Failures in squirrel control will follow
the use of strychnine barley when rain
washes off the poison coating or when
squirrels refuse such bait. Failure may
also result when squirrels are feeding
heavily on filaree. It is believed that the
high tannin content of filaree will coun-
teract the effect of strychnine. This view-
point has been questioned by Burnett
(1932) — See p. 23 — whose experiments
with Wyoming ground squirrels led him
to believe that some individual animals
could build up a tolerance to strychnine
by repeated doses in amounts that were
not sufficient to kill. There are no studies
of this kind on the California ground
squirrel, but ordinarily about 20 kernels
are considered enough to kill the average
of this species.
Quail, pheasants, and domestic poultry
can withstand relatively large doses of
strychnine. They often refuse strychnine-
poisoned baits — but this is no excuse for
carelessness in placing poisoned grain.
Strychnine readily kills various wild ro-
dents and birds, and, if taken in sufficient
amount, can kill livestock. It is poisonous to
man, even in small amounts, and therefore
supplies of it should be locked up out of
reach of children or irresponsible adults.
Zinc phosphide bait. Zinc phos-
phide is a dark gray powder with a slight
smell of phosphorus. It is used on cereal
baits with a "spreader" of mineral oil,
petroleum jelly (vaseline) , or lecithin oil
(formulas 2 and 3) , which holds the poi-
son to the bait and helps to protect against
rapid deterioration in the presence of
moisture. In damp places the zinc phos-
phide slowly produces phosphine (PH 3 ) ,
a poisonous gas. Zinc phosphide baits
should be prepared and handled out of
doors or in a well-ventilated building.
Operators should wear gloves when mix-
ing or distributing baits.
The principal baits used with this poi-
son are whole barley, hulled barley, whole
oats (lightly rolled) , and oat groats (oats
without hulls and the kernels flattened).
Whenever possible, it is best to buy the
prepared zinc phosphide baits from a
county agricultural commissioner, who
also sometimes has clean oat groats for
use in preparing baits.
Zinc phosphide baits act directly when
the squirrels eat or hull the poisoned
grain. Such baits need not be stored in
the cheek pouches to poison the animals.
They may be used in spring and even in
winter — over a longer season than strych-
nine baits. Zinc phosphide is most effec-
tive when seeds of grain and of range
grasses are scarce. When such seed sup-
plies are easily available, the results of
poison distribution may be irregular or
poor. One pound of prepared grain pro-
vides about 50 baits, which are placed in
the same manner as strychnine-coated
grain. Zinc phosphide baits may some-
times endanger livestock and game. Dogs
have been killed by eating squirrels poi-
soned by zinc phosphide.
Warfarin. Recent experiments indi-
cate that warfarin, a substance used
mainly for control of "house" rats and
mice, is of value for control of ground
squirrels in places where there are local
concentrations, or about farm headquar-
ters where other poisons are hazardous.
Unlike the usual stomach poisons, war-
farin produces fatal internal bleeding
(hemorrhage) when eaten in sufficient
amount for several days. This substance
is sold under various trade names, but
[15]
Fig. 4. Equipment for using carbon disulfide in control of ground squirrels: stock can of carbon
disulfide, waste balls of jute, and milk can with tight-fitting cover in which waste balls are soaked
in the fluid. A stiff wire (not shown) hooked at one end is useful to lift out waste balls and place
them in burrows.
all packages show the active ingredient
as warfarin. For ground squirrels use is
made of the concentrated form (1 :200, or
5 per cent) , a mixture with flour or other
powder that is to be combined with bait
in a 1 :19 ratio by weight so that the final
mixture contains warfarin in a strength
of 1:4,000 (about 100 milligrams per
pound). On grain baits intended to use
with ground squirrels a mineral oil at
one quart per 15 to 20 pounds of bait
may be added to make the warfarin ad-
here to the grain; mixing is done as de-
scribed for zinc phosphide baits.
The final mixture with warfarin must
be exposed where the squirrels can feed
repeatedly so as to obtain the necessary
dose. A covered bait box with holes in
the sides large enough to admit squirrels
readily is satisfactory. If placed where
squirrels are accustomed to feed and play
they will soon discover the grain and
commence feeding.
Warfarin is no hazard to chickens or
turkeys, but dogs and cats will be killed
if they feed for several days on the bait
or if they repeatedly eat rats killed by
the poison. In some situations it may be
desirable to surround the bait box with a
wire fence of large mesh (hog-wire) that
will admit squirrels but exclude larger
animals and children.
There is little danger to human beings
from warfarin. If a child or adult eats
some prepared bait he should be caused
to vomit and a physician called at once.
Treatment includes transfusion with
whole blood of appropriate type and
giving Vitamin K by mouth or intra-
venously.
Carbon disulfide gas. Ground squir-
rels and some other rodents can be
controlled with poisonous gases. Gases
will also kill other animals living in squir-
rel burrows, such as skunks, cottontails,
burrowing owls, and snakes. The chemi-
cals which have been used include carbon
disulfide (CS 2 ; often called "carbon"),
[16]
carbon monoxide (CO), sulfur dioxide
(S0 2 ) , methyl bromide (CH 3 Br) , hydro-
cyanic acid (HCN), chloropicrin (CC1 3 -
(N0 2 ) and tetrachloroethane (C 2 H 2 C1 4 ).
Carbon disulfide is the gas most widely
used for ground-squirrel control. This
liquid chemical takes fire and burns
readily; it evaporates easily and quickly;
and, in the vapor or gas state, it is highly
explosive. Stocks of carbon disulfide
should be tightly stoppered to prevent
loss and should be stored out of doors in
a cool, shady place free from fire, sparks,
or exposed lights. The fluid corrodes tin
cans readily. Carbon disulfide is poison-
ous to man and to most animals, but it
does not readily kill fleas.
Since it is heavier than air, carbon
disulfide will settle in the lowest parts of
an underground burrow, and will not
pass higher than the burrow mouth unless
forced up with a pump. The fluid vapor-
izes and spreads more rapidly at high air
temperatures, but when it is put into bur-
rows during the warm dry season much
gas may be lost through surface cracks
in the ground.
Carbon disulfide is applied in two
ways: with a special pump to force the
fluid or gas into a burrow, or by soaking
waste balls in the fluid and then placing
them in the burrow. Formerly the gas was
ignited a few seconds after the disulfide
was pumped into the burrow, but this
practice has been given up, for the most
part, because it takes longer and is a fire
hazard. When conditions permit firing
the gas, however, somewhat better con-
trol seems to result. Ten or 15 seconds
after introducing the gas, a bit of paper
or a small kerosene-soaked rag on the
end of a stick or a stiff wire, 4 to 5 feet
long, is lighted with a match and inserted
into the open burrow. The operator
stands several feet to one side to avoid
being burned by the explosion from the
burrow mouth. Care should be taken not
to set fire to nearby grass.
Waste balls about 2 inches in diameter,
made of short, frayed strands of jute
(from grain sacks) , are sold in sacks con-
taining about 1,000 balls. Supplies of
waste balls can be bought from some
county agricultural commissioners. When
only a few are needed, wool trimmings or
scraps of cloth can be rolled up and tied
to make waste balls.
The balls are soaked in fluid carbon
disulfide (fig. 4) , and one is placed 15 to
18 inches down each burrow. Each waste
ball takes up nearly 2 fluid ounces, and
1 gallon of fluid will saturate up to 70
balls. Before being put into the burrows,
the waste balls should be drained for an
instant as they are lifted out of fluid.
Immediately after a burrow is treated,
the entrance should be closed with a
shovelful of earth and quickly tramped
down; or the entrance may be stuffed
tightly with newspaper.
Several types of pump have been used
in the past to force carbon disulfide vapor
into squirrel burrows. The Demon Rodent
Gun (fig. 5) has replaced most other
Fig. 5. The Demon Rodent Gun for pumping
liquid carbon disulfide into rodent burrows.
[17]
pumps. This machine consists of a metal
cylinder containing fluid carbon disul-
fide. The central pump connects to a
flexible hose ending in a spray nozzle.
The nozzle is placed 15 to 18 inches down
the burrow, and the entrance is plugged
with a shovelful of earth or a wad of
crumpled newspaper to prevent loss of
gas. Then a single stroke of the pump
forces out 2 fluid ounces of the fluid and
vaporizes much of it just inside the bur-
row. After the hose has been withdrawn,
the burrow entrance is closed by tram-
pling the earth or forcing in the news-
paper. Some operators use only 1%-
ounce dosages (by partial stroke of the
plunger) in the early spring and the full
2 ounces later in the year. Some farmers
first go over the area and close all bur-
rows by using mattocks or shovels, then
apply the gas several days later to those
that have been reopened by squirrels.
Others consider that this method takes
too much time except on clean-up work.
Burrows under trees should be treated
only by use of the rodent gun. Applica-
tion of waste balls dripping with carbon
disulfide may produce a concentration of
the chemical that would injure or kill a
tree by absorption through the roots.
Some earlier vapor pumps required nu-
merous strokes, but forced carbon disul-
fide vapor more or less throughout the
burrow; the Demon Rodent Gun requires
only a single stroke, but tends to leave
varying amounts of the liquid close to
where the nozzle was inserted and does
not force the vapor so far down the bur-
row.
Sulfur dioxide gas. Sulfur is burned
in the presence of air to generate sulfur
dioxide. Various machines to produce
and force this gas into squirrel burrows
have been built and used by California
farmers. One recent design uses a gaso-
line torch with a sulfur chamber in front
of the flame; the draft created by the
torch flame forces the fumes through a
nozzle which is inserted in the burrow.
Another type is a cylindrical tank through
which air is forced by a hand-driven
rotary blower over the burning sulfur;
fumes from the sulfur pass into a tapered
nozzle or a large hose inserted in the
squirrel burrow.
The whitish fumes, besides killing
squirrels below ground, will reveal leaks
from cracks in the ground surface or
from other entrances connected with the
same tunnel system. Then the operator
can shovel earth to close the leaks. Some
users of sulfur dioxide report good re-
sults, but others are of the opinion that
this gas is not effective enough.
Other gases. Calcium cyanide, in
flake or dust form, placed in or pumped
into a damp burrow, generates hydro-
cyanic acid (HCN) , a deadly gas rapidly
fatal to all forms of animal life. Between
1925 and 1930, calcium cyanide was tried
by official agencies for ground-squirrel
control in California; but the results were
not satisfactory.
Another gas fumigant is methyl bro-
mide, used in recent years for clean-up
operations on ground squirrels after
other methods have been employed. It is
supplied as a fluid under pressure in
heavy steel cylinders or in 1-pound cans.
It requires a special release valve and a
trained operator to apply and is more
expensive than some other gases.
Carbon monoxide is contained in the
exhaust gases of an automobile, and
where field conditions permit driving to
the sites of squirrel burrows, a hose from
the exhaust pipe may be used to force the
gas into the burrows, which should then
be closed in. Running the engine for a
few seconds forces the gas throughout the
burrow. This method has been used under
trees where carbon disulfide might dam-
age or kill the trees.
Trapping. Traps may be used to re-
move small local populations of squir-
rels — as in dooryards or on ditchbanks —
or to clean up those that escape poison or
eas. This work may be done at any season.
The wooden box-type gopher trap, with
some changes (fig. 6), is useful in re-
[18]
Fig. 6. Trap for ground squirrels (made from
wooden pocket gopher trap) as set; smaller
figure shows rear of trap.
moving ground squirrels about residences
and in places where other control meth-
ods are unsuitable (Becker, 1940). The
back of the trap is replaced by %-inch
straps of iron, half of the trigger loop is
removed, and the bottom of the loop is
straightened so that bait is held above
the ground level; also the trigger holding
the spring on top of the trap is changed
to release when pulled forward toward
the trap front. Entire walnuts, citrus
fruits, and melon rinds proved effective
baits in southern California. Several traps
placed within a few feet of one another
on top of the ground have completely
cleaned out small colonies of squirrels in
a few days.
This type of trap has the following ad-
vantages over the spring steel trap : It can
be baited, hence is more attractive to
squirrels. It is more selective, catching
fewer other animals. It kills the squirrel
instantly, and it probably does not leave
trap-wise squirrels, such as those which
escape from steel traps.
The Young trap, a rectangular cage of
hardware cloth with a drop door at either
end and a central trigger pan in the floor,
also has been used successfully for ground
squirrels.
The steel jump trap (Oneida No. or
1) was formerly used for ground-squirrel
control. It lies flat against the ground and
can be buried flush with the surface near
a feeding place or in a burrow entrance.
The trap should be anchored by a stake
driven into the ground through the ring
at the end of the trap chain ; the stake also
marks the trap location. Bait such as
barley may be scattered over the setting.
Squirrels taken in any of these traps
should be handled with gloves to prevent
fleas from getting on the operator. Squir-
rels taken alive may be killed by drown-
ing while still in the traps.
Shooting. Small numbers of squirrels
in fields or other open places may be de-
stroyed with a long-range .22 caliber rifle
and sometimes with a shotgun.
Natural enemies. Ground squirrels
have various natural enemies that aid in
reducing the numbers of these rodents.
Wherever practicable, these enemies
should be protected and encouraged.
They include the coyote, badger, weasel,
wildcat, red-tailed hawk, golden eagle,
rattlesnake, and gopher snake. Badgers,
weasels, and snakes capture the squirrels
in their burrows. Wildcats and coyotes lie
in wait near the burrows until the squir-
rels come out in search of food. Coyotes
and rattlesnakes may not be practical as
aids in squirrel control, but the others
named are very useful in this capacity.
Some observations in San Diego
County give evidence of the service of
hawks and golden eagles. Four nests (two
golden eagle, one red-bellied hawk, and
one red-tailed hawk) with young con-
tained a total of 14 ground squirrels, 9
jack rabbits, and 2 pocket gophers. The
dead rodents in the nests represented the
surplus which the old birds had carried
to their young, in addition to food eaten
on the days of observation. Hawks and
eagles may, therefore, kill more rodent
prey than they and their young can eat.
The large predatory birds are important
aids in rodent control; they work every day
in the year and without expense to man.
An occasional hawk may take chickens,
[19]
but most of these birds are entirely bene-
ficial.
Other methods. Small isolated or-
chards of deciduous fruits or nuts may
be protected from ground squirrels by
smooth cylinders of tin fastened about
the tree trunks. This tinning, if started
about 2 feet above the ground and con-
tinued upward for 2 or 3 feet, will keep
the ground squirrels out of the trees un-
less there are drooping branches on
which they can climb.
Flat disks of sheet metal about 2 feet
in diameter are often used to protect
leaves of young roadside trees from being
eaten by ground squirrels. A hole in the
center of each shield admits the trunk of
the tree; and a radial cut, from circum-
ference to center, enables the shield to
be placed in position, below the first
branches.
Seed corn can be treated with coal tar
to protect it from ground squirrels dur-
ing germination. For this purpose, 1
tablespoon of coal tar is added to a gallon
of boiling water. When the mixture has
cooled somewhat, the corn is stirred in
and allowed to remain several minutes;
germinating qualities will not be im-
paired.
Costs of control
The amount of infestation, the method
used, the current prices for labor, the
speed and skill of laborers, and the kind
of area treated all affect the cost of con-
trol. Large-scale operations on flat lands
are less expensive per acre than those on
small land units or in rough foothills with
brush and rocks.
Some recent prices for materials were:
alkaloid strychnine, per ounce, $1.49
(1952) ; zinc phosphide, per pound $1.27
(1947) ; carbon disulfide, per gallon, 70
cents or higher (1948) ; waste balls, per
thousand, $3.00 to $3.50; strychnine-
coated grain, per pound, 12 cents (1952) ;
and warfarin (0.5 per cent), per pound,
$2.15 (1951). Present-day variations in
the prices of materials and in the cost
and quality of labor make it impossible
to give costs per acre.
OTHER GROUND SQUIRRELS, in northeastern California,
may damage alfalfa, pasture, and grain.
The Oregon ground squirrel {Citellus
oreganus oreganus) inhabits most of
Lassen, Modoc, and eastern Siskiyou
counties. The related Belding ground
squirrel (C. oreganus beldingi) lives in
the high Sierra Nevada south to Fresno
and Inyo counties. This squirrel is of
stocky build (fig. 7), with a short tail
and plain brownish-gray coloration. The
head and body are about 8V2 inches long,
and the tail is about 2% inches. The
animal is mainly an inhabitant of grass-
lands, and practically never climbs. It sits
bolt upright when alerted, hence is often
called "picket pin." It has a shrill trilling
whistle that carries for a long distance.
Habits
The burrows resemble those of other
ground squirrels but are smaller and not
so deep in the ground. The food is chiefly
grasses, pasture vegetation, and the leaves
and stems of alfalfa and grain. Most
Oregon ground squirrels hole up in July
and emerge from hibernation by mid-
March or earlier, even when there is still
much snow on the ground. The average
litter is about 8, and the young appear
from mid-May at the lower altitudes
(3,000 feet) until early June at higher
levels.
Control
The Oregon ground squirrel may be
controlled by poisoned oats (formula 4)
or poisoned dandelion greens.
The poisoned grain should be scattered
by hand or with a spoon on hard bare
ground, or along and near squirrel run-
ways. It is less likely to be found and
[20
«*£^
Fig. 7. Oregon ground squirrel of northeastern California. Head-and-body
about 8V2 inches, tail, 2'/2 inches.
FORMULA 4
For Oregon Ground Squirrel
Oats, recleaned 20 quarts
Strychnine (powdered alkaloid) 1 ounce
Bicarbonate of soda (baking soda) 1 ounce
Thin starch paste 3/4 pint
Heavy corn syrup 1/4 pint
Glycerine 1 tablespoon
Saccharin 1/10 ounce
Mix the strychnine, baking soda, and saccharin together dry. Prepare the starch
paste by mixing 1 heaping tablespoon of dry gloss starch in a little cold water until
smooth. Then pour into % pint of hot water, boil and stir until clear. Add the dry
strychnine, soda, and saccharin, then the corn syrup and glycerine; stir thoroughly.
Pour the hot mixture over the oats, turning and stirring until each kernel is coated.
Spread the coated grain out in a thin layer until the coating is thoroughly dried. Then
store in a can or sack, properly labeled as POISON, until used.
See page 7 — General Precautions with Poisons
[21
eaten if dropped in tall grass or on the
soft earth around burrows. About 1 table-
spoon should be scattered at each spot to
cover 1 to 2 square feet; one quart pro-
vides 30 to 35 baits.
For poisoned dandelion greens, use
formula 4 but substitute 5 gallons (bulk)
of freshly cut green dandelion plants with
roots, or fresh chicory, for the oats. Water
cress or alfalfa leaves are less satisfactory.
Two or three pieces of greens are put into
each burrow; this does not endanger
birds.
Carbon disulfide may be used to gas
burrows of this squirrel in the manner
described for the California ground
squirrel.
Around mountain cabins and in some
localities in eastern California and the
deserts it sometimes becomes necessary
to control other species of ground squir-
rels, and also chipmunks. In this group
are the golden-mantled ground squirrel
(Callospermophilus chrysodeirus) , the
antelope ground squirrel or chipmunk
(Ammospermophiliis leucurus) , and the
small striped chipmunks ( Eutamias) .
The general methods described for con-
trol of the California ground squirrel
(pages 13-20) will usually prove satis-
factory.
To rid a small area of a few squir-
rels, as about a mountain cabin, the
animals may be shot or trapped. Ordinary
wooden rat traps can be used, but care
must be taken when removing killed ani-
mals to prevent fleas or ticks, possibly
disease-bearing, from getting on the
operator. Gloves should be worn, and
clothing should be treated immediately
afterward with fly spray. Poisoned bait
should be used only when it can be placed
on the ground, under shelter of boards
or logs, where birds or pets cannot reach
it. Warfarin bait may be of value in such
places.
TREE SQUIRRELS — The gray squirrel and the eastern fox
squirrel sometimes raid fruit or nut trees.
In the foothills and mountains of Cali-
fornia there are two kinds of native tree-
inhabiting squirrels active during the
daytime: the gray squirrel (Sciurus
griseus) and the red squirrel (5. doug-
lasii) , also called chickaree or pine
squirrel. The head and body of the gray
squirrel measure about 10 to ll 1 /^ inches,
with the tail nearly as long; the coat
above is light steel gray in color, with
the under surface of the body and margin
of the tail white. The red squirrel is
smaller, 7% to 8Vt inches, with the tail
4% to 5Vi> inches long. Its coat is dark
brown above with a reddish tinge on the
back, and a black line along either side
borders the white or buff belly. The tail
is blackish with white-tipped margins.
The gray squirrel sometimes lives near
nut and fruit trees and may raid these.
One grower in Santa Cruz County re-
ported that in years of short acorn crops
the gray squirrels took many English wal-
nuts and also gnawed into boxes of
apples stored in the open. He captured
several squirrels in box traps, transported
them less than a mile from his ranch, and
released them; no further damage was
noted. Tin guards, like those described
for ground squirrels, would exclude tree
squirrels as well, provided there were no
low-hanging branches on the trees and
also that the trees were sufficiently far
apart so that the gray squirrels could not
travel overhead from tree to tree.
Red squirrels should be kept out of
mountain residences because some have
proved to carry diseases transmissible to
man.
Open seasons for hunting tree squirrels
have been declared in recent years by the
Department of Fish and Game for certain
districts or parts of districts. These may
change from year to year; some local
[22
open seasons have been established where
damage by tree squirrels was occurring.
If a farmer experiences damage in a place
where there is no open season, he should
ask the local game warden for a permit
for control by shooting.
A brownish fox squirrel (Sciurus niger
rufiventer) from the eastern states has
been introduced and become established
in several parts of California. It is griz-
zled brown above and rusty colored
below, and is nearly the size of a gray
squirrel. In the walnut orchards of the
San Fernando Valley it is now sufficiently
abundant to require annual control. Dam-
age also is done to oranges and avocados,
and lead-covered telephone cables are cut
by these squirrels. Control is accom-
plished chiefly by the use of wooden
(gopher) traps as altered for taking
ground squirrels (fig. 6), and in places
by shooting under permit.
For more information • • •
Additional material on ground squirrels may be found in the following references:
Becker, E. M.
1940. An effective ground squirrel trap. California State Dept. Agr. Bui. 29: 152. 1 fig.
Berry, C. E.
1938. Methyl bromide as a rodenticide. California State Dept. Agr. Bui. 27: 172-80. 5 figs.
Burnett, W. L.
1932. The action of strychnine on the Wyoming ground squirrel Citellus elegans. Colorado Agr.
Exp. Sta. Bui. 384: 1-19.
Fitch, H. S.
1948. Ecology of the California ground squirrel on grazing lands. Amer. Midland Naturalist 39:
513-96.
Grinnell, Joseph, and Joseph Dixon
1918. Natural history of the ground squirrels of California. California State Comm. Hort. Monthly
Bui. 7: 597-708. 5 pis. 30 figs. (Out of print.)
Howell, A. H.
1938. Revision of the North American ground squirrels. U. S. Dept. Agr., Bur. Biol. Survey.
North American Fauna 56: 1-256. 32 pis. 20 figs.
Jacobsen, W. C.
1923. Rate of reproduction in Citellus beecheyi. Jour. Mammalogy 4: 58.
Storer, T. I.
1930. Summer and autumn breeding of the California ground squirrel. Jour. Mammalogy 11:
235-37.
[23]
III. Pocket Gophers and Moles
These burrowing rodents are harmful to agriculture
throughout California, especially in fertile
areas intensively cultivated.
Pocket gophers (genus Thomomys;
fig. 3) are stout-bodied and short-legged,
with blunt heads, conspicuous incisor
teeth, and external, fur-lined cheek
pouches for carrying food. They have
brownish coats, small eyes and ears,
short, nearly naked tails, and long claws
on the front feet. The head and body
usually measure 6 to 8 inches; the tail is
3 to 4 inches long.
Although pocket gophers and moles
differ in both structure and mode of
work, their workings are sometimes con-
fused. The mole (fig. 16) differs from
the pocket gopher in the shape of the
head, the color of the coat, and especially
in having large front feet with stout
claws. The illustrations (figures 9, 10,
11, 16, and 17) show important dif-
ferences between their methods of work
and in the appearance of the tunnel open-
ings and the earth piles around the tun-
nels. Differences between the surface
workings of the two are described on
page 35.
Pocket gophers are distributed over
most of California and inhabit practically
all but very rocky areas; they are most
abundant in the better soils. More than
40 species and varieties of pocket gophers
live in California, but their habits and
the methods for their control are essen-
tially the same. The population varies
from place to place; large numbers may
be present in alfalfa fields where no con-
trol has been practiced, while lands with
sparse plant cover have few gophers.
Habits
Burrows. The pocket gopher is strictly
an inhabitant of the soil, living in burrows
of its own construction, never climbing,
Fig. 8. Pocket gopher. Important features are the blunt head, small eyes and ears fur-lined
cheek pouch at each side of mouth, long slender claws on forefeet, and scantily haired tail. Head-
and-body, 6 to 8 inches, tail 3 to 4 inches. Compare fig. 16.
24
Fig. 9. Method of the pocket gopher in pushing earth out of a lateral tunnel, by use of the
forefeet and head; later the exit will be closed with earth for some distance down the lateral
tunnel. Compare fig. 17.
and only seldom coming out on the sur-
face of the ground. It digs clean-cut round
tunnels, about 2 inches in diameter. These
are more or less parallel with the surface
of the ground, usually at depths of from
6 to 14 inches, but deeper in places. The
earth from these (figs. 9 to 11) is pushed
out on the surface through short lateral
tunnels made at frequent intervals (or
forced into abandoned tunnels). This re-
sults in a series of rounded surface
mounds which, by their position, usually
give a clue to the location of the main
tunnel. When putting earth out of a lateral
tunnel, the gopher pushes the loads of
earth into a more or less crescent-shaped
pattern ; and when the lateral is closed, a
central depression in the mound usually
indicates the location of the mouth of the
lateral.
Fresh mounds are often dark because
of the moisture in the earth that has been
recently pushed out. Any grasses or herbs
covered over by a mound are blanched
(by loss of chlorophyll) after a few
days, which provides another indication
of its age. Trapping is most productive
in fresh workings.
In addition to the lateral tunnels used
to push out earth, the gopher makes short,
almost vertical laterals in coming to feed
on surface vegetation. These often are
closed with earth that does not rise above
the adjacent ground surface.
Gophers dig deeper tunnels in connec-
tion with their nests, and may dig short,
steeply pitched "sumps," possibly to
drain adjacent tunnels. The nest is usually
in a chamber about 8 inches in diameter;
it is constructed of fibers of grasses and
'-,**&*
Fig. 10. Live pocket gopher at mouth of lat-
eral tunnel. Loads of earth from below ground
have been pushed out to the front and sides
so the mound has a crescent-like form with the
opening at the center. A feeding exit would
have less earth around the entrance.
[25]
other plants, shredded like fine excelsior.
Food is often stored beside the nest or in
other enlarged chambers of the tunnel
system.
The burrow system of a pocket gopher
may be many yards in length (fig. 11).
Ordinarily each system is inhabited by a
single gopher, although young may re-
main in the tunnel occupied by the
mother for a time after leaving the nest.
The systems of adjacent gophers may be
connected, but connecting tunnels and
even portions of the workings of a single
animal are often plugged firmly with
earth. When a gopher is trapped out of
a tunnel system another animal may later
move in and occupy that system. Moles or
mice occasionally use gopher burrows.
Pocket gophers are active throughout the
year (even in mountain areas, where they
work beneath the snow and put the sur-
plus earth in tunnels in the snow), and
fresh workings may be found in any
month. Surface activity is less on dry
areas during the hot summer months; at
this season new mounds may be entirely
lacking on unirrigated lands of the in-
terior valleys. The animals are also less
active during and just after a heavy rain.
Breeding. On pasture lands and on
uncultivated and unirrigated areas there
is evidently a limited breeding season
some time after the beginning of the
rains, when green forage becomes avail-
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COUPLING
"fcrdT
Fig. 14. Probe for locating pocket gopher tunnels to insert poisoned baits. The shaft may be in
one piece or divided by a pipe coupling for convenience in carrying when not in use.
[31
Fig. 15. Use of probe for placing pocket gopher baits; when pushed into the ground, if the
probe suddenly drops about 2 inches, a main tunnel has been located; then the probe hole is
made large enough to insert a poisoned bait.
this probe, can treat several hundred
holes, over as much as 40 acres, in a single
day. The probe is easiest to use when the
soil is damp and soft down to the level
of the main tunnels and less easy when
the ground is hard. It is unsatisfactory
in sand. In adobe soil that cracks when
dry, the probe drops as easily into a crack
as into a burrow. In finely cultivated
fields the dry surface soil should be
scraped back before closing the hole,
which may be done with a clod of earth.
The best time to use the probe is in the
fall during the first cool weather or just
after the first good rains, and in the
spring months. Burrows are easier to
locate when the mounds are conspicuous,
before green vegetation becomes tall and
abundant. The land should be gone over
thoroughly at this season. Alfalfa fields,
due to the abundant and continuous food
supply, are someties harder to treat effec-
tively than orchards or open fields.
Gophers are most apt to gnaw or girdle
orchard trees during late summer, after
the surface of the ground has become dry
and green vegetation is scarce. Thus at
the season when the gopher is doing the
most serious damage, and speedy destruc-
tion is most to be desired, the probing
method is least easy, and the operator
may have to dig down to the main run-
ways to place poisoned baits. In a garden,
nursery, or lawn where it seems desirable
to use poisoned baits rather than traps,
formula 5 or 6 may be employed.
[32]
In dry ground, where use of a probe
is not practicable, the main runs of the
gopher can be opened up in the same
manner as for setting traps. By use of a
slender pointed stick, one of the poisoned
baits is placed a foot back in each run-
way, which is then tightly closed. The
hole should be opened 2 days later; if the
bait is gone and the hole remains open,
the gopher is probably dead.
Approximate costs for controlling go-
phers in alfalfa by poison baits have
been estimated recently by Miller (1953) .
Assuming a 5-acre field averaging 20
gophers per acre, the strychnine alkaloid
($29.00 per pound) would cost 38 cents
and the carrots (5 cents per pound; 25
per cent wastage) only 25 cents. Labor
to prepare and place the baits, including
retreatment to kill gophers surviving the
first application, would require about 4
hours; at $1.25 per hour the cost would
be $5.00. The total per acre cost would be
$1.13 and for each gopher destroyed
about % cent for materials and 5 cents
for labor. A field largely or entirely free
of gophers will yield appreciably more
crop, when the increased value will more
than offset the cost of control.
Poisonous gases are not often effec-
tive against pocket gophers. Factors
which make the gas method less efficient
are the extent of the burrow system, the
chance for leakage of gas through the
softer earth of laterals, the closeness of
the main runs to the surface of the
ground, and the fact that gophers may
quickly plug off their burrows when a
poisonous gas is detected and so escape
destruction. Some workers have reported
success with carbon disulfide or calcium
cyanide (flakes or dust) , and a few have
used methyl bromide or chloropicrin. In
general, control by gases has not proved
economical or as efficient as other meth-
ods already described. The exhaust gas
from an automobile, which contains some
carbon monoxide, may be used for
gophers in the manner described for
ground squirrels (page 18). Various
gopher "bombs" have been offered for
sale, which, when lighted and placed in
the burrows, generate a gas intended to
overcome gophers. In general they have
not proved satisfactory.
Flooding. In most sections of Califor-
nia where irrigation is practiced, crop-
lands and orchards are periodically
flooded. On alfalfa this is done regularly.
At such times the gophers are either
drowned or forced out by the incoming
water. If their tunnel systems include runs
in the levees, they may avoid the water
by entering these. Some, driven out into
the open, seek the higher borders of the
field. At this time they may be easily killed
by a good dog or by a stroke of the irri-
gator's shovel.
Exclusion. Small flower or vegetable
gardens or orchards, adjacent to wild
lands over which the gardener has no
control, sometimes need special protec-
tion against the entrance of gophers either
by burrows or by overland migration.
A fence of small mesh wire or of sheet
metal or concrete extending about 24
inches below the ground surface and
about 10 to 12 inches above the ground
will usually protect against gophers. In
lighter soils greater depth may be desir-
able. If the fence is built to 36 inches
above ground, it will also exclude rabbits.
Cementing ditches is effective where
gophers are active in burrowing through
the banks. A power company that had had
much trouble with pocket gophers in a
canal bank dug a vertical trench 4 inches
wide and 6 feet deep lengthwise through
the middle of the bank. The earth was
loosened with a crowbar and removed
with a narrow post-hole shovel. Then the
trench was filled with a "lean" mixture
of cement and sand. Concrete was also
used to protect a small irrigation ditch
having a 7-foot surface from gophers,
weeds, and leakage. First a %-inch coat
of 1 to 7 cement and then a i/i-inch sur-
face layer of 1 to 3 cement was applied
to the sides and bottom. Such costly pre-
ventive measures are advisable only
[33
where the usual control methods are in-
effective.
Young trees may be protected against
gnawing by gophers if a cylinder of wire
netting (1-inch mesh or smaller) about
12 inches in diameter and 18 inches tall
is sunk in the hole around the tree when
it is planted; the top of the wire should
be a little under the surface of the ground
to avoid difficulty later in cultivating
around the tree.
Trenching is successful for small-scale
operations. A steep- or vertical-walled
ditch 18 inches wide by 24 inches deep
is dug around the plot that is to be pro-
tected against gophers. Open-topped 5-
gallon cans, spaced at intervals of 25 feet,
are sunk so that their tops are level with
the bottom of the ditch. Gophers getting
into the ditch will be likely to fall into
the cans, from which they cannot escape.
Encouraging natural enemies. The
barn owl and gopher snake are useful
aids in gopher control. The owl nests in
barns, steeples, palm trees, and holes in
cliffs or earth banks. Its diet is almost
entirely of rodents, often mainly of pocket
gophers. After digesting a meal, an owl
regurgitates the indigestible portions as
a "pellet" dropped below its roost. Anal-
yses of pellets from many roosts show that
pocket gophers are often the chief item
of diet (Smith and Hopkins, 1937; Evans
and Emlen, 1947) . One pair of owls may
take 3 to 6 gophers daily when feeding
their young. This owl rarely eats birds
and never kills poultry. The gopher snake
commonly eats gophers in fields and or-
chards, but sometimes takes eggs from
wild birds or from hen-houses. Every
gopher eaten by either of these animals
means one less for the farmer to catch.
MOLES are not rodents; their work is often confused with that
of pocket gophers, but different control measures are needed.
Moles are often garden pests. They are
not rodents, but belong to an entirely
different order, the Insectivora. Their
habits, food, and the methods for their
control are different from those of
gophers. Both moles and pocket gophers
^•m%.^mztm
live in the soil, make underground tun-
nels, and put up earth mounds on the
surface. The workings of these two ani-
mals are confused by many people.
The mole (genus Scapanus; fig. 16)
has a slender, conical snout, no external
Fig. 16. The mole. Distinctive features are the slender snout, short needlelike teeth, large fore-
feet and claws, velvety fur, and short tail. Head-and-body about 5 inches, tail 1 Vi inches. Compare
fig. 8. Beyond the mole is a surface tunnel or run.
34
eyes or ears, small, needlelike teeth, and
forefeet with large palms and heavy
claws. The silvery black fur is of velvety
texture and quite short. Unlike a pocket
gopher, the mole has no cheek pouches
of any sort.
Moles are common in the northwestern
humid coast belt of California south to
Monterey Bay; some live in the river bot-
toms of the lowlands and in other places
with damp soils, as in foothills and moun-
tain meadows, and locally in gardens and
citrus groves of southern California. Ir-
rigated pastures are providing places for
moles to live where they could not previ-
ously survive. They are absent from much
of the agricultural area of the state.
Two entirely different kinds of work-
ings are made by moles : tunnels or runs
just below the ground surface, and deep
burrows; the first type is more extensive.
In searching for food a mole moves along
very close under the ground surface and
pushes up a low rounded ridge (fig. 16) ,
leaving a tunnel or run below, through
which the animal may travel once or re-
peatedly. The gopher's tunnels are much
deeper and cannot be observed from
above except at the entrances. The deep
burrows of the mole, like the main tun-
nels of gophers, are farther below the
ground surface; the earth from such ex-
cavations, instead of being pushed out
of an open tunnel, is forced up from be-
low, there being always a central core
of loose earth, so that the surface of
the mound resembles a miniature volcano
(fig. 17). The runs and surface mounds
disfigure lawns, golf greens, and flower
beds. In making them the mole may
loosen or uproot small plants or cut the
roots of larger ones.
Moles partly compensate for this dam-
age by eating soil insects and worms,
which are their principal food. But they
also eat sprouting seeds and bulbs to a
limited extent, and they cut the roots of
some plants when making their runs or
tunnels.
Fig. 1 7. Mole hill and deep burrow of a mole.
Successive loads of earth are forced up as a
plug through the lateral tunnel to form an
irregular surface mound; the tunnel is never
open. Compare fig. 9.
Control methods
Trapping. For one or a few moles,
the persistent use of traps is recom-
mended. The ordinary Macabee trap will
sometimes catch a mole if set in a lateral
tunnel, but the modified Macabee is more
effective. Several special types are also
available (fig. 18). "Choker loop" traps
are used successfully against moles in the
Pacific Northwest, in the Middle West,
and in Europe; but these usually are not
available in California stores. In this
type, two loops (or diamonds) of wire
or metal are forced into the earth to en-
circle a run, and the trap is sprung by a
trigger pan touching the top of the ridge.
Two other traps commonly offered by
the hardware trade in California are the
Out-o'-Sight (with jaws) and the Red-
dick (with spears) . Both types are pushed
down to straddle the surface runway of
a mole; each has a trigger pan to be
pressed against the top of the earth over
the run. All these traps are released by
the upward pressure of the mole's body
against the earth over the run. The Out-
o'-Sight is a "scissor- jaw" type, and the
Reddick has several downward-directed
spears. The spring of the first causes the
two pairs of jaws to clamp the animal
firmly and fatally; the spears of the sec-
[35
Fig. 18. Mole traps as set for use. Left, choker
loop type (Nash); center, spear (Reddick); right,
lateral-jawed (Out-o'-sight).
ond are driven downward through the
earth and into the mole's body. The
choker loop is generally reported to be
the most effective; the spear type, least
so. The latter punctures the mole's skin,
a disadvantage if the pelt is to be saved.
To determine, before setting a trap,
which runs are in use, press down the
soil here and there on several surface
runs; if in use, the mole, in passing, will
raise the ridge again. Thereupon, press
the soil down lightly once more and set
a trap, pushing it down enough so that
the trigger pan rests firmly against the
earth over the run. Upon the next round,
the mole's body will force the trigger up-
ward and release the trap. In dry weather,
wetting the ground over a run is reported
to encourage a mole to return through
the run; then a trap may be set.
Repellents and gases. Lye, para-
dichlorobenzene ("PDB"), or naphtha-
lene, introduced into mole runways, a
teaspoon every 10 or 15 feet, is sometimes
helpful in repelling moles (Scheffer,
1930). Calcium carbide (used for gen-
erating acetylene) has been tried for the
same purpose in damp soil — with limited
success. Calcium cyanide dust blown into
mole tunnels is thought to be of some
value in control. Carbon disulfide poured
or pumped into a deep runway will some-
times kill the mole tenant, but may in-
jure or kill near-by plants. Exhaust gas
from an automobile may be forced
through a hose into a mole tunnel.
Poison baits. Ground meat or earth-
worms dusted with strychnine and placed
in the runways have been tried for con-
trol of moles in England with some
success.
For more information . • •
Further discussion of pocket gophers and moles can be found in the following
publications :
Evans, F. C, and J. T. Emlen, Jr.
1947. Ecological notes on the prey selected by a barn owl. Condor 49: 3-9.
Miller, M. A.
1946. Reproductive rates and cycles in the pocket gopher. Jour. Mammalogy 27: 335-58.
1948. Seasonal trends in burrowing of pocket gophers. Jour. Mammalogy 29: 38-44.
1950. Eradication of pocket gophers. California Agriculture 4 (Dec.) : 8-10.
1953. Experimental studies on poisoning pocket gophers. Hilgardia 22:4.
Scheffer, T. H.
1930. American moles as agricultural pests and as fur producers. U. S. Dept. Agr. Farmers' Bui.
1247: 1-20 (Revised).
Smith, C. F., and C. L. Hopkins
1937. Notes on the barn owls of the San Francisco Bay region. Condor 39: 189-91.
[36]
IV. Miscellaneous Rodents and Rabbits
Meadow mice, kangaroo rats, muskrats, and rabbits
damage farms and gardens in many places.
Meadow mice
Meadow mice or voles (genus Micro-
tus; fig. 19), commonly called "field
mice," are blunt-nosed, with small furry
ears and a scantily haired tail, and are
covered with soft dense fur that is black-
ish brown or grayish brown. When
grown they measure 4 to 6 inches in
head-and-body length, with the tail 1%
to 2% inches long. They live in fields
or in ditchbanks covered with weeds or
grass, in meadows, in grain or alfalfa
fields, sometimes around haystacks, and
in orchards with covercrops or where the
grassy ground cover is allowed to remain.
Meadow mice live both on the surface of
the ground and in burrows. Most species
cut off the vegetation to form little path-
ways about an inch in breadth that extend
here and there through the grassland.
These connect with the many small bur-
rows which the mice make in the soil
(fig. 20). Such workings are often hid-
den when the grass cover is tall. If the
presence of meadow mice is suspected,
it may be necessary to part the grass tops
and search beneath the cover. The num-
ber of mice present may be inferred by
the amounts of freshly cut grass or of
droppings to be seen in the runways.
Meadow mice may cut green vegetation
(including alfalfa), injure standing
grain, damage hay in loose cocks or
stacks, gnaw the bark and roots of trees
surrounded by grass and weeds, and eat
root crops or bulbs. Alfalfa fields are
occasionally damaged by meadow mice
to the stage where no profit results to the
farmer. The damage is irregular in
amount, season, and place of occurrence.
Periodic increases in meadow mice have
been noted at certain places in California.
They have increased in some recent years
on farms near Tule Lake (Siskiyou
Fig 19 Meadow mouse. The fur is dense and soft, blackish to grayish brown, and the ears are
furry and partly hidden. Head-and-body length, 4 to 6 inches, tail 1% to 2% inches.
[37]
Fig. 20. Burrows and runways of meadow mice in an alfalfa field heavily infested with these
rodents. The mice kill many plants by cutting the roots and eating the stems and leaves.
County), in the delta region of Sacra-
mento and San Joaquin counties, and
locally elsewhere.
Preventive treatment, where it can be
used, consists of clean cultivation. Or-
chards with covercrops should be watched
for signs of damage by meadow mice,
and steps taken at once to control them
if necessary. Otherwise, clean cultivation
in orchards and the removal of grass and
weeds along fences, about farm build-
ings, and around piles of lumber will
reduce the shelter and food for these ani-
mals.
Control methods. Meadow mice may
be controlled either by traps or by poison.
For a small area or a few mice the best
plan is to use mousetraps baited with oat-
meal, rolled oats, or bits of apple or car-
rot or other root vegetables, and set with
the triggers of the traps across the run-
ways. The traps will then be effective on
mice running in either direction along the
surface paths. Sometimes unbaited traps
will serve. Traps should be visited at fre-
quent intervals, since these mice are ac-
tive by day as well as at night and the
efficiency of individual traps is increased
by frequent attention.
When meadow mice are present in
large numbers or over a large acreage,
it is necessary to use poison. Formerly
strychnine on alfalfa leaves was em-
ployed (formula 10), but recently zinc
phosphide on rolled barley or oats or oat
groats (formulas 7 to 9) has been used.
The poisoned bait — either grain or al-
falfa — is broadcast by hand (gloves
should be worn) so that it will scatter
on the runways and be found by the mice.
For heavy mouse infestations, amounts
up to 15 pounds of grain bait are used
per acre. One man can treat 15 acres a
day, walking back and forth across the
field and using marker stakes along the
field borders to cover the field adequately
and evenly. More than one treatment per
season may be necessary when the mouse
population is large.
Recently it has been found that toxa-
[38]
phene spray, as applied in alfalfa fields jalja or cereals intended to feed stock or
for control of cutworms and grasshop- make hay, and must not be applied to
pers, is effective in killing meadow mice. any crops intended for human food.
One dosage used was 4 pounds of tech-
nical toxaphene in 2% gallons of water Kangaroo rats
per acre applied by airplane; a lesser In lowland localities, where dry farm-
dosage might be effective. This chemical ing is practiced adjacent to wild land of
may be used on alfalfa grown only for desert or semidesert character, kangaroo
seed. Toxaphene must not be used on all rats (genus Dipodomys) sometimes dam-
FORMULA 7
For Meadow Mice
Steam-rolled barley or oat groats 100 pounds
Zinc phosphide, powdered 10 ounces
Mineral oil 32 fluid ounces
Warm the mineral oil and stir in the zinc phosphide until evenly mixed. Pour this
mixture, a little at a time, over the grain, stirring vigorously and continuously until
all kernels are evenly coated with the blackish zinc phosphide.
A batch of 10 to 20 pounds may be made in a large bucket or pan with use of a
wooden paddle or large spoon. Larger amounts are best mixed in a steel or wooden
drum (or clean cement mixer) with inside baffle plates and mounted on a shaft at an
angle so that it can be turned easily. The drum should be revolved long enough to
ensure even coating of all the grain.
Workers should wear gloves and work outdoors or in a well ventilated room; and
they must avoid breathing fumes from the poison mixture.
FORMULA 8
For Meadow Mice
Cracked whole oats without hulls 100 pounds
Zinc phosphide 16 ounces
Mineral oil 32 to 40 fluid ounces
Mix as for formula 7. The larger amount of mineral oil should be used under damp
or wet conditions to prevent deterioration of the poison on the grain. (Cracked whole
oats, or "Pocatello oats," either clean or poisoned, can often be purchased from county
agricultural commissioners.)
FORMULA 9
For Meadow Mice
Whole red oats 100 pounds
Zinc phosphide 16 ounces
Mineral oil 24 fluid ounces
Mix as for formula 7.
FORMULA 10
For Meadow Mice
Green alfalfa leaves or steam-rolled barley 100 pounds
Strychnine alkaloid, powdered 3 to 4 ounces
Dissolve the strychnine in 2 quarts of water; sprinkle over the bait, turning the
latter until all parts are moistened.
See page 7 — General Precautions with Poisons
r 39 1
Fig. 21. Kangaroo rat. At either side of the mouth is a fur-lined pouch. The body is brown or tan
above, pure white below; head-and-body about 4 inches, tail about IVi inches long.
age grain crops. These distinctive rodents
(fig. 21) have long hind legs and feet,
short small forefeet, long tufted tails,
brown or tan backs, pure white under-
pays, and a pair of external cheek
pouches (like those of pocket gophers).
The head and body are about 4 inches
long and the tail about 7% inches. Kan-
garoo rats live in short shallow burrows,
in sandy or soft ground. The entrances to
the burrows are usually closed with earth
during the daytime. There may be con-
siderable fluctuation in their numbers.
Over much of interior California these
rodents are of slight importance, having
been exterminated from many areas long
under cultivation. When necessary, they
may be easily controlled by use of strych-
nine-coated barley (formula 11) broad-
cast by hand near the burrows as is done
for ground squirrels.
Muskrats
The muskrat (Ondatra zibethica) is a
large aquatic rodent with a scaly, scantily
haired tail and dense, dark brown fur
(fig. 22). The head and body measure
9% to 12 inches, and the tail is 7% to
10 inches long. The animals live in
marshy places about lakes or along
streams, and they feed on green vegeta-
tion and roots. They dig large burrows
FORMULA 11
Barley (clean whole grain) 16 quarts
Strychnine (powdered alkaloid) 1 ounce
Bicarbonate of soda (baking soda) 1 ounce
Thin starch paste 3/4 pint
Glycerine 1 tablespoon
Saccharin 1/10 ounce
Mix the strychnine, baking soda, and saccharin together dry. Prepare the starch
paste by stirring 1 heaping tablespoon of dry gloss starch in a little cold water until
smooth, pour into % pint of hot water, boil and stir until clear. Add the dry strych-
nine, soda, and saccharin, then the corn syrup and glycerine; stir thoroughly. Pour
the hot mixture over the grain, turning and stirring until each kernel is coated. Spread
the coated grain out in a thin layer until the coating is thoroughly dried. Then store in
a can or sack, properly labeled as POISON, until used.
40
with underwater entrances and sometimes
construct small "houses" of plant mate-
rials in quiet waters.
Muskrats reached the Imperial Valley
soon after completion of the canal carry-
ing water from the Colorado River. In
the 1930's muskrats escaped from fur
farms or were transplanted and released
in several localities from Del Norte and
Shasta counties south to Santa Barbara
and Kern counties (Storer, 1937; Twin-
ing and Hensley, 1943). Today, in the
coastal counties, they are present only
in a few places and in small numbers,
so that no damage has resulted. In the
Sacramento Valley south to the Delta re-
gion, however, and in parts of Modoc
County, they have multiplied and spread
to become a serious agricultural problem
wherever irrigation waters are stored and
used. The "rats" burrow commonly in
dams and ditchbanks and about head-
gates or outlet boxes, resulting in breaks
in the earth banks with consequent loss
of water. Farmers are experiencing in-
creased trouble and expense to repair the
damage and reduce the muskrats, besides
suffering some loss in crops. A crude esti-
mate of the annual expense in the Sac-
ramento Valley is nearer $50,000 than
$25,000. In addition, rice growers suffer
premature drainage of producing fields
by undetected leaks and breaks in ditch
banks that result in lessened returns on
the crop.
In eastern states, where irrigation is
unnecessary, the native muskrats do no
particular damage to agriculture; but
wherever irrigation or drainage is needed
in the West, the animals can be a con-
tinuing source of trouble.
For almost a decade fur trappers have
taken substantial numbers for pelts. In
1952 they captured about 70,000 which
sold at 47 cents to $1.36 each, yielding
about two-thirds the total fur income in
California. Trappers naturally seek musk-
rats where abundant and in winter when
the fur is prime.
How to trap. Where muskrats must
be prevented from damaging irrigation
structures, No 1 steel traps should be set
in burrows, or better on the characteristic
"feeding mounds" in water 2 to 3 inches
in depth, and should be partly sunk in
the bottom material. The trap chain
Fig. 22. Muskrat. The tail is narrow, higher than wide, and scaly; the fur is soft, dense, and
brownish. Head-and-body 9Vi to 12 inches, tail Th to 10 inches.
41
should be passed over a slender pole
driven into the bank but leaning over the
water; the outer end of the pole should
have a crosspiece at the top to prevent
the trap chain from being slipped off. A
trapped muskrat will usually dive into
deep water, so that the chain slides along
the pole and the animal is quickly
drowned. Some trappers use a bait of
parsnip, sweet potato, carrot, or other
root vegetable, suspended on a small stick
so as to be about a foot over the trap
site; but many use no bait whatever.
Other methods of capture have been de-
scribed by Lantz (1923) and Storer
(1937).
In the past, ditch owners have em-
ployed trappers under permit to reduce
muskrats, but this has not been satisfac-
tory because trappers did not remain long
enough to reduce the animals to small
numbers. Encouragement of trapping for
fur, followed by employment of paid and
supervised trappers where necessary, will
give greater protection and ensure that
the rodents will do no serious damage to
crops.
Muskrats near Bakersfield were con-
trolled by pumping carbon disulfide into
burrows at a season when water had been
withdrawn from the canals and the tun-
nels were easily found.
RABBITS damage or destroy crops and trees. Control
measures are limited because some rabbits are
protected as game.
Rabbits (fig. 23) comprise two major
types: the hares (genus Lepus) , repre-
sented by the jack rabbits and the snow-
shoe rabbits that live entirely above-
ground, make no nests, and bring forth
their young fully covered with fur, with
their eyes open, and able to move about
at once; and the true rabbits, including
the cottontail and brush rabbits (genus
Sylvilagus) , that dwell in dense cover,
under stone piles and brush, or in bur-
rows, and leave their young in the nest
for a period of growth. More detailed
discussion of rabbits in California will
be found in Orr (1940) — See reference
list at the end of this section.
Black-tailed jack rabbits (Lepus
calif ornicus) occupy the lowland, foot-
hill, and desert portions of California,
but are sparsely represented in the humid
coast region. They are absent from the
higher mountains above the yellow-pine
belt, but present in the northeastern pla-
teau counties. They depend upon speed
and dodging to escape enemies. They live
chiefly in open places, seldom inhabiting
dense brush or thick woods. Females may
produce more than one brood a year.
The number of embryos varies from 3 to
8, and the actual litter is about four;
young are in evidence through much of
the year, but the greater number are pro-
duced in the spring months.
Jack rabbits make no nests, but indi-
viduals often have a more or less regular
retreat or "form" beneath a bush, where
the animal is somewhat sheltered from
the full heat of midday sun and yet can
watch for the approach of enemies.
Their food includes a wide variety of
plants, both wild and cultivated, the lat-
ter including grain, alfalfa, various truck
and field crops, and at times the bark and
tender shoots of small orchard trees. In-
deed, one of the commonest complaints
is of the difficulty in getting new orchards
or vineyards started on areas adjacent
to lands where jack rabbits are common.
Formerly jack rabbits were enor-
mously plentiful, especially in the San
Joaquin Valley and on the flatter areas
of southern California. Many spectacular
rabbit drives were held; most of the hu-
man population of a district would turn
out and surround a territory several miles
in extent, driving the rabbits toward a
[42]
central corral bordered by wing fences.
After being concentrated in such an en-
closure, the rabbits were clubbed to death
by the hundreds and even thousands.
Drives and other measures have re-
duced the population far below the earlier
numbers, yet there are still enough jack
rabbits in some places to do considerable
damage to the more intensive agriculture
of the present day. Their numbers vary
considerably from year to year. Farmers
should watch these animals on their land,
because, in years when they are abundant,
crop damage may be severe. Overgrazed
lands tend to have larger jack rabbit pop-
ulations than areas on which the grass
cover is higher. Black-tailed jack rabbits
are not protected by law and may be de-
stroyed at any time of year.
Snowshoe rabbits live in the higher
parts of the Sierra Nevada and in the
northeastern plateau region of Califor-
nia. There are two kinds adapted to liv-
ing on or in the snow during the winter
months: the large white-tailed jack rabbit
or Sierra hare (Lepus townsendi sierrae)
and the smaller snowshoe rabbit (L.
washingtonii) ; both are hares. These,
like the black-tailed jack rabbits, are sur-
face dwellers. Their feet being densely
Fig. 23. Common California rabbits. Left, black-tailed jack rabbit that lives in open country;
head-and-body 18 to 20 inches, ears about 6 inches long. Center, cottontail, common in streamside
thickets and pastures; length about 13 inches, ears 3 inches long. Right, brush rabbit, found close
to bushes and chaparral in the foothills; length about 1 1 inches, ears 2% inches long.
[43
covered with long fur, they can travel
readily on snow. The tail of the snow-
inhabiting rabbits is always white, and
the animals themselves are white in win-
ter. These animals live where there is
little intensive agriculture; they occasion-
ally nibble the twigs and bark of apple
trees during the winter months, and the
snowshoe rabbit damages young forest
trees.
Cottontail and brush rabbits are
true rabbits occurring over the lower alti-
tudes in California. Cottontails {Syl-
vilagus audubonii and S. nuttallii) are
more common in stream-side thickets
and pastures; brush rabbits (5. bach-
mani) inhabit brushy and chaparral-
covered slopes in the hills. Little is known
about their rate of increase or manner of
caring for the young. The litters are small,
averaging about 4, and are usually pro-
duced in the spring months. Both of these
rabbits will feed upon cultivated crops of
garden and field.
These rabbits are classed as game and,
over most of California, may be hunted
only between November 15 and Decem-
ber 31. In the southern part of the state
(Districts 4, 4%, 19, and 21), they may
be taken at any time. Owners, tenants of
land, or their agents may kill rabbits on
their properties at any time in any part
of the state, but rabbits so killed may not
be transported or sold during the closed
season (Calif. Fish and Game Code, Sec.
1340.5,1341,1342).
Rabbit control
Rabbits are not difficult to control on
small areas but present greater difficulty
on large ranches. The methods used are :
(1) exclusion, (2) shooting, (3) repel-
lents, (4) trapping, (5) poisoning, and
(6) encouragement of natural enemies.
Exclusion. A fence of mesh not greater
than 1% inches, buried to a depth of 6
inches in the soil and carried 24 to 36
inches aboveground, will exclude all rab-
bits if it is patrolled at intervals to see
that neither rabbits nor squirrels have
dug passages under the buried portion.
Such a fence, if constructed of poultry
netting (although a heavier-mesh fence is
better) and combined with barbed wire
above, will often serve as a stock fence for
horses and cattle. Sheep and hogs, how-
ever, are apt to damage a light wire fence,
and hogs especially may root at the base
of such a fence and so destroy its effec-
tiveness. Neither jack rabbits nor cotton-
tails will ordinarily jump over a 24-inch
fence. But a jack rabbit when pursued by
a dog may do so, particularly if the fence
does not have closely spaced barbed wires
above. Such a protective fence must, of
course, be provided with tight-fitting
gates and with sills or other means for
making sure that rabbits cannot dig be-
low the bottom rails of gates (fig. 24).
The gates must be kept closed except
when vehicles or persons are passing
through.
Small spring-closing gates are useful
in many places. A fence for exclusion is
best for small flower or vegetable gardens,
especially those adjoining large areas of
pasturage, grain, alfalfa, or wild land.
Any high-priced crop such as a small field
of seed stock warrants the type of fenc-
ing here described. If such a fence is
constructed there will be almost no dam-
age by rabbits. When a small plot of
ground needs to be enclosed for only a
few months, light stakes may be used to
support the wire, the bottom of which is
buried in a furrow; then, after the crop
has been harvested, the wire and stakes
may be removed to permit the use of culti-
vating machinery.
Around large acreages of farmland,
provision of rabbit-proof fencing is ordi-
narily impractical. The large rectangular
mesh used as the lower part of a stock
fence, where sheep or hogs are being pas-
tured, has openings of a size that permits
the passage of rabbits. Fencing for large
areas with wire of strength adequate to
restrain livestock and of mesh sufficiently
small to exclude rabbits will cost upwards
of $1.00 per rod for the wire alone.
[44
Fig. 24. Rabbitproof fence and gate. Bottom of fence wire is buried 6 inches or more and lower
meshes should not be more than 1V2 inches apart. Gate is covered with 1-inch poultry netting and
bottom of gate must be within IV2 to 2 inches of the ground.
Over many years, farmers in California
have used individual mechanical protec-
tors to guard the trunks of young orchard
trees against damage by rabbits. Earlier,
thin slabs of yucca and other veneer ma-
terials were employed. Ordinary sacking
has been tied around trees with effective
results. Poultry netting of 1-inch mesh,
20-gauge galvanized wire, 18 to 24 inches
in width, cut into strips 12 to 18 inches
long and formed into cylinders around
young trees, is a common means of pro-
tection. Such cylinders should be braced
so that rabbits cannot press them against
the trees and gnaw at the bark between
the wire meshes. A fine-mesh wire or
hardware cloth would also protect against
meadow mice, especially if pressed down
into the ground at the lower edge.
Shooting. Rabbits may be effectively
controlled by shooting. They feed mainly
in the early morning, late afternoon, and
dusk of evening, when a gun is most effec-
tive; but they may feed also at night, so
that this method is not entirely successful.
Ranchers troubled by numbers of cotton-
tails may obtain some relief by inviting
sportsmen to hunt over their lands dur-
ing the open season. On certain ranches
spring hunting of jack rabbits is pro-
moted; the hunter thereby obtains some
sport at a season when other game cannot
be shot, and the rancher some reduction
of jack rabbits. Such hunting, however,
will seldom clear a ranch of the animals.
Reduction of jack rabbits in the spring
is of especial benefit in limiting the num-
ber of young which might otherwise be
produced. Jack rabbits have been hunted
for market use, and this aids in reducing
their numbers, but there is some danger
of contracting the disease tularemia when
[45]
dressing wild rabbits. Any person skin-
ning wild rabbits for fur or food is advised
to wear rubber gloves.
Repellents. Various repellent sub-
stances have been used to prevent rabbits
from gnawing the bark and twigs of trees
and vines. An adhesive whitewash has
some value. A strychnine-poisoned wash
(formula 12) has been used in Idaho to
protect young orchards. It should not
be applied where domestic animals are
present.
Fresh blood, daubed on young trees,
has been tried as a repellent, but results
are often unsatisfactory. Sulfurized lin-
seed oil as a repellent has served well in
some cases, but in other trials in Califor-
nia it burned the bark and killed orchard
trees so that its use cannot be recom-
mended. Laths dipped in sulfurized (lin-
seed) oil and driven in a circle about
young trees are reported to be effective.
Repellent 96A sold by the U. S. Fish and
Wildlife Service, Pocatello, Idaho, is a
spray or wash that has given protection
to trees during the dormant season in
some places.
In England, Thompson and Armour
(1952) had best repellent results with a
solution of 8 pounds of commercial resin
in one gallon of denatured ethyl alcohol.
Compound ZDC, Repellent 96A, and
"bone oil" (distilled from animal bones)
were relatively useless.
Trapping. The jack rabbit drives of
earlier years were a type of trapping. In
the Middle West and East, cottontails are
sometimes captured in a special box trap
known as the Wellhouse (Lantz, 1924) . A
permanent rabbit trap, constructed with
a horizontal run of sewer tile 6 inches in
diameter and 4 feet long leading into a
vertical 12 x 6-inch tile T, with a heavy
cover, has been used in Kansas. The en-
trance, surrounded by stones and brush,
gives a natural appearance. Such traps
may help in capturing cottontails in
orchards.
Poisoning. Use has been made of
poison against jack rabbits in districts
where ranches are scattered and the
human population is sparse. Ordinarily,
it should not be employed against cotton-
tails or brush rabbits because these ani-
mals have value as game.
The poisoned bait may be any material
relished by rabbits, such as alfalfa leaves,
grain heads, or oats. Since rabbits, like
many other animals, both wild and do-
mestic, are fond of salt, poisoned salt has
also been used. Before control by poison
is attempted, clean prebait of several
kinds should be spread in places where
rabbits are doing damage to determine
which kind will be taken most readily.
If poisoning is deemed necessary, the
person responsible for the operations
should make certain that all necessary
precautions are taken to protect domestic
animals, harmless wild animals, and hu-
man beings from danger. For example, on
stock ranges where poisoning has to be
FORMULA 12
Poison Wash to Protect Young Trees Against Rabbits
Strychnine sulfate 1 ounce
Laundry starch 8 ounces
Glycerine 6 ounces
Water 31/2 quarts
Prepare the laundry starch by mixing cold and then boiling in 1 pint of water.
Dissolve the strychnine in the remaining water by boiling. Add the paste and glycerine.
Cool and paint on trunks of trees. Do not admit domestic animals to orchards where
this poison wash has been applied.
See page 7 — General Precautions with Poisons
r 46 1
employed because jack rabbits are so
numerous as to reduce the pasturage,
substantial fenced pens excluding live-
stock but permitting jack rabbits to enter
have been used for the exposure of poi-
soned materials. Before poisoning, clean
prebait should be offered to get the ani-
mals used to feeding at the site and make
certain that the poisoned bait material
will be taken when provided later.
In the event that an owner or tenant is
having difficulty with rabbits which can-
not be solved by any of the methods
indicated above, he should consult the
agricultural commissioner or the farm
advisor of his county for appropriate
methods.
Encouraging natural enemies. The
red-tailed hawk and golden eagle both
feed upon rabbits, and the gopher snake
is known to capture small ones. These and
other natural enemies that subsist on rab-
bits and rodents are valuable aids to the
farms.
For more information . • •
Additional information on miscellaneous rodents and rabbits is found in the fol-
lowing reference works :
Lantz, D. E.
1923. The muskrat as a fur bearer with notes on its use as food. U. S. Dept. Agr. Farmers' Bui.
869: 1-20 (Revised).
1924. Cottontail rabbits in relation to trees and farm crops. U. S. Dept. Agr. Farmers' Bui. 702:
1-14 (Revised).
Orr, R. T.
1940. The rabbits of California. California Academy of Sciences Occasional Papers 19: 1-277.
10 pis. 30 figs.
Storer, T. I.
1937. The muskrat as native and alien. Jour. Mammalogy 18: 443-60. Also in: California Fish
and Game 24: 159-75. 1938.
Thompson, H. V., and C. J. Armour
1952. Rabbit repellents for fruit trees. Plant Pathology 1: 18-22, 2 figs.
Twining, Howard, and A. L. Hensley
1943. The distribution of muskrats in California. California Fish and Game 29: 64-78.
In order that the information in our publications may be more intelligible it is sometimes necessary
to use trade names of products or equipment rather than complicated descriptive or chemical
identifications. In so doing it is unavoidable in some cases that similar products which are on the
market under other trade names may not be cited. No endorsement of named products is intended
nor is criticism implied of similar products which are not mentioned.
Cooperative Extension work in Agriculture and Home Economies, College of Agriculture, University of California, and United States Department of Agriculture
cooperating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. J. Earl Coke, Director, California Agricultural Extension Service.
15m-7,'53(A6951)AA
There's
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in
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RODENT CONTROL
WILDLIFE CONTROL
A. B. degree, graduate
work for an M. A. in zool-
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ments for a Ph. D. in zoology.
Those who intend to take ad-
vanced degrees in medicine, veter-
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ing, find that the preliminary require-
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Employers look on training at the Univer-
sity of California at Davis as a better educa-
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are high . . . leaders in their various fields make up
the faculty . . . facilities for studies and research
are complete . . . the University experimental farm is
one of the nation's finest . . . students are stimulated by
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For further information — see the College Entrance
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University of California - Davis, California
These are but a few of the rewarding careers that
training in Zoology prepares you for.
Zoology students on the Davis campus of the University of
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museum work, research work, technical work,
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