UNIVERSITY Of CALIFORNIA PUBLICATIONS
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
THE CONTROL OF RED SPIDERS
IN DECIDUOUS ORCHARDS
BY
E. R. deONG
BULLETIN No. 347
August, 1922
UNIVERSITY OF CALIFORNIA PRESS
BERKELEY, CALIFORNIA
1922
David P. Barrows, President of the University.
EXPERIMENT STATION STAFF
HEADS OF DIVISIONS
Thomas Forsyth Hunt, Dean.
Edward J. Wickson, Horticulture (Emeritus).
, Director of Resident Instruction.
C. M. Haring, Veterinary Science, Director of Agricultural Experiment Station.
B. H. Crocheron, Director of Agricultural Extension.
C. B. Hutchison, Director of the Branch of the College of Agriculture at Davis.
H. J. Webber, Subtropical Horticulture, Director of Citrus Experiment Station.
William A. Setchell, Botany.
Myer E. Jaffa, Nutrition.
Ralph E. Smith, Plant Pathology.
John W. Gilmore, Agronomy.
Charles F. Shaw, Soil Technology.
John W. Gregg, Landscape Gardening and Floriculture.
Frederic T. Bioletti, Viticulture and Fruit Products.
Warren T. Clarke, Agricultural Extension.
Ernest B. Babcock, Genetics.
Gordon H. True, Animal Husbandry.
James T. Barrett, Plant Pathology.
Walter Mulford, Forestry.
Fritz W. Woll, Animal Nutrition.
W. P. Kelley, Agricultural Chemistry.
H. J. Quayle, Entomology.
Elwood Mead, Rural Institutions.
H. S. Reed, Plant Physiology.
L. D. Batchelor, Orchard Management.
W. L. Howard, Pomology.
*Frank Adams, Irrigation Investigations.
C. L. Roadhouse, Dairy Industry.
R. L. Adams, Farm Management.
W. B. Herms, Entomology and Parasitology.
John E. Dougherty, Poultry Husbandry.
D. R. Hoagland, Plant Nutrition.
G. H. Hart, Veterinary Science.
L. J. Fletcher, Agricultural Engineering.
Edwin C. Voorhies, Assistant to the Dean.
DIVISION OF ENTOMOLOGY AND PARASITOLOGY
W. B. Herms H. H. Severin
C. W. Woodworth E. R. de Ong
E. C. Van Dyke G. H. Vansell
E. O. Essig J. F. Lamiman
S. B. Freeborn
* In cooperation with office of Public Roads and Rural Engineering, U. 8. Department of
Agriculture.
THE CONTROL OF RED SPIDERS IN
DECIDUOUS ORCHARDS*
By E. E. de ONG
CONTENTS page
Introduction 40
Life history notes and habits of the three species of red spiders 40
I Common red spider (Tetranychus telarius) 41
Rate cf oviposition 41
Winter habits 43
Food plants ? 44
II Brown mite (Bryobia praetiosa) 45
Variation in hatching dates of winter egg 46
Food plants 47
III Citrus mite (Paratetranychus pilosus) 48
Food plants 48
Why red spiders should be controlled 48
Effect of defoliation on the prune 50
Relation between irrigation and red spider injury 54
Bud development 54
Dispersal of red spiders 56
Preventive and control measures 58
I Common red spider (Tetranychus telarius) 58
Preventive measures : Abundant soil moisture. Selected cover crops
Cultural practices. Intercrops 58
Control measures 59
Spraying versus dusting 59
Sulfur mixtures for spraying 60
Recommended practice 60
Preparation of^sulfur pastes 62
Experimental spraying 64
Sulfur dusting 66
Sulfur fillers 67
Intervals between dusting 69
Substitutes for sulfur 70
Nicotine dusts .' 71
Natural enemies 72
II Brown mite {Bryobia -praetiosa) 72
Winter spraying: Crude oil emulsion 73
III Citrus mite (Paratetranychus pilosus) 78
Spray program 78
Cost of spraying and dusting 79
Summary 80
* This work was done in cooperation with members of the Pomology Division
of the College of Agriculture, University of California, to whom I am indebted for
the interpretation of horticultural data. I also wish to express my thanks to
Mr. H. B. Stabler, County Horticultural Commissioner of Sutter County, to the
orchardists of Sacramento Valley who have cooperated with me, to the student
helpers in this work, and others too numerous to mention. The photographs are
largely the work of W. C. Matthews.
40 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
INTRODUCTION
The so-called Red Spiders are among the most persistent and
injurious pests of California's horticulture, yet no other pests are
more frequently allowed to go unchecked. The loss which they cause
is frequently not noticed, for, unlike apples attacked by the codling
moth, the fruit of prune and peach trees injured by red spider is
marketable. The grower's loss, however, is just as real, but comes
in so subtle a way as to be overlooked by many. Studies of yields
and of bud development, in attacked and in normal orchards, have
shown an annual crop loss ranging from $100 to over $400 per acre.
Besides this immediate loss there is the injury to the buds that will
produce next year's crop. These are so weakened that they may
shatter off, or at least produce infertile bloom, while the sap is so
thinned that the tree may become more susceptible to frost. 14
LIFE HISTORY NOTES AND HABITS OF THE THREE SPECIES OF
RED SPIDERS
There are three species of plant mites, commonly called "red
spider, ' ' which have long been recognized as more or less serious pests
of deciduous orchards, viz :
(I) Tetranychus telarius Linn,* the species active through mid-
summer and fall, generally known as the two-spotted, summer, or
common red spider.
(II) Bryobia praetiosa Koch,f the brown or almond mite, pri-
marily of the almond and prune, attacks during spring and early
summer.
(III) Paratetranychus pilosus Can. & Fanz.,$ commonly found on
citrus and deciduous trees through spring and summer.
* Tetranychus telarius Linn, and T. bimaculatus Harvey are considered synony-
mous, while T. sexmaculatus Riley is considered a distinct species. The first two
are commonly found on deciduous trees and other plants throughout the state, and
T. sexmaculatus (yellow mite) is usually found on citrus trees.
t Bryobia praetiosa Koch, 4 synonym Bryobia pratensis Garman.
X Paratetranychus pilosus Can. & Fanz., synonyms Tetranychus mytilaspidis
Riley, T. citri McG. Garman recently called attention to the appearance of a new
mite in Connecticut.^ On comparison of this species with our citrus mite they
seemed so similar that both Ewing and Quayle have expressed the belief that the
two species may be considered identical. McGregor, however, considers the two
species distinct.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
41
I. COMMON BED SPIDER (Tetranychus telarius Linn.)
The common red spider is a web-spinning species, usually found
on the under side of the leaf, excepting on the almond tree, where the
mite feeds on either leaf surface. This mite may be found in the adult,
and even in the egg and nymphal stages, in almost any month of the
year in the southern part of Sacramento Valley. The spring and early
summer months are spent on weeds and hardy cultivated plants, such
as strawberries and violets. Migration to orchard trees occurs at the
death or maturity of the spring host plant. During the middle and
late summer the mites increase rapidly and cause severe and pro-
tracted injury.
TABLE I
Rate of Oviposition of Red Spider (T. telarius)
Number of eggs from different mites.
Date
Max.
Temp.
Mite
No. 1
Mite
No. 2
Mite
No. 3
Mite
No. 4
Mite
No. 5
June 30
83° F.
3
July 1
85
4
July 2
72
6
July 3
82
6
July 4
90
July
88
14
July 6
83
14
July 7
79
8
16
16
13
July 8
80
4
3
13
8
July 9
79
2
16
6
4
July 10
76
16
16
13
July 11
77
9
2
July 12
70
4
2
Total
33
28
64
51
42
42 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Egg. — The egg, barely visible to the naked eye, is generally found
on the under surface of the leaf or scattered through the web attached
to the leaf. It is round, and when first deposited almost colorless, but
becomes yellowish-red before hatching. The rate of deposition of eggs
is dependent upon food and temperature ; temperature also influences
the time required for hatching. Ovipositing is rare at maximum daily
temperatures of 40°-50° P., but beyond this the rate rises rapidly,
the highest being at maximums of 76° F. to about 100° F. At the
latter temperatures, 12 to 16 eggs per day were frequently deposited,
but at daily maximum temperatures of 70° to 75° F. the daily rate
was from 2 to 8. At a mean maximum temperature of 87° F. the
duration of the egg stage was 3 days, and a drop to a mean of 77° F.
lengthened the average of the egg stage to 6 days.
The totals do not indicate the entire number of eggs which a single
individual would normally deposit but are records of a few days only.
Other investigators 4 have found total egg-laying periods of 12 to 36
days and a total number of eggs per female mite ranging from 51
to 110. 8
The variations in rate associated with the above temperature
changes are in accordance with Ewing's results in Oregon during the
month of October. 4 Temperature records were not given in Ewing's
report but probably were not above 72° F. The maximum number
of eggs deposited daily under these conditions was nine and the mini-
mum one, the average being two and three twenty-seconds.
The duration of the egg stage is directly associated with the tem-
perature, as is shown in Table II. At a mean maximum temperature
of 87° F. the length of the egg stage is 3 days; a mean maximum
temperature of 77° F. lengthened the egg stage to 6 days. The num-
bers of eggs used are too small to give reliable data, but they agree
with McGregor's and Donough's results in South Carolina where
larger numbers were used. 8
Larva. — The newly hatched larva is almost transparent and color-
less but after it begins to feed it changes to a greenish color. It is
six-legged in this first stage and has a round body. Movement over
the leaf surface is very restricted.
Nymphal stages. — The mite has eight legs beginning with the first
molt. In the older stages the mites feed voraciously and wander about
freely. The time of development from the egg to adult is from five
to ten days at summer temperatures.
Adult. — The adult mite moves rapidly over the leaf surface and
the webbing. The female is larger, with a body more nearly rounded
BULLETIN 347] CONTROL OF RED SPIDERS IX DECIDUOUS ORCHARDS
43
than the male's. Both are so small as to be distinguished only with
difficulty by the naked eye. The adult is yellowish-green, usually
with one large irregular dark spot or a cluster of small ones on each
side of the upper surface (see Fig. 1, opposite page 44). The colors
of the common red spider, as shown in the figure, were chosen as
typical for the adult while feeding. These may vary with different
host plants, 4 but the most striking change results from starvation.
Mites which are hibernating or insufficiently fed usually assume a
yellowish or red color.
TABLE II
Duration of the Egg Stage of the Common Red Spider (T. telarius)
Length of egg stage Mean max. temp.
July 3
July 4
Julv 5
Eggs deposited
Eggs hatched
3 davs
87° F.
July 9
Eggs deposited
July 10
Julv 11
July 12
6 days
77° F.
July 13
July 14
Eggs hatched
July 17
Eggs deposited
Julv 18
Julv 19
Julv 20
6 davs
77° F.
Julv 21
July 22
Eggs hatche 1
Winter habits. — The common red spider passes the winter in the
adult and nymphal stage on winter-growing plants, or hibernates
among leaves or in the soil. Colonies of mites ranging from eggs to
adults may be found on the leaves of wild morning glory (Convolvulus
arvensis) during the first part of December. When the upper part
of the morning glory is killed by frost, the mites are forced to hibernate
in the soil, on underground suckers or among dead leaves. Hibernat-
ing mites have also been found in bean straw and on blackberry and
loganberry leaves where the canes were lying on the ground. Con-
trary to common belief, the writer has never found them underneath
the bark of trees. It is probable that those attempting to hibernate
in such situations would fall a prey to the predators that are usually
found there.
44 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Hibernation experiments. — To determine the location of hibernat-
ing mites during midwinter, soil from beneath plants which were
infested in the late fall was carefully removed, layer by layer, to a
depth of four inches and kept in a warm room. Each lot was placed
in a separate jar, the top edge of which was covered with tanglefoot
to prevent any mites entering from the outside. Seeds or uninfested
young seedlings of beans or other host plants were planted in these
jars, which were held at a temperature of 60° to 70° F. Infestation
was noted in almost every jar within a few days after planting, show-
ing that the mites were lying dormant in the soil or surface mulch of
leaves. The greatest number were found on the immediate surface,
although in one jar, with soil from a four-inch depth, a single mite
appeared. Experiments with bark from the trunks of trees gave
negative results.
Cold storage experiments. — Great resistance to freezing temper-
atures was shown by adults and the older nymphs of T. telarius.
Colonies from wild morning glory were placed in cold storage at
temperatures of 36°, 40°, and 50° P. for 21 days, and, when returned
to a temperature of 70° P., living mites were found in containers
from each temperature, the highest record being 66 per cent from
the room held at 36° F. One adult female mite from this lot was
placed on a young bean plant and in two days had deposited three
eggs, but she died on the fourth day. These eggs hatched in seven
days at a temperature of 65°-75° F., thus proving that the mite was
quite normal after an exposure of three weeks at 36° F. Four female
nymphs and one adult male from this same temperature were also
kept alive for several days.
A second and longer experiment was then tried with mites of the
same species, stored from November 9, 1920, to January 3, 1921, at
temperatures of 32°, 36°, and 45°-50° F. The per cent of living
mites found in the different series, when removed to a warm room,
was 8, 12, and 5, respectively.
In most localities in California where red spiders are serious pests,
it is probable that dormancy will alternate with periods of activity
during the winter. A few eggs may even be deposited on warm days,
but no material increase occurs until spring.
Food plants. — The list of recorded host plants of this mite is grow-
ing constantly; the following arc subject to more or less severe injury
in California: almond, apricot, bean (pink, whites, etc.), blackberry
(wild), cherry, cucumber, geranium, hop, Malva parviflora, melon,
morning glory, grapevine, pea, peach, pear, plum, poplar, pumpkin,
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Bulletin 347] control of red spiders in deciduous orchards 45
rose, squash, strawberry, violet, walnut (English and Northern Cali-
fornia Black), watermelon.
In addition to this list, the following host plants have been
recorded in California and throughout the world : abutilon, acacia,
alder (red), alfalfa, Angelica, apple, arborvitae, ash, aster, beet
(sugar), birch, Bouvardia, cabbage, calla, carnation, castor bean,
celery, chard, chrysanthemum, Cinchona, citrus, clematis, Clitoria,
clover (red and white), corn, cotton, cowpea, cosmos, currant, cypress-
vine, dahlia, eggplant, elm, fever-few, flax, fuchsia, Godelia, goldenrod,
gooseberry, heliotrope, hemp, Hevea. holly-hock, hop-tree, horse-chest-
nut, iron-weed, jimson weed, larch, lettuce, lily (Easter), Manettia,
Manihot, maple, maize, mignonette, monkey-flower, moonflower, mus-
tard, okra, onion, passion-vine, Papaya, pea (sweet), peanut, pecan,
pepper, phlox, pink, pigweed, potato (Irish and sweet), radish, rasp-
berry, sage, Sesoania acgjjptiaca, smilax, snowball, sunflower, Thun-
bergia, tomato, turnip, verbena, willow.
In addition to the above, McGregor mentions 83 host plants that
are native, wild species in South Carolina, 8 perhaps none of which
is included among the above names. A systematic search of native,
California, host plants which are at least occasionally infested would
no doubt add many plants to the above record.
II. BROWN MITE (Brijobia praetiosa Koch)
The brown mite is a non web-spinning species which winters in
the egg stage. The egg hatches as the buds open in the spring and
the young mite feeds on the growing leaves and the blossom. The
mites usually congregate on the young twigs, especially the leaf scars,
through the day and migrate to the leaves at night ; the other two
species are found almost exclusively upon the leaf blade. There is
no distinct migration from one type of host to another, as with the
common red spider, the entire year usually being spent on one host.
They may, however, crawl from plant to plant or be carried about
by the wind.
Egg. — The egg is deep red, frequently with a yellowish tinge,
round, and barely visible to the naked eye. Eggs may be deposited
singly, but if abundant, are usually grouped in conspicuous patches
on the underside of the limb. A favored situation for ovipositing is
the slightly roughened bark of two or three year old wood. They are
also common on fruit spurs, and leaf and fruit scars. The white shells
of hatched eggs and molted skins of young mites are nearly always
found among the eggs, as seen in Pig. 2.
46
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
The hatching of the winter egg varies with the host and is so inti-
mately associated with the development of the spring growth as to
imply some relationship between the activity' of the developing host
and the hatching time of the egg.
TABLE III
Hatching Dates of the Winter Egg of the Brown Mite (Bryonia praetiosa)
Host
Hatching Date
Bud Development
A'mond
Feb. 21-Mar. 1, 1920
One-third to full bloom. Leaf clusters
one-fourth to one inch long.
Plum (Grafted on
peach)
Mar. 24-Mar. 31, 1920
Bloom buds opening. Leaf clusters
three-fourths of an inch long.
Apple
Mar. 27-April 3, 1920
Cluster buds opened. Leaf clusters one-
third of an inch long.
The last generation of mites on the apple may complete their egg-
laying from two to four weeks later than on the almond, i.e., July
to August 15. Mites are occasionally seen in September, but females
are rare by the first of August. Hence a period of from seven to
eight months is spent in the winter egg on the apple. Egg-laying
on the almond in the vicinity of Davis is usually completed by July 1,
but on early defoliated trees this date may be as early as the last
of May. The winter egg stage on prematurely defoliated almond trees
may continue from eight to nine months, but on normal trees the
period would be about one month shorter.
Larva. — Bright red, body almost globular, six-legged. Contrary
to the habits of the larval form of the common red spider, it is very
active, moves about freely over the leaf-surface and usually retreats
to a roughened part of the bark through the day.
Nymph. — All stages of the mite after molting the first time are
eight-legged. The color changes from red to brown or greenish after
the mite has fed a short time. The complete life-history was not
determined, but under most conditions three to four days were spent
in each instar. Nymphs of the second and third instar were found
on March 6, but no hatching of the winter egg was observed prior to
February 20.
Adult. — Reddish-brown, with a green tinge. First pair of legs
greatly elongated. Body flattened. Male slightly smaller and with a
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 47
mor^ pointed body than the female. Adults of the first generation
are found from March 20 to April 5. Second generation matures by
April 16 and the third generation by May 10.
Fig. 2. — Eggs of brown mite (Bryobia praetiosa) on rough bark. The white
objects are molted skins and egg shells. (X 10.)
Food plants. — The list of host plants for this mite grows constantly
as does that for the common red spider. The following are cultivated
plants subject to more or less serious attacks in California : almond,
prune, plum, peach, pear and apricot.
Those reported in the eastern United States, Europe, and Australia
are: alfalfa, barley, buckwheat, cherry, cherry (sand), citrus trees,
clover, gooseberry, grasses, oats, peas, quince, wheat.
48
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
III. CITEUS MITE (Paratctranychus pilosus Can. and Fanz.)
The citrus mite spins a web, but less abundantly than the common
red spider. The life-history of this mite on deciduous trees is similar
to that of the brown mite in that the winter is spent in the egg stage
on the host of the previous summer. A full account of its life-history
on citrus trees has been worked out by Quayle. 10
Fig. 3. — Eggs of the citrus mite (Paratetranychus pilosus), showing strands
of webbing and stalk; greatly enlarged.
This mite is of a deep red color, with conspicuous white spines
arising from prominent tubercles on the body. The egg is slightly
striated, a flattened sphere in shape, with a prominent stalk about
twice the diameter of the egg in height. This stalk is bent over at the
tip and ends in a slight enlargement (see Fig. 3). Strands of webbing
frequently extend from the upper part of the stock to the leaf -surface
or to other strands.
Food plants. — This mite feeds principally on citrus trees but is
found on almond, pear, apple, peach, prune, and plum trees and on
evergreens, such as Pittosporum, in northern California.
WHY RED SPIDERS SHOULD BE CONTROLLED
The red spider, while feeding, extracts the sap and reduces the
amount of green coloring matter (chlorophyll) as is evidenced by
the pale color of the leaves. This causes, first, a mottled appearance,
Bulletin 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 49
Fig. 4.-
-Sample I. Dried prunes from normal trees (top) and defoliated trees
(bottom). This fruit was all grown in the same orchard and under similar con-
ditions, except that one part of the orchard was defoliated by red spider.
50 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
and, finally, a paling of the entire leaf surface, which is frequently
followed by defoliation. Restoration from this type of injury to a
normal condition is rapid if the mites are destroyed before the leaves
are badly damaged (see Fig. 10). This recovery has been more pro-
nounced after attacks by the brown mite than by the common red
spider, probably because the former is more exposed and hence readily
killed by contact sprays. The seriousness of the injury is due to the
fact that substances are found in the leaf, associated with the green
coloring matter, by the aid of which the plant manufactures sugar
and starch from which the plant itself is developed. These materials
constitute more than four-fifths of the dry weight of all fruit and
even of the tree itself. If this green coloring matter is destroyed, the
plant is unable to feed a growing fruit crop properly or to mature
a normal number of strong buds for the coming year. The tree which
in midsummer has pale yellow leaves or is prematurely defoliated has
suffered a very serious reduction in its producing powers even though
it may mature a small fruit crop. The fruit must of necessity be of
small size, with an unduly large amount of pit in proportion to pulp —
a grade which is always a drug on the market. Trees that have been
prematurely defoliated by red spiders or other causes and then irri-
gated in the late summer will usually throw out new leaves (see
Fig. 7, d) and possibly bloom, from buds developed for next year's
growth. Other buds which have not been forced into foliage may
have so much of their stored starch withdrawn that they will shatter
off. A late second crop of leaves will manufacture and store more
sugar and starch but it is doubtful if it can restore a tree to normality
or develop fruiting buds.
The foliage must be retained in a normal condition until the last
of September or the first of October to enable deciduous fruit trees
to yield maximum returns. Foliage injury in any or all parts of the
tree reduces, in proportion to the extent of the injury, the amount of
food which the tree can manufacture, as shown by Harvey and Mur-
neek 7 and other investigators. Such weakness is manifest in many
ways : by scanty foliage, probably resulting in sunburn, infertile
bloom, and weakened growth. Common orchard troubles, usually
attributed to the need of fertilizers, irrigation, and cross-pollination,
may in some instances be due to the injury caused by red spiders,
and should be studied from this standpoint.
Effect of defoliation on the prune. — Data on the red spider injury
to the prune crop were obtained through the assistance of H. P.
Stabler, County Horticultural Commissioner of Sutter County. He
collected average, orchard samples during the last two years from
BULLETIN 347] CONTROL OF RED SPIDERS IX DECIDUOUS ORCHARDS 51
which the following grades and weights were determined. The first
sample contained 300 primes and the second 400. The two types of
fruit in each sample were grown under the same conditions, except
that of defoliation by red spider in a part of the orchard. Sample I
had been dried at the orchard, sample II was received in a fresh con-
dition and afterwards dried.
TABLE IV
Comparative Weights
of Fresh and Dried PRr
xes from Normal axd
,
Defoliated Trees
Average weight Average weight
of fresh fruit of dried fruit
Xumber of
of fruits to lb.
Percentage
of pits
ounces ounces
%
Sample I
Normal
0.403
0.22 '
40
72
11.4
Defoliated
13.8*
Sample II
Normal
0.565
0.259
64
13
Defoliated
0.438
0.205
78
17
* The small proportion of pits found in sample I indicates a better quality of
fruit than that in sample II, which may account for the difference in the effect
on the two grades.
The trees defoliated by red spider in sample I showed a crop loss
of 44 per cent by weight and a drop of 3 points in the grading. At
this rate of decrease a crop of dried prunes (40-50 grade) of 3 tons
per acre would have lost in weight 1% tons. The remaining 1%
tous would have depreciated in value from 11% cents per pound for
40-50 's to 744 cents per pound for 70-80 's, a total loss of approxi-
mately $453 per acre. Sample II showed a total crop loss from red
spider injury of $139.18 per acre (on the basis of three tons of dried
fruit per acre"), including both the reduction in weight and the
depreciation in value of the remainder. The injury to next year's
buds, also due to the work of red spider, causes a further loss.
A photograph of the dried prunes of sample I is shown in Figure 4 ;
the six at the top are from normal trees. The proportionate number
of the different sized fruits in Figure 5 is roughly indicated by the
number of individual prunes. For example, ten out of every hun-
dred fruits, from the defoliated trees, were shriveled like the one at
the left of row three from the top. The four to the right of this one
indicate the high proportion of low-grade fruit from the part of the
orchard defoliated by red spider. These trees had been absolutely
52
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
/
2
3
4
o
6
Fig. 5. — Sample II. Fresh prunes from normal trees (top) and defoliated
trees (bottom) (see Fig. 6). This fruit was all grown in the same orchard and
under similar conditions except that one part of the orchard was defoliated by red
spider.
BULLETIN 3-17] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 53
Fig. 6. — Normal and defoliated prune twigs showing the condition of the
trees from which the fruit of Fig. 5, Sample II, was gathered. The fruit and
twigs at the top of Figs. 5 and 6 are from normal and the lower ones from defoli-
ated trees.
54
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
defoliated by red spider three to four weeks before harvest, while the
remainder of the orchard still retained a fair amount of foliage. The
entire orchard was dry and in poor condition. A small proportion of
large prunes will be noted in the fresh fruit sample but none in the
dried; this may have been caused by varying conditions in the two
orchards from which the samples were drawn, or a low sugar content
in the large prunes from defoliated trees may have resulted in a
greater drying loss.
RELATION BETWEEN IRRIGATION AND RED SPIDER INJURY
Attack by red spider and drought have long been associated together.
In certain districts, the only protective measure used is abundant
irrigation, a practice that is not, however, entirely adequate in many
instances. But irrigation must always be considered of primary im-
portance, for an orchard suffering from drought is already in an
unthrifty condition which may be intensified by an uncontrolled
attack of red spider. Spraying is a waste of money or at best of
slight advantage unless an adequate supply of moisture is assured.
Bud development. — Much of the experiment work at the Univer-
sity Farm was with almonds, and since the orchard bore irregularly,
it was necessary to judge results by a study of the development of
buds rather than of production. Buds were selected in October in
certain sprayed and irrigated plots, to determine their relative devel-
opment under different types of treatment. The methods of treating
the various plots and the percentage of moisture found October 9 in
the first six feet of soil are shown in Table V.
TABLE V
Description of Sprayed and Irrigated Almond Plots
No.
Date of
Treatment
A
June 10
B
June 8
C
June 19
D
June 8
E
June 5
F
G
July 20
14-inch irrigation — no spraying
Sprayed with lime sulfur solution 2 gallons, sulfur 5
pounds, dry prepared flour paste 2 pounds, in 100
gallons of mixture '.
Sprayed with crude oil emulsion 15 gallons of oil in 100
gallons of mixture
Sprayed with 5 pounds of sulfur, plus 2 pounds of dry
prepared flour paste in 100 gallons of mixture
Dusted with sulfur
6-inch irrigation — no spraying
Untreated
11.51
8.81
t
t
12.23
7.78
fSimilar to plot B.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 55
The efficiency and more details as to preparation and use of the
spray mixture used in plots B, C and D will be found in the discussion
of Tables VI and VIII. It should be noted that the control in plot C
is for the brown mite, no attempt being made to check the red spider
attack during midsummer. In plots B and D the application was made
too late to give much protection against the brown mite, but it did
control the common red spider. An irrigation of approximately six
inches of water was given in July to plots B, C, D, E, and F. From
the contour of the ground it was impossible to apply a uniform depth
of water over all the plots and judging from the amount of soil moist-
ure present in October, plot F received considerably more than the
average depth. The comparative data given are possible from the acci-
dental joining of irrigation and spraying plots. Plot A was chosen as
a typical well-irrigated plot from a series of experiments planned by
the Division of Irrigation Practice.
TABLE VI
Comparative Numbers and Weights of Buds from Sprayed and Irrigated
Almond Plots in October
(Orchard attacked by both the brown mite and the common red spider.)
Plot
No.
Type of treatment
Total No.
of buds
on spurs
Buds per
spur
Rank by-
No. of
buds
Total
weight
of buds
on spurs
Weight
per bud
Rank by
bud size
Rank by
average
of weight
and No.
A
Early summer
irrigation
336
3.36
1
1.6429
4.89
1
1.0
B
Slimmer spray
(Lime-sulfur)
254
2.54
3
1.1221
4.42
2
2.5
C
Winter spray
(crude oil)
275
2.75
2
1.1752
4.27
3
2.5
D
Summer spray
(sulfur)
177
1.77
5
0.7133
4.003
4
4.5
E
Summer dust
(sulfur)
191
1.91
4
0.5281
2.76
6
5.0
F
Late summer
irrigation
55
0.55
6
0.1834
3.33
5
5.5
G
Untreated
38
.38
7
0.0846
2.23
7
7.0
The selection of buds, for the data given in Table VI, was made
as follows : Ten fruit spurs were selected from each of ten trees in
the different plots, the buds were then removed from these, counted,
and weighed. The total number of buds from the ten spurs taken
from each plot is given in column three of Table VI, the average num-
ber in column four, while in column five the rank or order according
to the total number of buds as listed. That is, the plot showing the
56 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
highest number of buds ranks first in column five. The same plan is
followed in grouping the data for the weights. The bud development
on the first of October is seen with typical foliage in Figure 7.
It should be noted that this comparison is not alone between spray-
ing and irrigation but also shows comparative control values for two
species of mites, the brown mite, and the common red spider. Plot
C represents the control of the brown mite alone ; in plots B, D, and
E, this mite had almost ceased feeding for the year, hence the value
of the treatment applies almost entirely to the control of the common
red spider.
From the data in Table VI it will be seen that the trees were
suffering from a combination of drought and red spider injury, as
irrigation at the proper time, but without spraying, resulted in even
better bud formation than where the mites were controlled but where
the trees suffered from drought. The combined injury from drought
and red spider resulted in the feAvest number of buds, as shown in
plot G. The next lowest in rank was plot F. The latter had been
defoliated rather early in the summer and, when irrigated, sent out
a heavy foliage at the expense of next year 's buds ; many of the latter,
which had been formed earlier in the year, withered and dropped off
at this time. This plot had practically no bloom the following spring.
It should also be noted that the summary of bud development for
plot C, where the brown mite alone was controlled, was identical with
that in plot B, in which the attack of the brown mite was unchecked
except for a very short time, while the red spider was thoroughly con-
trolled. Apparently the attack of the brown mite is as serious on the
almond as that of the red spider, a theory which is in accord with the
belief of many almond growers.
DISPERSAL OF RED SPIDERS
The usual ways of dispersal of the different species of red spiders
are: (1) moving from leaf to leaf; (2) crawling over the ground;
(3) wind carriage either of the individual mite or of a falling leaf
bearing a colony of spiders; (4) accidental carriage by insects or
birds which have rested upon an infested twig and then flown to a
new host.
Wind carriage is perhaps the most important of the four ways
under California conditions. Tests with sticky tanglefoot paper have
repeatedly shown that mites may be carried hundreds of feet even
by a light wind. 11 But the fact that fche falling leaf may be a carrier
of entire colonies of mites has apparently been overlooked. This seems
to be a natural means of infestation and was adopted as the basis for
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
57
making cultures in laboratory practice. A small section of an infested
leaf, pinned to another plant, soon dries, thus forcing a migration of
the mites to the new host.
Migration is more pronounced with the common red spider than
with the other two species. This is especially noticeable during the
hottest and driest months of the year, July and August. Voracious
Fig. 7. — Prune twigs showing bud development in sprayed and irrigated plots.
(a) Winter application of crude oil emulsion to control the brown mite,
Bryobia.
(b) Early summer irrigation.
(c) Summer application of lime-sulfur solution, sulfur and flour paste to con-
trol the common red spider, T. telarius.
(d) Late summer irrigation, causing a second growth of foliage. These trees
did not bloom the following year.
(e) Check. No spraying or irrigation. Note Bryobia eggs on the spurs.
feeding withers the leaves of the host plant and if it be those of a
small annual the entire plant soon dies. The mites then migrate or
are blown away by the wind. Defoliation of tall trees and the brush
along levees facilitates wind carriage and the infestation may advance
hundreds of feet in a few hours.
58 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
PREVENTIVE AND CONTROL MEASURES
I. COMMON RED SPIDER (Tetranychus telarius Linn.)
The control of the common red spider is frequently considered to
be dependent alone upon spraying or dusting. There are, however,
certain farm practices whose preventive value should be recognized.
Such measures alone, however, cannot be depended upon to control
the red spider under conditions favorable to its development. But
increasing the thriftiness of the tree and delaying somewhat the attack
on cultivated crops may be of great value in supplementing the general
spray practice. The postponement of the attack even for two or three
weeks during midsummer lessens the injury to any orchard.
Preventive Measures
1. Abundant soil moisture. — The advantages of timely irrigation
are shown on pages 54-56.
2. Selected cover crops. — The cover crops commonly used in Cali-
fornia, including legumes, grains, alfilaree, mustard, and the grasses,
are almost free from attack by red spiders. The common red spider
has been found feeding and ovipositing upon bur clover (Medicago
hispida and M. arabica), but apparently it does not thrive upon
either species. Any of the above-named plants may, so far as has
been observed, be safely used as cover crops, where such practice is
desirable. They are of value not only for green manuring but may
prevent the growth of host plants of the red spider.
3. Cultural practices. — Early plowing or other cultural practices
which kill or retard the growth of early spring host plants, such as
wild morning glory, and the cheese weed (Malva spp.), may aid
materially in retarding the development of the common red spider.
The mites are much reduced in numbers through the winter but
multiply slowly during the spring months on weeds and cultivated
plants. Migration of the mites from the spring to the summer host,
whether orchards, hops, or a truck crop, does not usually occur until
the early host has matured or succumbed to their attacks. Killing or
checking the spring host may prevent a great increase of the mites
and thus lessen the number of migrants.
4. Selected intercrops. — Intercrops for young orchards, subject to
red spider attack, can be chosen with the view of minimizing the
attack. If this is not done and very susceptible intercrops are grown
in the orchard in regions subject to attack by red spider, both the
orchard and the intercrop may suffer more than if each crop were
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
59
grown alone. Tomatoes and the various varieties of corn and sorghum
have been found safe to use as intercrops.
The above remarks concerning host plants apply only to the
common red spider, not to the brown mite or to the citrus mite. For
the control of the latter two species, winter or summer spra}4ng and
abundant soil moisture constitute the main dependence.
Control Measures
Spraying versus dusting. — Sulfur applied as a dust has long been
used in California for the control of red spiders, but in recent years
the practice has become increasingly unsatisfactory. Criticism of
dusting as a means of applying sulfur dates back at least to 1903
when Volck recommended a potassium sulfid spray as a substitute for
dusting in red spider control in Sutter County. 13 Sulfur dusting is
more satisfactory in regions where the air is somewhat humid* and
the winds light — conditions which are common throughout much of
the coast region. Dusting is of decidedly less value in the dry, windy,
interior valleys where the percentage of relative humidity runs very
low as will be seen in Table VII. Under such conditions, dependence
must be placed largely on spraying, preferably using a spreading and
adhesive material in the mixture. These general deductions have their
exceptions where the reverse of the above climatic features are found
during certain seasons of the year or in certain restricted localities.
TABLE VII
Monthly Mean of Relative Humidity at 5 p.m. During June, July, and
August, 1919 and 1920
Location
June
July
August
San Jose { 1919
(Coastal Region) 1 1920
%
50
49
30
30
16
23
%
56
44
32
30
16
20
%
54
51
Sacramento J 1919
(Interior valley) 1 1920
29
27
PpH "Rlnff 1919
17
(Interior valley) 1 1920
18
The greatest advantage of dusting over spraying is the speed of
application ; where climatic or other conditions permit its use it is a
valuable supplement to the slower work of spraying. One man with
* A distinction must be made between the amounts of condensed moisture on
leaf surfaces relative to the adherence of dusts. Moisture in sufficient quantity
to wet the surface of the leaf or condense in drops may actually prevent the
adherence of sulfur and cause it to run off onto the lower edge of the leaf. A
damp surface, however, will cause a more uniform adherence than a dry one.
60 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
a hand duster may cover from two to ten acres per day, according
to the size of the trees and the thoroughness of the application. With
a power blower, two men may cover from 20 to 30 acres per day, pro-
vided there is not sufficient wind to prevent dusting all day. Spraying
is very much slower, the range being from two and one-half to six
acres per day.
A serious objection to the dusting method is that two or more
applications are usually necessary, even where success is obtainable at
all in this way. One careful application of a liquid spray before the
attack is severe at any point should give a long period of immunity
from injury and is frequently sufficient for the entire season.
Sulfur Mixtures for Spraying
Sulfur may be applied as a spray in a pure state but only when
combined with a spreading or adhesive substance, such as casein*
or glue water, forming the so-called "wettable sulfurs" which are
similar to the commercial sulfur pastes. It may also be applied as
a caustic spray in chemical combination with other substances, for
example, a lime-sulfur solution either with or without the addition
of pure sulfur. Sulfur applied as a spray has apparently the same
effect as when applied as a dust and adheres better. Its action is
slow but extends over a period of several days.
The caustic solution of lime-sulfur acts as a "contact spray, ' :
killing the mites within 24 hours, provided they are well covered.
This spray gives immediate relief from injury and hence should always
be used when the mite is in evidence. A spreader, such as casein or
flour paste, is especially useful with all caustic sprays to facilitate
wetting all the foliage.
Recommended practice. — The following formulas have been tested
and found very successful for the past two years at the University
Farm and in a number of private orchards in Sutter and other
counties of northern California. To get the best results they must
be applied at the right time and with painstaking effort to cover both
the upper and lower sides of every leaf. Careless, scanty, or hasty
spraying is of almost no value. A large almond tree will require from
eight to 12 gallons to thoroughly cover it; medium-sized prune trees,
from four to eight gallons per tree.
Sulfur spray, Formula 1, is to be applied just before the time when
the mites usually began feeding in the orchard in previous years, but
before they are found on the trees. Should the work be delayed until
red spiders are present, then use Formula 2.
* There are now other forms of commercial spreaders, developed since this
experimental work was completed, which may be substituted for calcium caseinate.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 61
Formula 1
Sulfur (sublimed or powdered) 5 pounds
*Calcium casemate £ pound
Water to make 100 gallons
Prepare a paste of the sulfur and casemate as directed on page
62. Add this to the spray tank with the agitator running. The pres-
ent commercial form of casein is given preference as it is a better
spreader than the others suggested and is very convenient to use.
The first of July is the usual date for this application in the Sac-
ramento Valley. It maj^ be necessary to apply it in June in orchards
where the spring host plants are very abundant or if drought has
hastened the death of the first host. If the work is done early and
thoroughly, it usually holds the mite in check for the entire season.
Contact sprays. — Actual infestations by the common red spider or
the brown mite require a quicker acting spray than sulfur alone.
A spray is required for this type of work that will kill the mites on
contact and also one which contains sufficient sulfur to act upon the
3 r oung mites as they hatch from the eggs. The following formula has
been found very satisfactory, but if desired, dry lime-sulfur or other
lime-sulfur substitutes may be used in place of the solution. These
materials are added in the liquid form after the paste is prepared as
under formula 1.
Formula 2
Lime-sulfur concentrate (31°-34° Baume) on almond 2 gallons
prune 1 gallon
peach 1 gallon
Calcium caseinate \ pound
Sulfur (powdered or sublimed) 5 pounds
Water to make 100 gallons
Heavy infestations of red spider resulting from delayed control
cannot be entirely checked by a single spraying. One application will
be effective for about three weeks, when it should be repeated, unless
the attack begins very late in the season. If possible, spraying should
never be delayed until injury from the mites is noticeable, as it is
almost impossible to get as good results from late spraying as if the
work had been done when the attack usually begins.
* The glue-water formula, flour paste, or other commercial spreaders, may be
substituted for casein, if desired, according to the formulas given on p. 62.
62 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Caustic sprays are more dangerous to the foliage than those of
sulfur alone, hence particular care should be taken to work at tem-
peratures below 100° F., if possible. It might be possible in limited
areas to spray when the sun is not shining directly on the foliage that
is being drenched. Much stronger concentrations can be used at such
a time.
It has been reported from the citrus districts of southern Cali-
fornia that the use of caustic sprays has killed the insects that feed
upon the red spider. This observation has not been substantiated in
northern California; moreover, parasitic or predacious enemies are of
little importance at the height of the mite attack.
Preparation of sulfur pastes. — (-a) Casein in its present commer-
cial form of calcium caseinate is very satisfactory for mixing with
sulfur in preparing liquid sprays. It aids in wetting the sulfur and
also acts as a spreader and adhesive on the leaf surface. One-half
pound of the commercial powder is sufficient for preparing 100 gallons
of mixture. Add the casein to one and one-half gallons of water, into
which stir five pounds of sulfur (powdered or sublimed) until a stiff
paste without lumps is formed, adding more water if necessary. Wash
this through a fine screen into the spray tank with the agitator run-
ning. The material is then ready for application.
(b) The glue-water formula suggested by Gray 6 has been widely
adopted but on account of the time required for melting the glue it is
less convenient than the casein preparation. The formula is :
Formula 3
Powdered glue f ounce
Hot water 1£ gallons
Powdered sulfur 5 pounds
Water to make : 100 gallons
After melting the glue in hot water, it is diluted to one and one-half
gallons and this water used in mixing the sulfur into a paste, free from
lumps. Wash the paste through a sieve into the spray tank, while
the agitator is running. For small amounts of this mixture the more
expensive form of liquid glue may be substituted for the dry glue,
thus saving the time and work required in melting the powdered glue.
(c) Prepared flour pastes, wet (jelly) or dry, such as bill posters
use are also good mediums to use in mixing dry sulfur with water.
Home made flour paste (see p. 70) is also a good spreader to use
with caustic solutions of lime sulfur. Two pounds of the powdered
paste or three pounds of the wet paste (jelly) is sufficient for 100
gallons of mixture. Flour pastes are less convenient than casein in
the preparation of sulfur mixtures.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
63
(d) All mixtures of "wettable sulfurs' ;i should be used the day
they are prepared. If allowed to stand over night there is a tendency
to '.'set" and form a solid layer which is difficult to bring into sus-
pension.
TABLE VIII
Efficiency of Sulfur in Different Types of Liquid Sprays as a Control of
the Brown Mite and the Common Red Spider
(Applied June 8 on almond trees.)
(Amounts used are for 100 gallons of spray mixture.)
Material used
Decrease in infestation*
Plot
No.
observed
6/18
observed
8/11
observed
9/20
Remarks
A
Lime-sulfur solution (31° Baume)
2 gallons (1-50) plus 2 pounds of
dry flour paste and 5 lbs. of flow-
ers of sulfur
%
96
%
90
%
90
Control satisfactory for
entire season.
B
"Soluble Sulfur" (Niagra Sprayer
Co.) 3 lbs. plus 2 lbs of dry flour
paste
92
60
15
Apparent control but
results not permanent.
Slight foliage injury.
C
Dry lime-sulfur (Shervvin & Wil-
liams) 13^ lbs. plus 2 lbs. of dry
flour paste
85
42
5
Control unsatisfactory.
Results not perma-
nent.
D
Dry lime-sulfur (Sherwin & Wil-
liams) 3 lbs., plus 2 lbs. of dry
flour paste
93
73
25
Apparent control but
results not permanent.
E
"Milled Sulfur" (California Spray
Chemical Co.) 5 lbs
92
88
80
Slower in action than
caustic sprays but re-
sults permanent.
F
Sulfur (flowers of sulfur) 5 lbs.
plus 2 lbs. of dry flour paste
88
88
90
Similar to No. 5.
G
Dry lime-sulfur (Sherwin & Wil-
liams) 5 lbs. plus 4 lbs. of wet
flour paste and 5 lbs. of flowers of
sulfur
95
86
90
Equal to that of No. 1.
H
Dry lime-sulfur (Sherwin & Wil-
liams) 3 lbs. plus 4 lbs. of wet
flour paste and 5 lbs. of flowers of
sulfur
93
85
70
Slightly inferior to
No. 7.
Check (untreated).
Percentage of live mites 95 to 98.
* These percentages are a comparison with the check plots in which the number
of dead mites is normally from 2 to 5 per cent.
64
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
A number of commercial sulfur pastes are now on the market which
can be substituted for the above mixtures. These pastes are mixtures
of sulfur and other substances so combined as to give a wettable sulfur
with adhesive qualities.
Fig. 8. — Almond trees at the University Farm sprayed with 2 gallons of lime-
sulfur solution, 5 pounds of sulfur, and 2 pounds of dry prepared flour paste in
100 gallons of spray. Sprayed June 8, photographed August 4. Compare with
Fig. 9.
Experimental spraying at the University Farm, Davis. — Orchard
and laboratory experiments with a large number of sprays were con-
ducted at Davis and in the immediate vicinity during 1920. This
work was done principally on almonds as there were no large blocks
of prune orchard available. The more promising experiments have
been tested during 1921 on prunes at the University Farm and in
private orchards with similar results. Hence the data in Table VIII
may be taken as indicative of the behavior of such sprays both on
prunes and almonds.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
65
The plots received but one irrigation, the latter part of July, so
that all suffered from drought. The number of trees to an experiment
ranged from 14 to 40. The trees are about the size of the average full-
grown almond. Dry prepared flour paste, such as bill posters use, was
\
4
W ' \ •X,
\iXJ
^ A -
Fig. 9. — Almond trees defoliated by the red spider, T. telarius. These trees
were grown under the same conditions as those shown in Fig. 8 except that they
were unsprayed. Photographed August 4.
combined with all the caustic sprays to increase their spreading quali-
ties. The sulfur spray in plot F was prepared with the same paste,
instead of glue water, so as to make the series comparable. The amount
of spray mixture per tree was from 8 to 12 gallons, 10 gallons being
the average.
66 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
These applications were made toward the close of the brown mite
attack and just as the common red spider was beginning to show in
the orchard. The foliage at this time was becoming yellow and begin-
ning to drop, the combined result of the brown mite attack and
drought. Within ten days after spraying, the plots where the caustic
solutions were used (letters A to D inclusive, and G and H) were
regaining their normal color. Plots E and F did not recover until more
than a month after treatment. The quick action of the caustic solution
resulted from its effect as a "contact" spray. A large percentage
of the brown mites were killed within 24 hours after spraying and witli
this heavy drain removed the green coloring matter was quickly
replaced in the leaves. The sulfur used in plots E and F acted more
slowly but just as surely, so that in the final summary all plots which
had received sprays containing approximately five pounds of sulfur
per hundred gallons were almost free from red spider. Plots B, C,
and D, which received less than five pounds of sulfur per hundred
gallons of mixture were much more heavily infested from August
until the end of the season. It would seem from these experiments that
the red spider may be checked on almonds by a single thorough spray-
ing at the beginning of the season. The type of spray mixture seems
to be of less importance than the amount of sulfur used. All sprays
containing approximately five pounds or more of sulfur per 100 gallons
resulted in a fair degree of success.
Sulfur Dusting
Powdered or sublimed sulfur has frequently proved satisfactory
as a control for the different species of red spiders, but only when it
adheres well and is in close proximity to the mite. Quayle has shown
that the distance over which it is effective is but a very small fraction
of an inch. 10 Lack of adherence, poor distribution, and the heavy
web of the common red spider are the causes of many of the failures
attributed to this manner of application.
Comparative laboratory tests of dry and liquid applications have
shown an almost uniform efficiency in the action of the two methods.
These tests were made with infested twigs sprayed or dusted on the
same day and in the same way as the work was done in the orchard.
The twigs were then placed in jars partly filled with water and each
jar set in the center of a sheet of tanglefoot fly paper. The latter
precaution prevented the loss of dead mites as they dropped from
the twigs. The laboratory temperature was practically that of the
orchard, the greatest difference in the two situations being that the
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 67
laboratory was free from currents of air sufficiently strong to dislodge
the sulfur from the leaves. Under these conditions, dusting experi-
ments requiring thousands of mites showed an efficiency within 1 or
2 per cent as great as that on sprayed twigs, while in the orchard a
difference of 12 to 40 per cent was common. Sulfur applied as a
dust in the orchard apparently does not adhere sufficiently long to
be effective, especially with the little pubescence common on almond
foliage.
Control by dusting seems more practical for the brown mite than
for the common red spider. The work is done earlier in the year
when the humidity is higher and the freedom from webbing in the
case of the brown mite makes it easier to bring the sulfur in intimate
contact with the mites. The most successful dusting at the Univer-
sity Farm was on a still, foggy morning in April on the brown mite.
This application of sulfur adhered through a three-day heavy north
wind, and ten days after the application there were, by actual count,
from 81 to 102 thousand grains of sulfur per square inch of leaf
surface. This one application controlled the brown mite for the entire
season. Other applications made on dry windy days showed scarcely
a trace of sulfur 24 hours after the dusting. Such applications are
of course worthless.
Sulfur fillers. — Sulfur for dusting purposes must be sufficiently
fine so that it will adhere well to foliage. Sulfur refiners are now
preparing a very finely powdered material for this purpose, but such
sulfur used alone has a tendency to pack and become lumpy.* This
material does not spread well either from hand or power blowers,
unless a very efficient agitator is used, as it has a tendency to clog the
machine and to spread very unevenly. This difficulty may be over-
come by mixing an inert material, such as hydrated lime or kaolin,
with the sulfur. The best proportionate amounts of sulfur and filler
were determined in the experiments reported in Table IX. This work
was done in a closed laboratory where the possibility of air currents
dislodging the sulfur was reduced to a minimum. Almond twigs
infested with red spider were dusted with a hand blower, and placed
in jars containing water. These jars were set on sheets of tanglefoot
paper so that a record could be kept of the mites as they died and
dropped off. The tests were made in duplicate with two check jars
for comparison.
* Eecent developments in the preparation of sulfur dusts are overcoming this
trouble, hence it may not always be necessary to use fillers.
68
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
TABLE IX
Comparative Efficiency of Sulfur with Varying Amounts of Filler
(Laboratory test — applied in August)
Proportions of sulfur
(1) Pure sulfur
(2) 90% sulfur— 10% hydrated
lime
(3) 80% sulfur— 20% hydrated
lime
(4) 70% sulfur— 30% o hydrated
lime
(5) 50% sulfur— 50% hydrated
lime
(6) Check (untreated)
Per-
centage
of red
spiders
killed
August
25
Per-
centage
of
mites
killed
August
28
90
98
90
98
85
97
77
85
65
65
95
(alive)
95
(alive)
Remarks
Spreads unevenly; packs on standing.
Spreads more evenly. Requires less
material to cover twigs than pure
sulfur.
Slow action. Effective in laboratory
but of doubtful value in orchard.
Slow action. Inferior to larger pro-
portions of sulfur.
Inefficient.
From the above data, it is concluded that dusting sulfur with a
ten per cent filler is superior to the pure material in that it spreads
more evenly and gives a better distribution than is secured with pure
sulfur. There was little difference of efficiency between the pure
sulfur and that with a ten per cent filler. It will also be noted that
the 90-10 dilution was more rapid in action than any of those con-
taining smaller amounts of sulfur. This point is very important, for
the great weakness in the use of dusting sulfurs is their tendency to
shatter off from the leaves. Hence a slow acting sulfur may not be
efficient because much of it is dislodged before becoming effective.
The 80-20 dilution was almost as efficient as the 90-10 after the lapse
of 7 to 12 days, but for shorter periods than this was less effective.
In orchard practice, therefore, the 20 per cent filler would not be
desirable unless applied when the leaves were damp or combined with
an adhesive material. The 70-30 dilution was very much slower in
action than the first three materials tested, and the 50-50 dilution
gave such poor results that it could hardly be considered worth
applying.
The cost of dusting material with a ten per cent filler should be
about the same as the cost of pure sulfur. It is doubtful whether
the decreased cost obtained by using 20 per cent of lime would be
an economical practice, unless the material can be applied under the
most favorable circumstances.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
69
Field tests of sulfur with varying amounts of filler confirmed the
laboratory results, viz., that a ten per cent filler increased the mechan-
ical action of dusting sulfur without a loss in efficiency, and that a
50-50 dilution was decidedly inferior to blends with higher percent-
ages of sulfur.
Intervals between dustings. — It is the common experience in sulfur
dusting that the applications must be repeated at intervals. The
length of time elapsing between applications varies with the adhesion
of the sulfur and to a certain extent with the temperature. At 75° F.
and above, the effect of sulfur is more toxic, and the rate of hatching
of the egg is shorter than at 70° F. and below. To determine the exact
intervals between spraying, the following experiments on the brown
mite were made on almond from April 20 to June 4. The sulfur used
was a mixture of a special dusting brand containing a ten per cent
filler of hydrated lime. The applications were made at intervals
ranging from 5 to 20 days and repeated from 2 to 9 times. All
work was done as uniformly as possible. The sulfur was applied with
the same knapsack blower in all the experiments, the work being done
between 7 :00 a.m. and 9 :00 a.m. This is usually the stillest time of
the day with a comparatively high humidity, conditions which favor
this type of work.
TABLE X
The Number and Frequency of Sulfur Dustings Necessary to Control the
Brown Mite (Bryobia) on Almond
(The first dusting in all plots was on April 20.)
No. of
applications
Days elapsing
between dustings
Observations
No.
May 17
Percentage of
mites dead
June 14
Relative number of
live mites
%
%
1
9
5
98
1
2
5
10
96
2
3
3
15
95
12
4
3
20
70
2
5
2
5
70
35
6
2
10
65
35
7
2
15
65
30
8
2
20
70
30
Check
(untreated)
4
96
70 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION
The data given in Table X show that at least three dustings are
necessary under the existing conditions, to gain a satisfactory control
of this species of red spider. A decided decrease in the degree of
efficiency is shown in all plots receiving but two applications.
A fair degree of control is seen in all of the first three applications
where the time between dustings ranged from 5 to 15 days, but
applications at 20-day intervals were inferior to those of 15 days
or less.
The data indicate that the range should be from 10 to 15 days
rather than 15 to 20 days, with 3 as the total number of applica-
tions. The sulfur adhered better on the plots dusted early in the
morning than when applied during a hot, windy time of the day,
particularly in midsummer. Sulfur in noticeable quantities was
present in plots one to three, inclusive, throughout the entire experi-
ment.
It should be noted that this work was done on the brown mite,
which is not a web spinner, thus making possible a much better dis-
tribution of the sulfur among the mites than in the case of the com-
mon red spider, which spins a heavy protecting web. The work was
also begun in the spring when the humidity is higher than in the
summer, consequently the sulfur adhered better than in drier weather.
For these reasons we would not expect so good control of the com-
mon red spider through midsummer as is shown in the above table.
SUBSTITUTES FOE SULFUE
It has long been recognized that there is an element of danger in
an indiscriminate use of sulfur on certain trees and plants. The
apricot and apple in certain districts are usually conceded to be par-
ticularly susceptible to "sulfur sickness." But fortunately the stone
fruits, which are the heaviest sufferers from red spider, are not very
susceptible to injury from this chemical. The caustic solutions of
lime or soda and sulfur are much more dangerous to foliage than
solutions of sulfur alone. But the application of weak caustic solu-
tions is seldom attended with foliage injury unless applied at tem-
peratures of 100° F. or more. Even in such cases it is quite probable
that the injury from an unchecked attack of red spider would be very
much greater than that resulting from spray injury.
Substitutes for sulfur that have been suggested are wheat flour
paste (cooked), consisting of one pound of flour to one gallon of water
and diluted at the rate of one part of paste to nine parts of water. 9
The time and work required to cook the paste have been a great deter-
rant to the use of the formula, but prepared flour pastes, which are
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
71
sold at prices comparable to that of a good grade of flour, make this
material of practical value. Where the use of sulfur is attended with
considerable risk, flour paste mixtures alone might be used with
advantage.
Spraying with water is sometimes resorted to in regions where
sulfur applications are objectionable. Such sprays are of value in
breaking down the web and washing large numbers of the mites off
the leaves, but of course are less effective than those containing at
least one gallon of lime-sulfur solution or its equivalents (see p. 74).
Linseed-oil emulsion has been recommended by Vinal particularly
for greenhouse work. 12 The oil is emulsified with soap and used at the
rate of one or two gallons per 100 gallons of spray. This mixture is
effective on cucumbers and violets but apparently has not been tested
as an orchard spray.
Fish-oil soaps have frequently been recommended and are efficient,
but foliage injury frequently occurs from their use. A resin wash
has been highly recommended by McGregor for use in South Caro-
lina, 8 but has received little attention in California.
None of these materials has come into general use, perhaps from
inefficiency or lack of convenience in preparation. Fish-oil soap, on
account of its variable nature and the danger which attends its use
on foliage, cannot be recommended. Nicotine applied in the sulfate
form has frequently been suggested, but after careful tests, was found
inefficient both as a liquid spray and dust. A summary of the results
with dusts is given in Table XI. Weak emulsions of the safer forms
of petroleum oils are also being used.
. ' TABLE XI
Experiments with Nicotine Dusts* on Adults and Nymphs of the
Brown Mite
(Laboratory experiments applied May 8, observed May 13, 1920.)
Material used
Number and percentage of
dead mites
Number and percentage
of living mites
2% "Nicodust"
480
68.4%
685
87.2%
610
89.6%
152
(.8% nicotine)
5% "Nicodust"
31.6%
88
(2.0% nicotine)
4% "Nicodust" (1.6% nico-
tine) and 60% Sulfur
12.8%
70
10.4%.
Check (untreated)
2%
98%
* The nicotine dusts used in these experiments were prepared by the California
Walnut Growers' Association, under the direction of Professor Ralph E. Smith.
They are composed of a carrier of hydrated lime or kaolin combined with definite
amounts of nicotine sulfate. The percentage of nicotine can be approximated
from the amount of nicotine sulfate. For example a 2 per cent nicodust is sup-
posed to contain 0.8 per cent nicotine.
72 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Banding trees with tanglefoot or other substances which might
prevent the mites from crawling up the trunk has proved of little
value. Such measures have been tried repeated^, but with so little
success that they are seldom used.
Natural enemies. — Red spiders are subject to attack by a number
of insects, including predacious thrips, coccinellids. a coniopterygid,
syrphid fly larvae, and predacious bugs. The last group only has
been recorded at all commonly during the height of the attack by the
common red spider, and then in too few numbers to be of practical
value. A bug (Triphleps tristicolor) has been noted more frequently
during the summer than any other predator.
The mites are so well adapted to the heat and aridity of the interior
valleys during the summer months and so prolific under these con-
ditions that no natural control has been found that can cope with them
at this season. Natural control by parasitic or predacious insects in
the early spring seems much more feasible. The mites are greatly
reduced in numbers during the winter and at the low temperatures
of March and April increase very slowly. One attack on the common
red spider at this time of the year was noted, apparently the work
of a predacious thrip, in which the mites in every colony examined
were killed and the contents of the eggs eaten. It was months after-
wards before a single colony of mites could be found in this orchard,
and during the entire year the red spider was less abundant in this
orchard than ever before noted.
II. BROWN MITE (Bryobia praetiosa Koch.)
Spraying experiments at the University Farm and at Durham
show that the most effective control for the brown mite is an applica-
tion of crude-oil emulsion or lime-sulfur solution for the winter egg,
during December, Januar}^, or the first of February. The oil emul-
sions have in all cases killed 99 per cent or more of the winter eggs and
the efficiency of the lime-sulfur solution has averaged 95 per cent.
Crude-oil emulsions were at first used at 12 or 15 per cent strength.
Later experiments have shown that these percentages are needlessly
high and that emulsions containing but four per cent of oil (four gal- #
Ions in 100) have been as effective as an ovicide at stronger dilutions.
The latter dilution might be inefficient against insect eggs or scale
insects, which might be present, hence unless the object is to control
only brown mite eggs, it would be better in most instances to use ten
gallons or more of oil in 100 gallons of mixture. Two disadvantages
Bulletin 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
73
TABLE XII
Comparative Value of Oil Emulsions and Lime- Sulfur Solutions as a
Control of the Brown Mite (Bryobia praetiosa)i in the
Winter Egg Stage. Orchard Test*
(Observed 3/17)
Date
Applied
Material
used
Dilution
No. of
trees
sprayed
Percentage
of eggs
hatching
Percentage
of hatched
mites
living
Percentage
of
efficiency
Remarks
Crude oil
15 gals.
%
%
%
Shot hole
1-19-20
emulsion
(Home-made)
of oil
in 100
24
1
30
99.7
fungus
unchecked
Crude oil
15 gals.
Shot hole
1-19-20
emulsion
("Ortho")
of oil
In 100
26
1
25
99.75
fungus
unchecked
Crude oil
15 gals.
Shot hole
1-20-20
emulsion
("Buggo")
of oil
in 100
6
1
25
99.75
fungus
unchecked
Lime-sulfur
10 gals.
•
Shot hole
1-20-20
(Home-made)
32° Baume
in
100
23
50
8
96.0
fungus
checked
Distillate
Q l A gals.
Shot hole
1-20-20
Emulsion
("Spra-
mulsion")
of
emulsion
in 100
10
14
90
87.4
fungus
unchecked
Distillate
Q% gals.
Shot hole
1-20-20
emulsion
("Zeno")
of
emulsion
in 100
12
16
90
85.6
fungus
unchecked
Distillate
7 gals.
Shot hole
1-20-20
emulsion
(" Distillate
Emulsion")
of
emulsion
in 100
12
18
90
83.8
fungus
unchecked
Check
Shot hole
(untreated)
90
92
fungus
unchecked
* Duplicate orchard and laboratory experiments in large series were made with
the materials mentioned, with results so similar that the above data may be con-
sidered as typical.
t Garman reports similar results on the egg of P. pilosiis. 50
74
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
in the use of crude-oil sprays are, first, their lack of fungicidal action
(for example on "Shot-Hole Fungus") and second, their somewhat
low efficiency on the twig borer (Anarsia linmtella). These faults can
be largely overcome by the use of a specially prepared crude-oil emul-
sion, which can be safely combined with lime-sulfur solution. This
mixture in the one year that it was tested showed the superior qualities
of both the oil and lime-sulfur preparations at the dilutions of four
gallons of crude oil and one gallon of lime-sulfur solution in ten of
the mixture.
Proprietary and homemade crude-oil emulsions proved equally
efficient. A satisfactory type of oil for the preparation of emulsions is
one testing 18° to 21° Baume. Very light crude oil was slightly less
efficient than the heavier types.
TABLE XIII
Comparative Value of Lime-Sulfur Solution and its Substitutes* and
Crude Oil Emulsion as a Control of the Brown Mite
(Bryobia praetiosa) in the Winter Egg Stage
(Laboratory tests applied January 18, 1921.)
No.
1
2
3
*4
5
*6
7
8
Material used
Observed February 23
Percentage of
eggs
hatching
Lime-sulfur, 1 in 10
Lime-sulfur, 1 in 10; Nicotine
sulphate, 1 in 800
Barium-sulfide, 26 lbs. in 100
gals
Barium-sulfide (B.T.S.), 64 lbs.
in 100 gals
Dry lime-sulfur, 20 lbs. in 100
gals '
Dry lime-sulfur, 44 lbs. in 100
gals
Crude oil emulsion, 4%
Crude oil emulsion, 4%; Lime-
sulfur, 1 in 10
Check (not sprayed)
Percentage of
hatched
mites dying
65
70
60
55
60
65
4
95
10
15
5
10
2
Observed March 1
Percentage of. Percentage of
eggs
hatching
80
95
95
65
85
90
12
95
hatched
mites dying
100
100
91
83
81
100
12
* The substitutes for lime-sulfur solution are here compared on their poly-
Blllfide content. On this basis it requires 6.4 pounds of dry barium-sulfld or 4.4
pounds of dry lime sulfur to equal 1 gallon of lime-sulfur concentrate, 33° Baume.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS
75
HOME-MADE CEUDE OIL EMULSION
Formula 4
Crude Oil - 4 to 12 gallons
Fish Oil Soap (hard) 4 to 7 pounds
Fish Oil Soap (liquid) 1£ to 3 gallons
Lye or caustic soda 12 to 16 ounces
Water to make - 100 gallons
No injury resulted to the tree from airy of the crude-oil sprays,
except that the one applied just before blooming slightly injured the
tips of the new leaves. It will probably be found safer to apph r oil
Fig. 11. — Development of almond buds January 20, when the second series
of winter sprays was applied to kill the winter egg of the brown mite {Bryooia
praetiosa) .
sprays while the tree is quite dormant. Light oils, such as distillates,
including both homemade and commercial emulsions and those listed
in Table XII as "Zeno" and " Spra-Mulsion, " were decidedly inferior
to crude-oil emulsion.
The time for applying crude-oil emulsions is during the dormant
period of the tree. There is no apparent advantage in late spraying,
as there is with lime-sulfur solutions. Crude-oil emulsions have been
applied during December, January, and the first week of February
and even after the buds were opening on almonds, without any injury
to the tree. It is probably safer to spray with oil on almonds and
prunes during December and January rather than later in the year.
76
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Avoid spraying with oil when the trees are very dry, for example,
following a heavy north wind. At such time there is greater possi-
bility of absorption of oil by the trees since the amount of moisture
in the twigs has been reduced.
TABLE XIV
Results of the Combined Effect of Sprays and Mite Attack
(Observed May 21)
Material used
Applied
Months
after
application
Percentage
of
decrease in
infestation
Remarks
Crude oil emulsion,
1/19
4
98
Foliage a deep green, except
15%
where mite is present. Side
next to check row shows a
15% increase in infestation.
Lime-sulfur, 1-10
1/20
4
92
Foliage a lighter green than
where crude oil was used, but
of a better appearance than
early spring app^cations.
Lime-sulfur, 1-50
3/6
2.5
88
Leaves small; beginning to fall.
Distillate oil, 7% ....
1/20
4
70
Leaves small and yellow, com-
parable to check row.
Lime-sulfur, 1-50
5/8
0.5
99
Leaves small but regaining a
deep green color.
Check (untreated)....
3
Leaves small and vellow; de-
foliated very rapidly.
Lime-sulfur solution used at winter strength (one in ten for concen-
trates testing about 33° Baume) was inferior to crude oil in killing
the egg of the brown mite, but since this spray is also of value in
controlling "Shot-Hole Fungus" {Cercospora circumscissa) and the
twig borer (Anarsia lineatella) , when applied just prior to blooming,
its total value in most orchards might be greater than crude oil. The
percentage of mite eggs actually killed by the lime-sulfur solution is
low, ranging from 10 to 70 per cent, but it will be seen from Table XII
that a large number of mites which hatched died within a few days
after emerging. The total efficiency (i.e., including both the eggs and
young mites that were killed) of the lime-sulfur spray applied at
different times is as follows :
December 29, 1919, total efficiency 92.3 per cent
January 20, 1920, total efficiency 96.0 per cent
February 11, 1920, total efficiency 97.8 per cent
The latter date is from seven to ten days before blooming. It will be
seen from this that the later in the winter this spray is applied the
more effective it is in control.
Fig. 10. — (Color plate.) Natural color of foliage restored after checking the
brown mite (Brijobia) attack. The twig at the right was photographed thirty-six
days after being sprayed. The twig at the left shows the typical color of the
uiisprayed trees. Spray applied May 8.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 77
Spring and early summer control of the broivn mite is best effected
by the caustic spray solution as recommended for the common red
spider (page 61). Dusting with sulfur is more effective against this
Fig. 12. — Development of almond buds February 11, when the last series of
winter sprays was applied to kill the winter egg of the brown mite (Bryobia
praetiosa.)
mite than against the common red spider, but is slower in action than
the lime-sulfur solution. The almond tree responds very quickly after
the mites are killed and within a month's time it regains its normal
color (see Fig. 10).
78 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
III. CITEUS MITE (Paratetranychus pilosus Can. and Fanz.)
The control of the citrus mite on deciduous trees is similar to that
of the brown mite. Winter sprays applied to the egg are desirable
but, if control is delayed until spring, the use of a dilute lime-sulfur
solution with the addition of five pounds of powdered sulfur is the
quickest means of control (see p. 61).
SPRAY PROGRAM FOR DECIDUOUS ORCHARDS
Almond:
To kill the winter egg of the brown or almond mite spray in
December or January with crude-oil emulsion 4 to 12 per cent strength
(formula 4, p. 75).
If the twig-borer or shot-hole fungus is present in addition to red
spider eggs, use a combination of crude-oil emulsion and lime-sulfur
(see p. 74), or lime-sulfur solution alone at winter strength (or its
substitutes), preferably just before the buds open (Table XIII and
pages 73 and 76).
For the spring attack of the brown mite, spray with lime-sulfur
or its substitutes (formula 2, p. 61), or dust every 10 or 15 days with
sulfur (pp. 66-70).
The common red spider is controlled in the same way as on the
prune in the interior valleys.
Prune:
Interior valleys. — Spray all orchards subject to infestation during
the first two weeks of July, or earlier if attacks in previous years
have begun before this date. Do not wait until the red spiders are
seen before beginning to spray.
Use formula 1 (p. 61), or dust carefully three times (usually)
every ten or fifteen days, with sulfur containing a ten per cent filler.
The latter method is frequently less satisfactory than spraying,
especially if much web has been spun (pp. 59-60, 66).
If the mites are well established in any part of the orchard, spray
with formula 2 (p. 61).
Should the brown or almond mite attack, use the methods recom-
mended for this mite under Almond.
Coastal regions. — Prunes in these localities are more subject to
attack by the brown mite than in the interior. For orchards that are
commonly infested or where the winter egg is found in abundance,
use the measures recommended under Almond.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 79
The common red spider is not so troublesome as in the interior
valleys, but may require control, especially in dry years. For this
mite use the same sprays as in the interior, though dusting is more
successful in these localities than in the interior valleys.
Peach :
Red spiders seriously injure peaches in parts of the interior, par-
ticularly the San Joaquin Valley. This injury is principally due to
the common red spider. Use the control measures recommended for
prunes in interior valleys (p. 78).
COST OF SPRAYING AND DUSTING
Spraying.— To thoroughly wet a full-grown tree in foliage will
require from 7 to 11 gallons of spray, the latter amount for large
trees like almonds. Allowing 70 trees to the acre and an average of
9 gallons per tree, 630 gallons per acre would be required. With
sulfur at 5 cents a pound, lime-sulfur solution, 20 cents a gallon,
and calcium casemate, 23 cents a pound, the cost of the materials in
100 gallons of spray and per acre is as follows :
Per 100 gallons
Formula I $0.36
Formula II 0.56 to 0.76
Formula III 0.26
Formula IV 0.82 to 1.48
The cost of application fluctuates with the price of labor and the
type of apparatus used. A power sprayer requires a team and two
or three men to operate it, according to whether a "spray gun" or
the regular nozzle is used, and possibly the cost of a supply wagon
must be included. The amount of spray applied per day varies from
800 to 1800 gallons per day.
Dusting. Knapsack Duster
Sulfur (15 to 20 pounds per acre) $0.75 to $1.00
Labor cost (1 man, $2.50 per day) (2 to 10 acres per day) 0.25 to 1.25
Per
acre
$2.28
3.58
to
4.84
1.67
5.20
to
9.37
'Range of total cost per acre for labor and materials $1.00 to $2.25
* Acreage covered based on trees of five years ' age or more. It should always
be noted in the comparison of the cost of dusting versus spraying as a control for
red spider, that for most situations it is necessary to make two or three applica-
tions of dust to one of a liquid spray, and even then the repeated dustings may
be less efficient than one thorough spraying.
80 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Power Duster
Sulfur (20 to Qo pounds per acre) _— $1.00 to $3.25
Labor cost (2 men and 1 team per day, $5.00)
(Machine operation, per day, $1.00)
$9.00
Labor per acre (20 to 30 acres for an all-day run) 0.30 to 0.45
Range of total cost per acre for labor and materials $1.30 to $3.70
SUMMARY
Three species of red spiders are common in deciduous orchards :
the common red spider or two-spotted mite which attacks the trees
during midsummer ; the brown mite or almond mite, which passes the
winter in the egg stage on the tree, feeds from the first of March to
August ; and the citrus red spider, also found on the tree during the
winter in the egg stage.
The common red spider winters in a dormant condition or feeds
upon hardy weeds and cultivated ^plants. There is a slow increase in
numbers during the spring on wild morning-glory, Malva, and other
weeds. Migration to orchard trees occurs in June or the first of July.
Red spider attack results in pale, yellow leaves, followed by defolia-
tion. Trees so affected are incapable of producing the required food
for maturing the year's crop and buds for the following year.
Red spider attack decreased the crop value in two prune orchards,
$113 and $453 per acre, respectively. This included loss in weight of
crop and reduction in grade.
Drought is usually associated with red spider injury. Thrifty trees
with abundant moisture are less liable to this type of loss, but ample
soil moisture is not absolute protection against red spider attacks.
Timely irrigation or the control of either the spring or summer
attacks of red spiders caused a marked increase in the number and
the size of fruit buds in orchards attacked by red spider.
Spraying as a control for the common red spider should be done at
the time the attack usually began in previous years, even though the
mites cannot be seen. The spraying must be done very carefully,
using five pounds of sulfur (made into a paste with calcium casemate
or glue water) to 100 gallons of water.
Should the spraying be delayed until the red spider is abundant,
two or more applications at three weeks' intervals are usually neces-
sary.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 81
Sprays, applied after the common red spider is numerous in the
orchard or for severe attacks of the brown mite or citrus mite, should
contain one gallon of lime-sulfur concentrate (or its substitutes) to
100 gallons of mixture.
The control of any species of red spider by spraying requires
timely and extremely careful applications. Careless, scanty spraying
is almost worthless. Cover both sides of the leaves on every part of
the tree, using from five to ten gallons per tree. An early applica-
tion, usually about the first of July, is of far more value than late
sprayings.
Sulfur dusting is of the greatest value in regions of light wind and
moderate humidity. Use a 10 per cent filler of hydrated lime in all
dusting sulfurs and apply every 10 or 15 days. Three applications
are usually necessary unless all conditions are very favorable.
The brown mite is most easily controlled in the egg stage by winter
spra} T s of crude-oil emulsion or lime-sulfur solution, winter strength.
The oil emulsion may be applied at any time during the winter, but
lime-sulfur is more effective as the cluster buds are opening.
Lime-sulfur in the dry or solution form is also of value in control-
ling the peach twig borer and shot-hole fungus. If the latter pests are
present, it is better to use lime-sulfur than crude-oil emulsion, even
though the latter is more effective against the egg.
82 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION
BIBLIOGRAPHY OF LOCAL INTEREST
i Banks, Nathan.
1900. "The Eed Spiders of the United States." U. S. Dept. Agr., Div.
Ent., Tech. Ser. 8, pp. 65-79, figs. 1-16.
2 DE Ong, E. R.
1918. "Control of Red Spiders." Mo. Bull. Calif. St. Com. Hort,, vol. 7,
no. 3, pp. 111-118.
1919. "The Red Spider." Mo. Bull. Calif. St. Com. Hort., vol. 8, nos.
11-12, pp. 679-680.
1921a. "Controlling the Red Spider." In Univ. Calif. Jour. Agr., vol. 7,
no. 1, pp. 3-4, 23-24.
1921b. "Suggestions for the Control of Red Spiders in Deciduous Or-
chards." Mo. Bull. Calif. St. Dept. Agr., vol. 10, nos. 5-6, pp.
186-191.
1922. < < Summary of Red Spider Control. ' ' Mo. Bull. Calif. St. Dept. Agr.
(in press).
s Essig, E. O.
1913. "Injurious and Beneficial Insects of California." Mo. Bull. Calif.
St. Com. Hort., vol. 2, nos. 1 and 2. Reference to red spiders,
pp. 6-10, figs. 9-11.
1915. "Injurious and Beneficial Insects of Calif." Supplement to Mo.
Bull. Calif. St. Com. Hort., vol. 4, no. 4, pp'. 13-17.
1922. "The European Red Mite." Mo. Bull. Calif. St. Dept. Agr., vol. 11,
no. 4, pp. 409-411.
* Ewing, H. E.
1914. "The Common Red Spider or Spider Mite." Oregon Agr. Exp. Sta.,
Bull. 121, 95 pp.
1921. "New Nearctic Spider Mites of the Family Tetranychidae. " Proc.
U. S. Nat. Museum, no. 2394, vol. 59, pp. 659-666.
s Garman, Philip.
1921. "The European Red Mite (Paratetraiiychus pilosus Can. & Fanz.) in
Connecticut. ' ' In Jour. Econ. Ent., vol. 14, no. 4, pp. 355-359.
5° Garman, Philip.
Same, hi Report Conn. St. Entomologist, 1921, pp. 146-152.
6 Gray, Geo. P.
1918. "Wettable sulfurs." Mo. Bull. Calif. St. Com. Hort., vol. 7, no. 4,
pp. 191-192.
7 Harvey, E. M., and Murneek, A. E.
1921. "The relation of Carbohydrates and Nitrogen to the Behavior of
Apple Spurs." Oregon Agr. Exp. Sta. Bull. 176, 47 pp.
BULLETIN 347] CONTROL OF RED SPIDERS IN DECIDUOUS ORCHARDS 83
s McGregor, E. A.
1916. "The Red Spider on Cotton and How to Control It." U. S. Dept.
Agr., Farmers' Bull. 735, 12 pp., 10 figs.
McGregor, E. A., and McDonough, F. L.
1917. "The Red Spider on Cotton." U. S. Dept. Agr. Bull. 416, 72 pp.
9 Parker, W. B.
1913. "The Red Spider on Hops in the Sacramento Valley of Calif ornia. ' '
U. S. Dept. Agr., Bur. Ent., Bull. 117, 41 pp. 9 figs., 6 pis.
1913. "Flour Paste as a Control for Red Spiders and as a Spreader for
Contact Insecticides. ' ' U. S. Dept. Agr., Bur. Ent., Circ. 166,
5 pp., 2 figs.
io Quayle, H. J.
1912. "Red Spiders and Mites of Citrus Trees." Univ. Calif. Agr. Exp.
Sta., Bull. 234, pp. 483-530, 35 figs.
1913. "Some Natural Enemies of Spiders and Mites." In Jour. Econ.
Ent., vol. 6, no. 1, pp. 86-88.
ii Stabler, H. P.
1913. "Red Spider Spread by Winds." Mo. Bull. Calif. St. Com. Hort.,
vol. 2, no. 12, pp. 777-780.
12 Vinal, Stuart C.
1917. "The Greenhouse Red Spider Attacking Cucumbers and Methods for
Its Control." Mass. Agr. Exp. Sta., Bull. 179, p. 33.
is Volck, W. H.
1903. "Sulfur Sprays for Red Spiders." Univ. Calif. Agr. Exp. Sta.,
Bull. 154, 10 pp.
1913. < < The Control of Red Spiders. ' ' Mo. Bull. Calif. St. Com. Hort., vol.
2, nos. 3-4, pp. 356-363.
14 Written, J. C.
1919. "Frost Control and Related Factors." Mo. Bull. Calif. St. Dept. of
Agr., vol. 8, no. 11-12, pp. 675-678.
15 WOODWORTH, C. W.
1902. "The Red Spider of Citrus Trees." Univ. Calif. Agr. Exp. Sta.
Bull. 145, 19 pp.
1903. Entomology. In Univ. Calif., Agr. Exp. Sta., Bpt., 1901-1903, pp.
104-110. (Red Spider Remedies, p. 105.)