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i!’   ‘Qffgfi December 1974

rifaf‘ r, H‘

 A REVIEW OF THE SPIDER-MITE
 PROBLEM ON GRAIN SORGHUM
AND CORN m WEST TEXAS

Tlie Texas A&M University System,
The Texas Agricultural Experiment Station, l. E. Miller, Diregtor
College Station, Texas

Summary and Conclusions

1. Intensive investigations during the 1972 and 1973 growing seasons revealed four
species of spider-mites infesting corn and grain sorghum in West Texas. These
were: Banks grass mite, Oligonychus pratensis (Banks); grass mite, O. stickneyi
(McGregor) (corn only); two spotted spider-mite, Tetranychus urticae Koch; and
carmine spider-mite, T. cinnabarinus (Boisduval) (grain sorghum only). Banks
grass mite was the most frequent species encountered on both crops.

2. The recent wave of spider-mite outbreaks on grain sorghum and corn in “West
Texas is apparently the indirect result of the evolution (or importation) of a genetic
race (biotype) of spider-mites (presumably O. pratensis) which can utilize these
host plants more efficiently.

3. Spider-mite outbreaks are closely correlated with reproductive maturity in the
plant and appear to be separated, temporally and spatially, from populations of
effective natural enemies. Available evidence indicates that, in general, infesta-
tions need not be aggravated by pesticides in order to qualify as economic
disasters.

4. Because of the genetic adaptability of spider-mite populations, those artificial
control measures (includingthose directed to other pests) which can impose
harsh selective pressure (e.g., chemical acaricides and/or insecticides, resistant
host plants, sprinkler irrigation) on the population should not be expected to
provide a long-term solution to the problem. These techniques should be ex-
ploited to provide a short-term remedy until more sophisticated management
techniques (particularly integration of these with biological control) can be de-
veloped.

5. It is suggested that future research be directed toward developing a theoretical
perspective for the grain sorghum agro-ecosystem in order to insure a scientifi-
cally expedient approach to developing pest-management programs.

6. Finally, unless a well founded research program is realized, profitable grain-
sorghum production on the High Plains of West Texas may suffer the same
devastation from spider-mites as did the El Paso area.

Contents

Summary and conclusions, page two; history of Spider-Mite outbreaks, taxonomic
considerations, page three; biological notes, page four; ecological considerations, page
six; control considerations, page ten; directions for future research, page eleven;
references cited, page fourteen.

Acknowledgments

I am grateful to P. L; Adkisson for the administrative and financial support given
during the course of the investigation and to my assistants, R. D. Kirby, C. L. Jones, S.
K. Peoples, F. R. Raubfogel and P. Iohnson, for their aid in conducting the necessary
field and laboratory work. I thank C. R. Ward, G. L. Teetes, T. L. Pate, P. I. Lyerly and
I. T. Pitts for their cooperation in much of the field investigations. Critical review of the
manuscript was obtained from C. R. Ward, D. G. Bottrell, I. W. Smith, Ir., and G. L.
Teetes. E. W. Baker and H. B. Boudreaux confirmed my taxonomic determinations
and reviewed the section of the manuscript concerning taxonomy of spider-mites.
Determination of specimens was rendered by I. B. Chapin (S tethorus spp.) and W. H.
Ewart (Thysanoptera). I am extremely grateful to G. W. Frankie for awakening me to
concepts of island biogeography. Lastly, I am indebted to the many growers, particu-
larly I. D. Heinrich and I. Armstrong, for providing the necessary acreages in which
the field studies were conducted.

 

A Review of the Spider-Mite Problem
on Grain Sorghum and Corn in
West Texas

  
   
    
  
   
  
  
  
   
  
 
  
  
  
   
  

_ Spider-mites (Acari:Tetranychidae) have re-
ently become severe pests of grain sorghum
, orghum bicolor (L.) Moench) and corn (Zea mays (L. ))
 the Trans-Pecos and High Plains of Texas. In cer-
y ' areas (e.g., El Paso Valley) some growers have
iefrained from planting these crops because of the
(ttendant spider-mite problem. Furthermore, it ap-
ears that the spider-mite problem has grown more
‘ evere each year throughout West Texas.

p. Intensive investigations during the 1972 and
; 973 growing seasons revealed that this situation was
more complex than originally suspected and that a
different, more fundamental approach was needed
 for providing an ecologically sound and economically
p. rewarding solution. The purpose of this paper is to
i‘ summarize the available knowledge germane to the
 situation and to suggest directions for future research
5 to follow. Also, certain theoreticalexplorations rela-
 tive to pest management in the sorghum agro-
l ecosystem are presented.

A History of Spider-mite Outbreaks

. Trans-Pecos

g Pate and Neeb (1970, 1971) summarized the his-
p‘ tory of spider-mite outbreaks on corn, grain sorghum
f. and forage sorghums in the Trans-Pecos area, espe-
Icially El Paso, Reeves and Pecos Counties. The
 spider-mites in question were presumably Banks
grass mite [Oligonychus pratensis (Banks)], although
. other species could have been involved (Ehler, 1973).
 Spider-mites reached pest status on corn in 1967
f) and on grain sorghum in 1968. Similar outbreaks
g» occurred on both crops in 1969 and have continued
I thrdugh the present. Pate and Neeb also reported
. that ‘spider-mites were difficult to control with stan-

- ‘Former assistant professor, The Texas Agricultural Experiment
. Station (Department of Entomology) Texas A&M University Sys-

tem; present address: Department of Entomology, University of
i California, Davis, California 95616.

r Mention of a trademark or a proprietary product does not consti-

, tute a guarantee or warranty of the product by The Texas Agricul-

. tural Experiment Station and does not imply its approval to the
exclusion of other products that also may be suitable.

L. E. Ehler*

dard insecticides and acaricides although they pre-
sented no direct evidence to indicate spider-mite re-
sistance to these materials.

A concomitant decline in grain sorghum produc-
tion in this area was reported by Ward (1973) who
speculated that a major cause for this was the lack of
successful chemical control of spider-mites. Ward
also noted that spider-mites in this area may possess
genetic resistance to the available chemical acaricides
and insecticides.

High Plains

Outbreaks of spider-mites were first reported on
grain sorghum on the High Plains in 1967 (Huddles-
ton et al ., 1968) and have increased in severity since
then (C. R. Ward, personal communication). The in-
tensity of spider-mite infestations on the HighPlains
has generally been less than in the Trans-Pecos re-
gion.

Several chemical acaricides and insecticides have
been effective in controlling spider-mites on grain
sorghum (Ward et al., 1972; Teetes, 1973) although
the degree of control is often erratic. Ward et al. (1971)
presented limited evidence which indicated spider-
mites were developing resistance to pesticides al-
though no direct evidence is available to corroborate
this conjecture.

High Plains grain sorghum is especially in
danger of spider-mite outbreaks since the average
crop usually receives at least one application of
organo-phosphate insecticide for control of greenbug
[Schizaphis graminum (Rondani)] and/or corn leaf
aphid [Rhopalosiphum maidis (Fitch)]. These treat-
ments have, in general, been made since 1968 when
explosive outbreaks of greenbug first occurred. Thus,
aphid control could yield the attendant phenomena
of resistance and secondary outbreaks of spider-
mites.

Taxonomic Considerations

Ehler (1973) demonstrated that spider-mites on
grain sorghum and corn, previously considered to be
Banks grass mite [Oligonychus pratensis (Banks)],
could in fact be other species or represent multiple-
species infestations. Since different species of
spider-mites may show differing susceptibilities to
acaricides (Smith and Bryan, 1951; Leigh, 1963) and

3

because different species on the same host may inflict
different kinds of damage t0 plants (Flaherty and
Huffaker, 1970), it is necessary to obtain proper iden-
tification of the spider-mites in order to control, ob-
serve or experiment with them. It is the purpose of
the following sections to describe suitable techniques
for preparing spider-mites for study and to present
keys and diagnostic characters for separating species
encountered on grain sorghum and corn in Texas.

Preparation of Specimens

Spider-mites can be collected and stored in either
lacto-phenol (3 parts lactic acid, 1 part phenol cryst-
als) or in ethyl alcohol. Specimens should be
mounted in a modified Hoyer’s medium (Pritchard
and Baker, 1955) which can be obtained by mixing the
following ingredients in sequence:

Distilled water . . . . . . . . . . .50 grams
Gum arabic (flakes) . . . . . . .30 grams
Chloral hydrate . . . . . . . . .200 grams
Glycerine . . . . . . . . . . . . . . . .20 grams

Specimens are usually mounted according to their
sex. Females should be mounted dorso-ventrally
whereas males must be mounted laterally for proper
study of the aedeagus. The latter process is easily
accomplished by placing a 22 mm’ coverslip on the
preparation and, while observing the specimen
under the microscope, gently sliding the coverslip in
the proper direction until the specimen's aedeagus is
in proper lateral profile. For semi-permanent mounts
and to facilitate use of the oil-immersion objective,
preparations should be placed on a hot plate or slide
warmer and kept ca. 50°C until the medium has hard-
ened. Finally, the mount can be ringed, preferably
with Glyptal®, for further protection. Specimens
should be examined with the aid of a phase-contrast
microscope although most diagnostic features can be
discerned with a light microscope. Boudreaux and
Dosse (1963a) recommend an alternative method for
preparing Tetranychus females for study; however,
the above described technique is usually adequate.
Diagnosis

The systematic position of spider-mites has been
described by Pritchard and Baker (1955) and can be
summarized as follows: spider-mites belong to the
family Tetranychidae which consists of those pro-
stigmatid mites having the paired basal segments of
the chelicerae fused into the pouch-like lobe
(stylophore) in which are anchored the proximal ends
of the movable digits or stylets (Tetranychoidea) and
in which the fourth palpal segment bears a distinct
thumb-claw process and the dorsum of the body
bears not more than 16 pairs of setae. These charac-
teristics can be used to separate spider-mites from
other groups of mites. F

Identification
Since, during 2 years of intensive surveys, only
four species of spider-mites have been collected on

4

   

corn and grain sorghum in Texas (Ehler, 1973), a sim- 
ple key, presented below, should serve to distinguish 3
specimens of those species. A detailed account of the 
morphology of spider-mites has been given by
Pritchard and Baker (1955) and is not presented here; i
illustrating material from their work is referred to as
needed, however. Oligonychus males are required for
species determinations whereas Tetranychas females
are adequate for identification to species level.

Key to Species of Spider-Mites Known to Occur on
Corn and Grain Sorghum in Texas‘

1. Empodium claw-like and equal in length to
proximo-ventral hairs (Figure 305); peritreme
straight distally and ending in a simple
bulb . . . . . . . . . . . . . . . . . . . . . . . . . .(Oligonychus) 2
Empodial claw shorter than proximo-ventral
hairs, often not visible (Figure 387); peritreme
recu rved distally (Figure 3) . . . . . .(Tetranychus) 3

2. Knob of (male) aedeagus not over V4 as long as
dorsal arm of shaft (Figure 306) pratensis (Banks)
Knob of aedeagus ca. V; as long as dorsal arm of
shaft (Figure 297) . . . . . . . . ..stickneyi (McGregor)

3. Color of live summer females green (usually with
2 dark food spots); dorsal integumentary folds
(striae) bearing semi-oblong lobes; eggs usually
white . . . . . . . . . . . . . . . . . . . . . . . . . . ..urticae Koch
Color of live summer females carmine, often with
4 dark spots; dorsal integumentary folds bearing
triangular lobes; eggs usually with a trace of
red . . . . . . . . . . . . . . . . . . . .cinnabarinus (Boisduval)

Biological Notes

Oligonychus pratensis (Banks), Pritchard and Baker
(1955)

Commonly known as Banks grass mite (others
include date mite and timothy mite), O. pratensis was
originally described as Tetranychus pratensis (Banks,
1912) from timothy grass near Pullman, Washington.
Other names include Paratetranychus pratensis Banks,
P. simplex Banks, P. heteronychus Ewing, and
Tetranychus simplex Banks. Pritchard and Baker (1955)
proposed the new combination Oligonychus pratensis
(Banks).

Diagnosis. Pritchard and Baker (1955) state that
in O. pratensis the distal knob (or enlargement) of the
aedeagus is ca. twice the width of the stem of the
knob, the axis of the knob forms a distinct angle with
the axis of the shaft, the dorsal margin of the knob is
nearly straight with the tip slightly down or curved or
angulate, the anterior projection of the knob is
bluntly angulate, the posterior angulation acute and
there is no constriction of the shaft. The knob is ca. 1A
the length of the dorsal arm of the shaft, whereas, it is
V3 the length of the dorsal arm in O. stickneyi. Also,
the knob is smaller than in O. stickneyi . In males, the

1Figures refer to those in Pritchard and Baker (1955); illustrations of
T. urticae and T. cinnabarinus are in Boudreax (1956) and Boudreaux
and Dosse (1963a).

i proximo-ventral hairs of tarsus I are fused to form a
claw-like structure. Females of O. pratensis and O.
stickneyi are indistinguishable.

Distribution. Banks grass mite (BGM) has been

collected throughout the corn and grain sorghum
 producing regions of Texas, including the Rio Grande
Valley (Walter and Wene, 1956; Dean, 1957), and
 Trans-Pecos, High Plains and Central Texas (Ehler,
. 1973). Pritchard and Baker (1955) reported that BGM
y. occurred in California, Oregon, Washington, Utah,
- Kansas, New Mexico, Louisiana and Florida. Addi-
~ tional records include Missouri (Thewke and Enns,
' 1970), Arizona (Tuttle and Baker, 1964), and Georgia
I (Flechtmann and Hunter, 1971). Records from out-
 side the U.S. include Mexico (Beer and Lang 1958,

Estebanes and Baker, 1966), Central America (Baker
and Pritchard, 1962), and South Africa (Meyer and

. Ryke, 1959).

These records indicate that BGM is perhaps na-
tive to North America although this conclusion re-

I mains tentative until the tetranychid fauna of South
; America, other areas of Africa and Asia are more
thoroughly described. The South African record

would then represent an introduction from North

America. Present evidence substantiates this

hypothesis, i.e., BGM was not collected in Mauritius

' and the Congo (Baker and Pritchard, 1960), not in the
v Philippines (Rimando 1962), Australia (Womersley,
° 1940), Brazil (Ehara, 1966, Flechtmann and Baker,
A 1970, Paschoal, 1970) and not in Paraguay (Aranda
_. and Flechtmann, 1971).

Host Plants. Banks grass mite is generally re-

‘ stricted to monocotyledenous plants, particularly
 grasses. Malcolm (1955) listed over 80 species of gras-
U; ses in 17 genera which were infested by BGM. Pritch-

1 ard and Baker (1955), Tuttle and Baker (1968) and
i McGregor and Stickney (1965) also present host plant
' information for BGM. In Texas, Ehler (1973) reported
. O. pratensis on corn and grain sorghum. An up to date
] host list is not presented here.

f Oligonychus stickneyi (McGregor), Pritchard and
I Baker (1955)

This spider-mite was originally described as

Paratetranychus stickneyi and designated a grass

I mite" by McGregor (1939). Pritchard and Baker (1955)
‘ proposed the new combination of Oligonychus stick-
1 neyi (McGregor).

>4 Diagnosis. According to Pritchard and Baker

I (1.955) males of O. stickneyi can be recognized by the

knob of the aedeagus, which is ca. Va the length of the

i‘ dorsal arm of the shaft. In addition, the anterior mar-
’~ gin is broadly rounded, the caudal is acutely angu-
¥ late, and the axis of the knob forms less than a 30

degree angle with the axis of the shaft. The shaft is

a sharply constricted before the dorsal bend. As in O.
’ pratensis males, the proximo-ventral hairs of tarsus I

are fused and form a claw-like structure. The knob of
the aedeagus in O. stickneyi is larger than in O. praten-
sis and in the latter species the knob is not over 1/4 the
length of the dorsal arm of the shaft. Presently, only

males can be used in species determinations. Females
of O. pratensis and stickneyi are indistinguishable.

Distribution. Pritchard and Baker (1955) report
grass mite occurring in California, Arizona, Florida
and Mexico. Ehler (1973) reported this species in
Texas (El Paso County). The writer is aware of no
reports of grass mite in areas other than the U.S. and
Mexico. This would indicate that this spider-mite is
native to North America.

Host Plants. Pritchard and Baker (1955) studied
grass mite from corn, Bermuda grass and maiden
cane grass. McGregor (1950) recorded grass mite
from 11 different grasses. This mite has been collected
from corn in Texas but not from grain sorghum.

Tetranychus urticae Koch, Boudreaux and Dosse
(1963b)

This species is the common two-spotted
spider-mite which has been commonly referred to in
the literature as Tetranychus bimaculatus Harvey and
T. telarius (Linnaeus). Boudreaux (1956) gave species
rank to T. telarius ( = urticae) and the closely related T.
cinnabarinus which previously comprised the T.
telarius complex. Boudreaux and Dosse (1963b) then
proposed T. urticae as the proper name for two-
spotted spider-mite. These authors also presented a
complete synonymy.

Diagnosis. According to Boudreaux (1956), T. ur-
ticae summer females are basically green (occasionally
yellowish or dark green) and possess ”semi-oblong"
lobes (often semi-circular). In T. cinnabarinus, the
summer females are carmine and the lobes are
pointed and usually triangular. Diapausing females
of T. urticae, collected from greenhouses during the
winter, may be carmine (H. B. Boudreaux, personal
communication). Females are adequate for making
species determinations. For determining males,
study of the aedeagus is required; since the attendant
diagnostic features are difficult to observe and be-
cause artifacts are easily encountered, it is suggested
that only females be used for species determinations.

Distribution. Two-spotted spider-mite is a cos-
‘mopolitan species which can be collected from most
cultivated crops and many wild hosts. In Texas, Ehler

(1973) collected T. urticae on corn and grain sorghum i

although most collections were from corn. In view of
recent systematic revisions, much of the literature
pertaining to the host records of T. urticae (and T.
cinnabarinus) is open to question since it is not possi-
ble to determine presently which species was actually
being studied. For this reason, host records are not
presented for either species. Such records for T.
telarius (= T. cinnabarinus + urticae) are given by
Pritchard and Baker (1955).

Tetranychus cinnabarinus (Boisduval), Boudreaux
(1956)

This species is known as carmine spider-mite
(also "red spider") and for many years was consid-
ered to be conspecific with what is now considered
T. urticae Koch. The binomen T. cinnabarinus was

5

proposed by Boudreaux (1956); Boudreaux and Dosse
(1963b) presented a complete synonymy.

Diagnosis. Boudreaux’s (1956) description of T.
cinnabarinus can be summarized as follows: Basic
color of summer female carmine (restricted to area
caudad of eyes; body anterior of eyes yellowish),
usually 4 dark spots laterally, dorsal integumentary
folds (striae) with triangular lobes (some may be
semi-circular). The shape of the lobes of the striae and
the basic color of the summer females will serve to
distinguish T. cinnabarinis and T. urticae. Boudreaux
described differences in male aedeagi of these
species; however, this distinction is often difficult to
observe. Hence, this issue is not considered here.

Distribution. Pritchard (in Boudreaux, 1956) re-
ported T. cinnabarinus from the U.S., Europe, Israel,
Turkey, Argentina and Japan. It is likely that this
species is cosmopolitan. .

In Texas, Ehler (1973) reported T. cinnabarinus
taken from corn in College Station and recent collec-
tions reveal the species to occur on grain sorghum in
this area. Carmine spider-mite occurs sporadically on
grain sorghum on the High Plains.

Frequency

During the 1972 and 1973 growing seasons, sam-
ples of spider-mites were taken from grain sorghum
and corn in 3 areas: High Plains (Lubbock, Hale,
Lamb, Deaf Smith, Swisher and Castro Counties),
Trans-Pecos (El Paso, Pecos and Reeves Counties),
and College Station (Brazos County). One sample
was taken per field per growing season, normally
when spider-mites were relatively abundant. Usu-
ally, 10-20 specimens per sample were examined and
determined to species level.

In samples from 11 High Plains corn fields, 8
contained O. pratensis, 2 contained T. urticae and 1
contained a mixture of these species. In Trans-Pecos
corn, all 5 fields sampled were infested with O.
pratensis whereas 2 of the 5 contained multiple-
species infestations, i.e., O. pratensis + O. stickneyi
and O. pratensis + O. stickneyi + T. urticae. The latter
fields were virtually destroyed by the spider-mites
(see Figures 3 and 4). In College Station, 3 corn fields
were sampled; 2 were infested with T. cinnabarinus
and 1 was infested with O. pratensis. Banks grass mite
occurred most frequently in West Texas corn al-
though researchers and growers should anticipate T.
urticae and O. stickneyi, particularly in combination
with O. pratensis.

All samples from 17 High Plains grain sorghum
fields contained O. pratensis, including multiple-
species infestations with T. urticae (1 case) and T.
cinnabarinus (2 cases). In Trans-Pecos, only 5 fields of
grain sorghum were sampled — all contained only O.
pratensis. In College Station, samples from 3 grain
sorghum fields each contained a different species,
i.e., O. pratensis, T. urticae and T. cinnabarinus. Banks
grass mite was the predominant species on grain
sorghum in West Texas. (However, in greenhouse

6

sorghum, T. urticae often replaces O. pratensis as the 

predominant species.)

Ecological Considerations

Ecological investigations can proceed in a sophis-
ticated manner only when the organisms to be
studied can be segregated taxonomically. In the pre-
vious section, such taxonomic considerations were
detailed. The purpose of this section is to summarize
available ecological data relative to spider-mites and
the factors which affect their population fluctuations.

Sampling ..
General aspects of sampling tetranychid popula-
tions have been reviewed by Huffaker et al. (1970)
who reported numerous techniques for estimating
population parameters. Ieppson (1951) used the
method of counting adult females in the field — a
technique used in most of the present studies. The
density of females (number per plant) is a sufficient,
index of population trends. Natural enemies and
other associates can usually be counted directly (pref-
erably on a per plant basis) since they are larger in size
and fewer in number. For the study of population
dynamics, it is best to express mite population den-
sity in terms of number per plant since number per
leaf, used alone, could easily lead to artifacts in the
data due to spatial pattern of the mites. For purposes
of gathering efficacy data for chemical agents, Ward et
al. (1972) employed a stratified sampling technique
which was sufficient for assessing chemical control.

Phenology

Observations relative to seasonal occurrence of
the spider-mites in question have dealt largely with
Banks grass mite. This species is present throughout
the growing season on Iohnsongrass (Sorghum
halepense) and can be found infesting this plant in late
fall and early spring. Presumably, BGM overwinters
on Iohnsongrass although hiburnal diapause has not
been established. (BGM infests numerous grasses in
Texas; Iohnsongrass, which is widely distributed,
was the major host plant sampled. Hence this discus-
sion relates largely to this plant.)

Malcolm (1955) reported that fertilized BGM
females (and a few males and immatures) overwin-
tered although he did not report diapause in the
species. Malcolm also demonstrated that BGM over-
wintered near the base of the previously infested
plant in the debris and top soil. Samples which were
taken from previously heavily infested corn and grain
sorghum fields in West Texas during the winter
months did not yield any BGM when processed via a
Berlese funnel. These samples included stalk, leaf
litter and top soil. The data indicate that, in West
Texas, BGM does not successfully overwinter in pre-
viously infested fields, although more intensive sur-
veys may yield contrary evidence. In conclusion, any
perennial grass (e.g., S. halepense) should serve to
perpetuate temporal continuity in BGM populations,
whereas annual species (e.g., corn, grain sorghum)
would seem less suitable.

if Incidence on corn and grain sorghum.

Pate and Neeb (1971) observed a relationship

I between rate 0f spider-mite population increase and
 stage of maturity of the grain sorghum plant. In the
present study, this phenomenon was documented
.2 for both corn and grain sorghum.

In grain sorghum, spider-mites are usually pres-

.1 ent at low levels during the vegetative stages of

owth and increase rapidly, often exponentially,

' after the plant has reached reproductive maturity
s (Figures 1 and 2). In general, this rapid increase in
L density occurs during the dough stage of seed mat-
1 uration, usually when ca. 5O percent of the seeds are
. at hard dough. However, as is apparent from Figures
a 1 and 2, there can be considerable variation in the
a expression of this phenomenon.

The general relationship between spider-mite

l. surges and plant or seed maturity on grain sorghum
1 has been observed regardless of year, plant varieties,
r cultural practices (e.g., fertilizer, irrigation) and pes-
l ticide history of the crop. These data imply a strong
‘ correlation between spider-mite population growth

and the physiological condition of the host plant. At
this developmental stage, concomitant variables in

1 the plant include sugars, alcohol-extractable proteins

and alkali-extractable proteins (Wall and Blessin,
1970).

In corn, the rapid increase in spider-mite density

If was closely associated with the tassel (bloom) stage

(Figures 3 and 4). However, since only 2 fields were

 observed, it is premature to offer a precise conclusion
y: relative to this point.

. Natural Control

The ecology and natural control of spider-mite
populations has been treated exhaustively by Huf-
faker et al. (1969, 1970), McMurtry et al. (1970) and van
de Vrie et al. (1972). These authors summarize and
critically analyze the two central hypotheses concern-
ing spider-mite outbreaks, namely, reproductive

A ' stimulation by pesticides and fertilizers versus inhibi-

tion of predator efficiency via pesticide interference.
In essence, either phenomenon can act singly or can
interact with the other to engender serious outbreaks
of spider-mites. In the following discussion, these
hypotheses are considered relative to spider-mite
outbreaks on corn and grain sorghum in West Texas.

Predators. In the present study, only insect and
acarine predators of spider-mites were observed. In-
sectan parasites (parasitoids) and bacterial.pathogens
have not been reported from spider-mites (Huffaker
et al. 1969); also, no pathogenic agents have been
isolated from the spider-mites in question although
these are likely to accur.

The following predators have been observed
feeding on either adults or developmental stages of
spider-mites (especially BGM) on corn and/or grain
sorghum in Texas:

Coccinellidae

Stethorus punctum (LeConteP

Stethorus atomus Caseyz, 3

Hippodamia convergens Guerin-Menevillez
Olla abdominalis (Say)2

Anthocoridae
Orius insidiosus (Say)2, 3
Orius tristicolor (White)3
Thripidae
Scolothrips sexmaculatus (Pergande)2, 3
Chrysopidae
Chrysopa rufilabrus Burmeisterz
Chrysopa carnea Stephens?’

Phytoseiidae
Amblyseius (= Typhlodromisfiallacis (Carmen):
Typhlodromus mesembrinus Dean2
Tydeidae
Pronematus ubiquitus (McGregor)2
Cecidomyidae
(?) Arthrocnodax sp.3

Pertinent biological features of these and other
spider-mite predators have been outlined by McMur-
try et al. (1970).

Evaluation of predation. In the absence of experi-
mental evidence which would quantify the role of
predators in controlling or regulating spider-mite
populations, we can only make correlative assess-
ments of the relationship, that is, the numerical re-
sponse of predators to the increase in density of prey
(Figures 3 and 4).

The available evidence corroborates the observa-
tions of Pate (personal communication) that the de-
layed response, primarily reproductive, of the pre-
dators is not sufficient to subjugate spider-mite out-
breaks and, hence, to avoid economic loss. This is
illustrated in Figures 3 and 4, which describe the
predator-prey interaction in two corn fields near El
Paso during 1972. In general, spider-mites (a variable
mixture of O. pratensis and O. stickneyi in these cases)
were present at very low densities throughout the

growing season and increased rapidly when the _

plants reached reproductive maturity, or, 50 percent
bloom. Most of the predators present (i.e., Orius,
Chrysopa and Scolothrips) did not show a strong num-
erical response to the prey density increase, whereas
Stethorus almost completed one generation. The data
suggest that Stethorus, especially because of its host
specificity, might be an effective natural-control
agent if manipulated properly.

In grain sorghum, similar phenomena occur and,
generally, the same natural enemies are involved. In
this study, densities of spider-mites seldom reached
levels sufficient to maintain the number of enemies
found in corn near El Paso.

zObservations of Dean (1957).
zObservations of the writer and associates.

 

50‘ 15'
1a 2a
1-
% 40- 12-
% 
(f)
H 30- a 9-'
<
5 5
m <
~» a
d 20*  6-
2 2
Z
E 10" E 3-
E
I BOOT IBLOOMI oouon MATURE] aoor | BLOOM oousu mnum: ]
I I I
so ea a2 92 10s 12o 5a 65 as 10o
DAYS AFTER EMERGENCE DAYS AFTER EMERGENCE
1501 1°00
1 b w 2 b
g 1201 5 soo-
5‘ S
\ n.
Q 9°" ‘E 100*
é é’
u; m
‘L 60- “. so-
g 2
z 5
< 30' lu 5-1
lg! 2
!BO0T IBLOOMI ooucau MATURE !BOOTJIBLOOM oouen MATURE ]
40 6O 80 105 140 4° 59 75 105 13°
DAYS AFTER EMERGENCE DAYS AFTER EMERGENCE
50‘ 2501
1° 2c
i; 40- E 200-
E 2
\ a
3 30- S 150-
;‘ §
r; ti‘
. 2°“ . 100-
O O
z z
z Z
“ 10- ‘
g g 50-
! aoor |awo|v|| DOUGH | mnuns ] BOOT |s|.oou| oouoa [imruns]
4O 6O 8O 105 14O 5,4 7% a 7 "'0
DAYS AFTER EMERGENCE DAYS AFTER EMERGENCE
Figure .1. ‘Popililation ‘trends-of spider-mites (predominantly O. Figure 2. Population trends of spider-mites (predominantly O.
pratensis) in High Plams grain sorghum fields at Slaton (a) and pratensis) in High Plains grain sorghum fields at Abernathy during
Springlake (b and C) during 1972. 1972 (a) and Slaton (b) and ldalou (c) during 1973.

8

MEAN NO. PER PLANT

Pesticide influence. Numerous lines of evidence
can be used to evaluate the influence of pesticides
 (namely, acaricides and insecticides) on spider-mite
outbreaks, either via pest stimulation or natural

K I enemy inhibition, or both. The general conclusion
reached here may or may not be applicable to
" localized areas since, in certain areas, chemical con-

1 trol has been erratic (suggesting local genetic differ-

‘ ences) and in other areas spider-mite outbreaks may
A i be associated with chemical control of aphids on grain
' . sorghum.

Pate and Neeb (1971) reported that, during 1967,

i spider-mites were a problem in the Trans-Pecos area
. regardless of the use of organo-phosphate acaricides.

In fact, the two corn fields sampled in 1972 near El

“ Paso (Figures 3 and 4) were literally destroyed by
_' spider-mites and neither field received a pesticide
a application of any sort. These observations strongly
suggest that spider-mite infestations in the Trans-
] Pecos area need not be exacerbated by pesticides in
A order to qualify as economic disaster.

QLIGONYCHQS g9

§IETHORUS

50-

25-

CHRYSO PA

 

SCOLOTHIRIPS

 

JUNE JULY AUG.

I Figure 3. Population trends of spider-mites (a mixture of O.
pratensis and 0. stickneyi) and natural enemies on field corn near

Clint (El Paso Co.) during 1972. Asterisk denotes 50 percent tassel

in plants.

1041
  99

10’-

F i *
E 1 I
a! STETHORLI§
n:
E so-
0'
Z
E
I I
u:
ORI S
S 25- 
"" I I
CHRYSOPA
25- 
scoioriimg I '
25"

JUNE l JULY I AUG.

Figure 4. Population trends of spider-mites (a mixture of 0.
pratensis and 0. stickneyi) and natural enemies on field corn near
Fabens (El Paso Co.) during 1972. Asterisk denotes 50 percent
tassel in plants.

In High Plains grain sorghum, spider-mite (pre-
dominantly O. pratensis) infestations do not appear,
in general, to be aggravated by pesticides. For exam-
ple, Huddleston et al (1968) reported outbreaks of
spider-mites during 1967 — one year prior to initia-
tion of massive chemical control measures for green-
bug. Secondly, and more definitively, in a series of
experiments conducted by the writer, application of
the insecticides disulfoton (LC, 0.1 AI/A), disulfoton
(G, 0.5 AI/A), parathion (EC, 0.5 AI/A) and phorate
(G, 0.5 AI/A) did not engender increases in spider-
mite density over the nontreated control. In fact, in
most instances, the spider-mite density was signif-
icantly reduced and seldom resurged to a level com-
parable to the control. In most of these studies,
natural enemies of the spider-mites were absent or at
low levels and did not become abundant until
spider-mites had reached near economic injury
levels. These data establish that the hypothesis of
pesticide influence on spider-mite outbreaks cannot
be accepted.

To summarize, available evidence indicates that
spider-mite outbreaks are (1) conditioned by the
physiological condition of the host plant, pesticide

9

influence not withstanding, and (2) are temporally
and spatially separated from effective natural
enemies such that virtual crop destruction often oc-
curs before natural enemies can exert sufficient con-
trol.

Abiotic factors. Of the abiotic mortality factors act-
ing on spider-mite populations, only rainfall has been
considered in any detail (e.g., see Ward, 1973). Ob-
servations by the writer indicate that rainfall and at-
tendant phenomena (so termed because thunder
showers are usually accompanied by dust, hail,
wind, etc.) can drastically reduce spider-mite den-
sity. Such a case is illustrated in Figure 5 in which
pest density after thunder showers was as great as 90
percent less than before the rain. These correlations
have not been experimentally verified.

l l

30-:

25-

n
O
I

MEAN NO. FEMALES/PLANT
8 6-‘
I l

5Q

I I T
15 2O 25 3O

flit
A
O

AUGUST

Figure 5. Populations trends of spider-mites (predominantly O.
pratensis) on grain sorghum near Fabens (El Paso Co.) during
August, 1972. Arrows denote rainfall of ca 1.0” on Aug. 12 and ca
4.0” on Aug. 26.

Control Considerations

A proper appreciation for the taxonomic prob-
lems (if any) and ecological complexity of a pest prob-
lem is prerequisite for developing a solution which is
both ecologically sound and economically rewarding.
A remaining topic to be discussed, then, is control of
spider-mites on corn and grain sorghum. Also, some
specific directions for future research are presented.
In the present case, control actions are desired;
hence, the concept of integrated control (Smith and
Reynolds, 1966) would seem most appropriate and
should be the final desiderata.

Chemical control

Although chemical control of spider-mites has
been erratic and appears to be breaking down (Ward,

10

1973), insecticides and acaricides will likely remain 
the most potent short-term tactic for controlling r

spider-mites. However, in view of the genetic adap-
tability of spider-mites (Helle and Overmeer, 1973), it
can be expected that genetic counter-selection by
spider-mite populations will render effective chemi-
cal materials ineffective in a few years or in a few
generations. Adaptability (proirnpt response to chem-
ical selection in this case) in spider-mites is particu-
larly enhanced by the haplo-diploid sex-
determination mechanism (arrhenotoky) (Helle and
Overmeer, 1973) which allows for immediate expres-
sion of mutations in the male, whether mutations are
recessive or dominant. Hence, there is a prompt in-
teraction between mutation and selection. In the
present case, all spider-mite species involved are pre-
sumed to be arrhenotokous.

It is imperative that exclusive reliance on chemi-
cal control be avoided and that chemicals be applied
only when necessary and in a prescribed manner.
With respect to the latter point, it is questionable
whether or not aerial applications of acaricides on
mature corn can lead to sufficient spray coverage so
as to produce economic control. In California, O. G.
Bacon suspects that such applications do not result in
sufficient coverage for economic control (personal
communication).

Resistant Host Plants
Pate and Neeb (1971) noted that spider-mite in-

‘festations were more severe on corn and grain sor-

ghum than on forage sorghums. These observations
suggest that breeding varieties which are resistant to
spider-mites would offer potential for controlling
them. Such studies are now in progress (G. L. Teetes,
personal communication).

Such resistant varieties (exclusive of those whose
resistance mechanism is largely tolerance) may be
relatively short-lived because of the genetic adapta-
bility of spider-mites. For example, Banks grass mite
has apparently evolved a genetic race (biotype) which
can utilize both corn and grain sorghum more effi-
ciently. In this case, BGM was reported on corn as
early as 1954 (Walter and Wene, 1956) in the Texas Rio
Grande Valley and during the early 1950's on wheat
on the High Plains (Daniels et al., 1956). This species
occurred sporadically and then reached major pest
status on corn and grain sorghum in West Texas
during the late 1960's. (In this respect, we must not
discount the possibility that such a race of BGM was
accidentally imported from another region.)

Biological Control

The use of natural enemies to maintain spider-
mite population density at sub-economic levels,
either through importation of exotic predators and
pathogens or manipulation of native enemies, is a
viable approach, and should be further exploited.
Since, in the opinion of the writer, proper theoretical
perspective for such tactics relative to the grain sor-
ghum and/or corn agro-ecosystems is partially lack-

 
  
   
 
  
  
  
  
  

mg, the entire issue will be treated in the next major
section.

. Unexploi ted Techniques

A number of biological processes in spider-mites
are presently being studied and, presumably, some
of these should have practical applications in control.
In a series of papers, Cone et al. (1971a, 1971b and
’ 1972) have presented evidence for a sex pheromone
I in T. urticae. The behavior of mature males of this
species parallels that observed by the writer in O.
. pratensis. It is feasible that manipulation of spider-
mites using pheromones could be used to control
A outbreaks or to preclude their development.
i, Pathogens of spider-mites include fungi and vi-
ruses (McMurtry et al., 1970) although extensive re-
{ search has not been directed toward these agents for
. control. The writer is aware of no published reports of
pathogens of O. pratensis and O. stickneyi, the major
pest species in West Texas. Microbial control of
spider-mites offers some promise and should
l5lil<ewise be exploited in the present case.
Sprinkler irrigations have been used to suppress
spider-mite densities by Hudson and Beirne (1970)
and Kinn et al. (1972). This approach may be espe-
cially apropos in the present case since the limited
evidence available indicates spider-mites in West
. Texas are extremely susceptible to rainfall. However,
'Ward (1973) cautions that spider-mites may counter
(by adapting to such selective pressures.

i Economic Thresholds
 Economic threshold (action level) can be consid-
, ~ ered to be that population density (and/or other ap-
t propriate parameters) at which control measures
y‘ should be initiated to keep pest density from reaching
an economic injury level. Theoretically, at the
» economic threshold, there is a substantial probability
r that, if left uncontrolled, pest density will exceed the
f, economic injury level.
_ Ward et al . (1972) demonstrated that, in general,
’ when spider-mites on grain sorghum were controlled
I after the soft dough stage, there were not significant
= increases in yield over the non-treated control. These
a data indicate that spider-mites need not
' be controlled subsequent to soft dough unless mites
‘ invade and web the panicum or severly weaken the
; plant thereby predisposing it to stalk-rot and other
f pathogenic organisms. Predictability of the latter
' phenomena is in need of study. Predictability of
spider-mite phenology and population increase on
grain sorghum (see previous section) should enhance
_. the specification of 5a suitable economic threshold.
it Presently, an experimentally deduced economic
1 threshold is lacking. Development of such a level is
' desirable although the writer questions massive ex-
perimentation to determine an economic threshold in
‘ those areas where spider-mites are presumed to be
§ resistant to available chemical acaricides and insec-
-' ticides. Economic aspects of spider-mite control on

corn are, at best, fragmentary and in need of research
(see Ward et al. 1972).

Integrated Control

An integrated approach “to controlling or manag-
ing these spider-mites should be the final desiderata.
This is because no single artificial control technique
can be expected to withstand the genetic counterat-
tack to be anticipated from spider-mite populations
when subjected to harsh selective pressure. Sec-
ondly, biological control cannot be expected to be
fully developed nor appreciated for several years
and, in the event of successful biological control, this
balance can be readily disrupted by chemical meas-
ures applied for control of other primary pests and
occasional pests. Hence, suitable selective artificial
control measures need to be developed.

Some Directions for Future Research

Sophisticated pest-management tactics de-
signed to stand the test of time should have as their
foundation a thorough understanding of the ecology,
particularly population dynamics, of the pests and
their associates. It is not surprising that the practice of
biological control, which can lead to permanent sup-
pression of a pest, is firmly supported by ecological
theory. However, these very principles and practices
which pertain to population dynamics and biological
control need examination in light of the grain sor-
ghum agro-ecosystem‘; and, if current concepts are
of limited use, some new considerations are in order.
It is particularly important to have a handle on
theoretical ideas since,_ without them, we will not
know precisely which parameters to measure, much
less, how to measure them.

Population dynamics

Traditional approaches to the study of insect
population dynamics generally involve a standard set
of sequential operations. These can be listed as fol-
lows: (1) sampling of appropriate stages of the life
cycle and of mortality factors associated with each
stage; (2) construction of age-specific life tables from
the census figures; (3) analysis of the life table data

from several successive generations, usually via a a

key-factor analysis; (4) testing to determine if mortal-
ity is dependent or independent of host density; and
(5) generation of a population model for explaining
and predicting population events (Harcourt, 1969;
Varley and Gradwell, 1970).

This approach to the study of insect population
dynamics is not totally applicable to dynamics of
insect-pest populations on grain sorghum. This is
primarily because these methods are applicable to
univoltine species (Luck, 1971; Ito, 1967), whereas

“Most of the remarks in this section deal with grain sorghum since
there is sufficient knowledge available to speculate on insect-plant
or mite-plant relationships in this crop. Most of the remarks prob-
ably apply to corn also.

11

most pests of grain sorghum are multivoltine; often
the generations overlap such that age-specific life
tables cannot be constructed. Also, the classical ap-
proaches (e.g., Varley and Gradwell, 1968) require
that a population be studied in the same location for a
number of years. A field of grain sorghum one year
may be a field of cotton the next. In short, the ap-
proach is inapplicable not only to grain sorghum
pests but also to pests of other temporary or annual
agroecosystems, e.g., cotton, corn (see also Way,
1973).

In view of these circumstances, we should seek
alternative methods of analysis and/or modify the
existing methods to suit our needs.

For pests with overlapping generations (e.g.,
spider-mites, greenbug and other aphids) the
methodology advanced by Hughes and Gilbert (1968)
is useful. The approach entails gathering data (usu-
ally expressed in fecundity schedules) on aphids,
their predators, parasites and hyperparasites and
other parameters which markedly influence the sys-
tem. In the end, a computer model is generated
which, if the input data are realistic, can be used to
predict population events.

For pest species with discrete generations
(whether univoltine or multivoltine) on grain sor-
ghum (e.g., Heliothis), perhaps a modification of ex-
isting methodology is in order. For example, age-
specific life tables can be constructed for these popu-
lations and can be replicated in time and space. How-
ever, the analysis of the life-table data must take on a
new dimension. Since the classical key factor analysis
is of little value in this respect, a simple visual analysis
of the life tables can aid in demonstrating any consis-
tent features, such as a major loss of individuals be-
tween two successive stages. Also, the analysis of
intragenerational processes as opposed to
inter-generational processes may be of more value,
especially from the utilitarian standpoint, since pests
may be bivoltine or trivoltine and may cause
economic loss in each generation. Modeling these
sorts of relationships has not been exploited.

It is apparent that the study of insect population
dynamics in temporary agro-ecosystems is a virgin
field of applied ecology. The special theory and
methodology underlying this discipline are lacking in
many cases and it appears that those involved in
research on grain sorghum pests (for example) are in
a prime position to develop its basic principles. We
cannot expect ecological theorists to do it. This, then,
should be one major long-term research effort.

Classical biological control

The importation of exotic natural enemies to sub-
jugate native or an exotic pest species is an estab-
lished tactic in pest management. It is appropriate
that workers in Oklahoma (e.g., Jackson et al., 1971;
Rogers et al., 1972) are exploiting this technique, par-
ticularly since greenbug, corn leaf aphid and sor-
ghum midge are exotic insects. However, such ap-

12

plied biological control should be given some special
consideration in view of the temporal and spatial
properties of the grain sorghum agro-ecosystem and
the pests contained therein.

Lloyd (1960) noted that most successful cases of
biological control occurred on pests of perennial
plants (e.g., citrus, olive, coffee, eucalyptus, larch,
etc.) and further indicated that the method may be
limited to pests of such perennial or stable-crop
agro-ecosystems. Newsom (1970) and Ridgway
(1972) further document the fact that this apparent
limitation to successful biological control is substan-
tiated by the record of successful cases (see‘ also
Southwood and Way 1970). Hansberry (1968) further
concludes that ”it seems very unlikely that we can
ever expect biological methods to control field and
truck-crop pests."

Despite the fact that most of the attempts at and
successes of biological control have involved pests of
perennial crops, it should not be concluded, on ana
priori basis, that the method is not suited for pests of
temporary or annual agro-ecosystems. This is be-
cause no one has presented a valid theoretical reason
why importation of exotic natural enemies for control
or pests in such agro-ecosystems should not be at-
tempted.

In this case, an underlying reason for the lack of a
suitable explanation can be traced to the fact that the
practice of importation of natural enemies has grossly
preceeded the theory. That is, present-day guidelines
for importation of natural enemies of foreign pests
have been largely established as a result of
biological-control work in more permanent, peren-
nial ecosystems. Thus, as in the case of population
dynamics of insect pests in short-cycle agro-
ecosystems, the theoretical guidelines for classical
biological control in these situations seem deficient.
We should not assume that the properties of a suc-
cessful biological-control agent on olive or citrus will
necessarily be coincident with an exotic enemy for
a pest of grain sorghum

In recent years, some theories of island biogeog-
raphy put forth by MacArthur and Wilson (1967) have
been used to enhance theoretical perspective with
respect to non-insular habitats which are analogs of
oceanic islands (i.e., habitat islands). Following the
suggestion of MacArthur and Wilson (1967), several
authors followed by relating principles of island
biogeography to mountain tops (Brown 1971), host
plants (Ianzen 1968), individual oak trees (G. W.
Grankie, personal communication), caves (Culver
1970), continental areas (Vuilleumier 1970) and geo-
graphic distribution of oaks (Opler 1974). Of the sys-
tems remaining, it may be of value to consider the
geographical area occupied by a grain-sorghum field
(for example) to be a habitat island or ”functional
island" (see Force, 1972).

MacArthur and Wilson (1967) emphasize two
evolutionary strategies: K-strategy, in which a

species evolves toward more efficient utilization of
environmental resources; and r-strategy, in which

 selection favors a rapid rate of population increase.

The temporary agro-ecosystem can be viewed as

i‘ a functional island. That is, grain sorghum is planted,
i cultured and harvested in one season and the plant
i remains are plowed down at the end of the season.
a The following year, the same processes occur, often
§ in another location. Thus, the pest species and their
{ natural enemies may be forced to play a hide-and-
A seek game with their respective food source, and it
 follows that those species best adapted (pre-adapted)
a» for this mode of existence (e.g., colonizing species)
‘ will be successful in exploiting such a temporary re-

quisite.
Force (1972) noted that, in a disturbed eco-

 system (e.g., agro-ecosystem), the r-strategist (or

colonizing species) will be numerically dominant and
further suggested importation of such enemies to
effect biological control. Pianka (1970, 1972) and Price
(1973) have given further details on identification of
r-strategist species. Some major pests of grain sor-
ghum in West Texas, namely, greenbug, corn leaf
aphid and spider-mites, can perhaps be classed as

 r-strategists and it is suggested that future research
 be directed toward developing operational methods

for identification of r-strategist enemies which, pre-
sumably, would be suitable agents for biological con-
trol of pests of grain sorghum.

In this regard, the following can be considered a
working hypothesis: in a short-cycle crop such as
grain sorghum, those natural enemies capable of col-
onization of the habitat and rapid exploitation of a
temporary requisite (i.e., the prey) can be suitable
biological-control agents. Some attributes of such
enemies likely include: (1) competent dispersal (e.g.,
aphid parasitoids passively carried within the body of
the alate aphid); (2) appropriate host or prey finding
mechanisms once the enemy has arrived in the sys-
tem; and (3) a comparatively high reproductive rate.

Recent evidence (Ehler et al, 1973) from an
analogous crop (cotton) gives added insight into how
the strategy of rapid exploitation of cabbage looper
[Trichoplusia ni (Hubner)] can be accomplished. In
this case, polyembryony (a host-specific parasitoid),
polyhedrosis (a nuclear polyhedrosis virus), and
polyphagys (a complex of general predators) repre-
sent attributes enabling the given enemies to take
advantage of a temporary requisite. Presumably,
substantial biotic mortality is the result.

To summarize, we should question the concept
that the necessary properties of a successful
biological-control agent for a pest of a stable crop will
be coincident with those of an agent for a pest of grain

‘Although general predators may be of value in biological control
programs, it is suggested that introduction of such agents be given
critical scrutiny before establishment since these enemies could
possibly disrupt established predator-prey systems (e.g., by pref-
erentially feeding on other predators) and thus engender serious
flarebacks.

sorghum. Second, we should attempt to delineate the
properties of a suitable biological-control agent for
pests of grain sorghum such that biological control
can become a more predictive science.

Natural control of natural enemies

Varley (1970) urged the development of life ta-
bles for predators and parasites to enhance modeling
of these agents and, by implication, to aid in explain-
ing why certain enemies are not efficient. The studies
of Hassell (1969a, b) apparently represent the first
definitive explanation of the factors which contribute
to population fluctuations in a natural enemy.

This line of research is particularly relevant to
natural enemies of pests of grain sorghum in West
Texas. For example, G.L. Teetes (unpublished data)
demonstrated that when H ippodamia convergens Guer.
adults and developmental stages were hand removed
twice daily throughout the growing season, there
were no significant increases in prey density (green-
bug) over plots which retained the normal comple-
ment of this enemy. As a consequence, Kirby (1974)
constructed life tables for the predator and demon-
strated that, in general, 80-90 percent of the genera-
tion mortality occurred during the age interval com-
prised of eggs and first and second instar larvae.
Kirby's results were consistent in time and space and
can be used to aid in explaining why these beetles are
seemingly inefficient predators.

Kirby also observed that a variable portion of the
mortalit of eggs was due to predation by other
species Beg, Orius insidiosus (Say)]. If this phenome-
non becomes a predictable event and is of sufficient
magnitude, it may be less profitable to seek those
exotic aphid-feeding coccinellids whose ecological
characteristics are similar to H. convergens since their
ecological impact may be similarly affected. In this
case, a parasitoid may be more suitable since certain
natural enemies (hyperparasites) of these species
exert their effect after the host (pest) has been
parasitized and thus even high rates of hyperparasiti-
zation (e.g., Walker et al., 1973; Frazer and van den
Bosch, 1973) would not necessarily be conducive to
disruption of biological control. It is suggested that

these concepts be treated as working hypotheses and ‘

be the object of future research.

Manipulation of natural enemies

Apart from considering those factors which ef-
fect direct mortality of natural enemies in the agro-
ecosystem, it may be expedient to investigate rela-
tionships within or outside of the system which,
through intelligent manipulation, could enhance the
impact of natural enemies. Such manipulation may
involve providing an adequate supply of an alternate
host during critical periods (e.g., Doutt and Nakata,
1973) or providing artificial nutrients for enemies
(e.g., Hagen and Tassan, 1970).

In the case of spider-mites on corn and grain
sorghum, natural enemies often respond to mite out-
breaks too late to exert sufficient control. In the El

13

Paso Valley, S. atomus is a case in point. This species
shows a delayed reproductive response to increases
in mite density. If adult beetles could be stored on
alternate hosts nearby the crop, perhaps prompt im-
migration and more efficient predation would result.
The writer has observed this species in the desert
areas which surround the agricultural region in the
valley, where it is closely associated with desert

spider-mite (Tetranychus desertorum Banks) on creo-
sote bush. Since this inoculum of predators is spa-
tially separated from corn and grain sorghum in the
area, it is suggested that future research be directed
toward bridging this gap, perhaps through planting
suitable plants in the immediate vicinity of crops, so
as to ultimately enhance the efficiency of the preda-
tors. 

"r -.

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