A2457

L186

Selected Factors Influencing

The Abundance of
Banks Grass Mite
in Sorghum

      <
 é     ‘Y’
Willi’ 2  1978

Texas  University

Texas Agricultural Experiment Station
Neville P. Clarke, Director
College Station, Texas
The Texas AGM University System

B-l I86
April I978

CONTENTS

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Effects of Grain Sorghum Phenology
on Spider Mite Populations . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Effects of Pesticide Applications
on Spider Mites Infesting Sorghum . . ._ . . . . . . . . . . . . . . . . 3
Effects of Water Stressed Sorghum
on Spider Mite Populations . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Effects of Sorghum Phenology on Leaf Sugar Levels
and Spider Mite Population Levels . . . . . . . . . . . . . . . . . . . . 4
Effects of Pesticide Alpplications
on Spider Mite Popu ations in Grain Sorghum . . . . . . . . . . 5
Effects of Water Stressed Sorghum
on Spider Mite Population Levels . . . . . . . . . . . . . . . .  . . . 7
Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

SUMMARY

Banks grass mite, Oligonychus pratensis (Banks),
population densities changed in correlation to host
phenology. Prior to sorghum boot development, spider
mite numbers were low. These population levels in-
creased, however, as the plant approached reproductive
maturation. Maximal population densities developed at,
or just prior to, host plant senescence.

Total soluble leaf sugar concentrations fluctuated in
response to the physiological state of sorghum develop-
ment. Prior to the boot stage of development ca. 1.2 mg
sugar was present per 6 cm’ leaf tissue; whereas, at boot
stage, sugars increased to ca. 2.8 mg/6 cm2 leaf tissue.
This increase may have been caused by a temporary de-
crease in metabolic activity in the reproductive portion
of the plant prior to blooming thereby removing less
sugar from the leaves. Soluble leaf sugars decreased to
ca. 1.3 mg/6 cm“ leaf tissue during the grain develop-
ment stages. Data indicated that sugars were translo-

cated from the leaves to the developing seeds, resulting
in decreased leaf sugar concentration. As the plants ap-
proached maturity, less sugar was required for grain fill,
resulting in an increase in leaf sugars. Slight variations in
total soluble leaf sugar concentrations occurred in leaves
at different locations on the plant. These variations may
have resulted from shading of lower leaves.

Application of parathion to sorghum apparently re-
sulted in increased spider mite numbers significantly
greater than on non-treated plants. Parathion-treated
sorghum plants supported more spider mite colonies
with more unit area per colony than did non-treated
plants. But spider mite numbers were less per unit area
of colony in parathion-treated plants than non-treated
plants. The data indicated that parathion application re-
sulted in spider mite dispersal from the localized dense
colonies. This dispersal resulted in a release of the
reproductive-inhibition associated with crowding. Signif-
icantly higher spider mite numbers occurred on water-
stressed sorghum plants than on sorghum receiving suf-
ficient moisture by irrigation.

SELECTED FACTORS INFLUENCING THE ABUNDANCE
OF BANKS GRASS MITE IN SORGHUM

David H. Kattes and George L. Teetes*

In 1967 and 1968, infestations of Banks grass mite,
Oligonychus pratensis (Banks) Acari; Tetranychidae,
were observed in several counties of the Texas High
Plains (Gate and Bottrell 1971). Such infestations were
generally confined to older plants. Pate and N eeb (1971)
observed the Banks grass mite to be more serious each
year in the Trans-Pecos area of Texas. This was due, in
part, to increased mite population levels as well as failure
to obtain adequate control with acaricides approved for
use on sorghum.

Presently, the mite problem is not so severe on the
Texas High Plains as it is in the Trans-Pecos area. How-
ever, with continuing insecticide use for greenbug con-
trol and increasing resistance to selected pesticides,
these mites pose a major threat to sorghum production.
To better understand the mechanisms causing spider
mite outbreaks, information is required regarding the
interaction between mites and their host plant and the
environmental factors influencing both (Van de Vrie,
McMurtry, and Huffaker 1972).

MATERIALS AND METHODS

Effects 0f Grain Sorghum Phenology on
Spider Mite Populations

This study was designed to monitor seasonal
changes in mite population density in sorghum and to
investigate the vertical stratification of leaf sugars and
vertical migration of mites. The study was conducted on
the Donald Terrel farm located in Hale County near
Plainview, Texas. The field contained 145 rows, 45.7 m
long on 101.5 cm centers. Grain sorghum hybrid “Niag-
ara Oro T” was planted May 18, 1975, at a rate of ca. 10.1

. kg seed/ ha.

I I

One application of parathion at 0.6 kg AI/ha to the
entire test area was required July 14, 1975, to prevent
plant damage due to greenbug infestation. The single
treatment was effective, and no further treatments were
required.  -

*Bicounty extension entomologist, Mitchell and Scurry Counties, and
associate professor, Department of Entomology, Texas AirM Univer-
sity, respectively.

KEYWORDS: Sorghum/Banks grass mite/Oligonychus pratensis
(Banks)/insecticides, host phenology/plant stress/Texas.

Each week, 1O plants were selected at random, and
the number of female mites and mite predators were
recorded on each leaf of each plant. Using a modified
anthrone reagent method (Sunderwirth, Olson, and
Johnson 1964), total soluble sugar concentrations were
determined for leaves of 5 of the 1O plants. Each leaf was
cleansed in the field using a water saturated cloth, and a
6 cm’ disc was removed from either side of the midrib
at the approximate midpoint of the leaf. Samples of leaf
tissue were obtained via a hole punch. Leaf discs from
1O leaves of each plant were placed in a vial containing
5 ml of 8O percent ethyl alcohol. The vial was then sub-
merged in a mixture of acetone and dry ice (liquified

CO2).

The vials were returned to the laboratory and sub-
merged in boiling water (97.7°C) for ca. 3O minutes. This
procedure extracted soluble leaf sugars which entered
the alcohol solvent (Sunderwirth, Olson, and Johnson
1964). A 1 ml aliquot of the alcohol-sugar solution from
each sample was transferred to a separate vial. The re-
maining 4 ml were discarded.

Ten ml of the anthrone reagent were added to each
vial containing a 1 ml sample of the sugar solution. Such
vials were capped and placed in a 95°C water bath for 15
minutes. Various shades of blue developed as a result of
varying sugar concentrations in the sample. Heated so-
lutions were placed‘ under tap water and cooled to near
room temperature. The concentration of sugars was de-
termined using an Evelyn colorimeter calibrated at 620
mu which measured the optical density of the bluish-
colored solution.

A series of concentrations (O.1, 0.2, 0.3, O.4, 0.5,
O.6, O.7 mg/ml solvent) of glucose in 8O percent ethyl
alcohol served as standards, as well as a test of the suita-
bility of the anthrone reagent and colorimeter. The opti-
cal densities of the various concentrations of glucose
were directly proportional to increasing sugar concen-
trations. The reagent was discarded if variance of greater
than 5 percent occurred in the standard curve.

Effects of Pesticide Applications on
Spider Mites I nfesting Sorghum

A test was conducted in 1974 to study a possible
mechanism of insecticide induced outbreaks of spider
mites on grain sorghum in the High Plains of Texas. The

test was located 0n the Donald Terrel farm near Plain-
view, Texas.

The test field consisted of 138 rows, 333.8 m long
with 101.6 cm row centers. The grain sorghum variety

“McNair 650” was planted May 5, 1974, at ca. 10.1 kg of

. sorghum seed per ha. Fertilizer (11-37-0) was applied

prior to planting at a rate of 156.8 kg/ha, and 244 kg/ha of
anhydrous ammonia was applied post-emergence.

Applications of disulfoton (3.2 kg AI/ha) July 18 and
26, 1974, were necessary for reduction of greenbug
populations. On July 5, 1974, parathion was applied at a
rate of 3.4 kg AI/ha to 10 adjacent rows using a Hahn
Hi-Boy®, high clearance spray machine. The 1O rows
were located 33.5 m into the field from the west side.

Mite counts were obtained from 1O randomly
selected plants from both the parathion-treated area and
the surrounding check. Active stages of mites and mite
eggs were counted separately. Counts of the active
stages of mite development were not segregated.

Additional information collected included the
number of spider mite colonies per plant, the size of
each colony, and the number of mites in each colony.
Predator counts also were recorded.

Effects of Water Stressed Sorghum on
Spider Mite Populations

To determine the effects of moisture stress on the
total soluble sugar content of sorghum leaves and sub-
sequent effect on mite numbers, a test was conducted at
the Texas Agricultural Experiment Station at Halfway,
Texas. The test area consisted of 52, 101.6 cm sorghum
rows, 243.8 m long. The grain sorghum variety “McNair
650” was planted May 20, 1975, at an approximate rate of
7.8. kg sorghum seed/ha. Two applications of parathion
insecticide were necessary to prevent damage by green-
bugs on July 17 and August 7.

The test was divided into plots containing 26 rows.
One-half of the plots were irrigated when the soil
moisture declined to a level recommended by C. W.
Wendt* (personal communication), based on moisture
levels and plant growth stage. These levels were based
on the phenological state of plant development and

available soil moisture. The alternate plot was irrigated.

shortly after emergence and received no additional water
except from rain.

Soil moisture levels were estimated using Irrome-
ter® tensiometers. A tensiometer was placed on rows 8,
16, and 24 of each plot at approximately 61.0, 121.9, and
182.9 m, respectively, into the field. The tensiometers
were read and calibrated weekly.

Total leaf sugar content was determined by sam-
pling 1O randomly selected plants from each plot. Ana-
lytical procedures were the same as those previously de-
scribed. 

Mite numbers were counted weekly on 1O marked
plants of each plot. The plants were selected on alternate
rows beginning on the third row from the exterior of

*Charles Wendt, soil and water research scientist, Texas AérM Univer-
sity Agricultural Research and Extension Center, Lubbock.

each plot. Each plant marked was approximately 8O feet
from its closest, neighboring marked plant.

RESULTS AND DISCUSSION

Effects of Sorghum Phenology on Leaf Sugar Levels
and Spider Mite Population Levels

Data collected in 1974 (Figure 1) indicated peak
spider mite densities concomitant with host senescence,
which agreed with previously reported research (Ehler
1974). However, data collected in 1975 indicated that
peak numbers occurred during early dough stage (Figure
2). Discrepancies in mite population densities may be
attributed to abiotic factors associated with the study
area. Frequent, light rains occurring during August 1975
possibly caused the sudden decrease in mite numbers
(Ward 1975).

Total soluble leaf sugar concentrations of sorghum
fluctuated during the reproductive phase of grain sor-
ghum development (Figure 3). Prior to the boot stage of
plant development (July 14), total soluble leaf sugars av-
eraged 1.2 mg per 6 cmz of leaf tissue. D. Kreig* (per-
sonal communication) suggested that this may have re-
sulted from a temporary decrease in metabolic activity in
the plant’s reproductive portion prior to blooming.

100

Mean No. Mites / Plant
.4
O

0.1

Jul19 23 31 Aug 5 15 Oct 3
Bloom Dough Maturity

Figure 1. Seasonal abundance of spider mites infesting sorghum in the
Texas High Plains, 1974.

*Daniel Kreig, plant physiologist, Texas AbM University-Texas Tech,
University Cooperative Research Unit, Lubbock.

At the blooming stage (July 21 and 28) total soluble Effects 0f Pesticide Applications 0n

t leaf sugars decreased t0 1.3 mg per 6 cm2 leaf tissue. Spider Mite Populations in Grain Sorghum
Between plant bloom and maturity (July 28, 1975 to Au- 9
gust 25, 1975) total soluble sugar concentration remained
stable. This stability in soluble sugar concentrations was
indicative of sugars produced by the leaves being trans-
ported to the developing seed (Eilrich et al. 1964).

Spider mite numbers increased following the appli-
cation of parathion insecticide (Figure 5). Initially, large
spider mite numbers were present in the parathion-
treated area. However, disulfoton applied Iuly 18,
1974, to control greenbugs reduced spider mite numbers

Soluble sugar concentrations of individual sorghum on plants in both the Check and the parathion_treated
leaves Were else eemPaTed- Ne Significant (llllerenee (5 areas. An additional application of disulfoton on July 26,
Pereenl eenfidenee levels) eeeurred in Soluble Sugar een" 1974, reduced mite numbers in the parathion treatment
centrations between leaves. below that in the Che¢]<_

No correlation was found between the fluctuating

soluble leaf sugars and spider mite numbers (Figure 4). 15

12

1O

No. Female Mites/ Plant

0.5 0.7 0.9 1.1 13 1.5 1.7
mg Sugar/Bcmz Leat Tissue

Figure 4. Correlation between total soluble leaf sugars concentration
and spider mite numbers in sorghum.

Ave. No. Female Mites/ Leat
I

1000 I——I Parathion
#--* Check

0.1

Jul14 21 28Aug4 11 18 25Sept1 8 15
Boot Bloom Dough Maturity

Figure 2. Seasonal abundance of spider mites infesting sorghum in the
Texas High- Plains, 1975.

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Jun. Jul7 14 21 za Augd 11 1a 2s Septt a Jul 19 23 31 Aug5 15 Oct3
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Figure 3. Total soluble leaf sugar concentration changes in response to Figure 5. Seasonal abundance of spider mites infesting parathion-

phenology of sorghum. treated and non-treated sorghum.

 

Between August 5 and 15, spider mite numbers in
the parathion treatment increased from 5 to 357 per
plant (Figure 5). However, the number of spider mites
in the check area remained relatively stable (7 t0 6.4
mites per plant). On October 3, spider mite population
densities were 1,629 per parathion-treated plant and 494
per non-parathion treated plant. An application of pho-
rate (1.1 kg AI/ha) on October 5, 1974, decimated the
spider mite population and no further counts were
made.

Samples taken ‘on each sample data indicated that
spider mite predator populations were always less than
one predator per 1O plants. The literature does not indi-
cate the number of predators necessary to subjugate
spider mite outbreaks. Ehler (1974) suggested that pred-
ator response to increased mite numbers was delayed and
is not sufficient to reduce mite numbers before economic
levels were attained. Therefore, the predator density ap-
peared to be too low to cause substantial differences in
numbers of spider mites between the parathion-treated

and the non-treated areas.

    
 
   
  

I—-—I Paralhion
i———t Check

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Jul 19 2a a1 Augs I5 Ocla

Figure 6. Mean number of spider mite colonies per plant on
parathion-treated and non-treated sorghum.

IO
I——I Parathion

i
*—-—i' Check
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Mean cm’ / M ile Colony

 

JU|I9 2a a1 Aug s I5 Oct:

Figure 7. Mean area per spider mite colony on parathion-treated and
non-treated sorghum.

Parathion-treated plants supported more spider
mite colonies than non-treated plants (Figure 6). The
area occupied per colony was also greater on the
parathion-treated plants than on non-treated plants (Fig-
ure 7). However, on several sample dates (i.e., July 31,
August 5 and 15) parathion-treated plants supported
fewer mites per unit colony space than the non-treated
plants (Figure 8).

Between August 5 and 15, mite numbers increased
on the parathion-treated plants (Figure 6). Mite num-
bers decreased, however, in the non-parathion treated
area during the same time interval. The less dense col-
onies on plants following the parathion treatment prior
to August 15 may have facilitated rapid population in-
creases by lowering fecundity inhibition. However,
spider mite colony density increased concomitantly in
the non-parathion treated area, resulting in fecundity
inhibition which impeded an increase in number by the
mites. Consequently, the magnitude ofincrease in mites
per plant between August 15 and October 3 was greater
on the parathion-treated plants than on the non-
parathion-treated plants (4.5- and 78.5-fold increases,
respectively). ‘

These data suggest that spider mite outbreaks fol-
lowing application of parathion possibly did not result
from predator inhibition. They also may suggest that
parathion treatment resulted in spider mite dispersal

   
  
 

I——I Paralhion

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Jul I9 2a a1 Aug s l5 Ocla

Figure 8. Mean number of spider mites per unit area of colony space
on parathion-treated and non-treated sorghum.

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from dense colonies, thus overriding the inhibition of
fecundity effects of crowding (Davis 1952). Davis (1952)
noted similar effects on Tetranychus multisetes
McGregor, after application of DDT.

Effects 0f Water Stressed Sorghum on
Spider Mite Population Levels

Soil moisture levels varied throughout the growing
season. Prior to July 28, frequent rains totaling as much
as 3.8 inches maintained soil moisture at high levels.
However, at the plant bloom stage of development (July
28), soil moisture had decreased to 50 centibars, and
irrigation of blooming grain sorghum was recommended.

Recommendation to irrigate was based on soil
moisture levels and the physiological stage of the crop.
Accordingly, one half of the test area was irrigated, and
the other half was not. Irrigation resulted in a substantial
increase of soil moisture in the watered area. On the
non-irrigated land, soil moistures increased slightly due
to a 0.1 inch rain on August 1, 1975.

After July 28, mite numbers on plants in the
stressed area increased (Figure 9), whereas mites were
not present on sampled plants in the irrigated area. Mite
numbers on plants in the non-irrigated portion of the
field increased until September 1, 1975, when they
reached 124.6 female spider mites per plant. Mean
numbers of mites on non-irrigated plants were signifi-
cantly higher (0.2 confidence levels) than mite numbers
on plants in the irrigated area on August 11, 18, and 25
and on September 1 and 8.

I—-I Stressed

*—* Irrigated
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Jul 20 Aug 4 n ll 25 Seph a l5

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h; Figure 9. Mean adult female spider mite population levels per week on
water-stressed and non-stressed sorghum.

The number of spider mites on plants in the irri-
gated area increased to 3.4 mites per plant on August 25,
1975. Spider mite densities decreased between Sep-
tember 8 and 15 in both areas (Figure 9). This decrease
was expected as mite populations reportedly decrease at
plant senescence (Ehler 1974). No substantial differ-
ences occurred between concentrations of total soluble
leaf sugars of sorghum plants in the irrigated and non-
irrigated areas.

In another experiment not described in this article,
several rates of urea fertilizer applied to sorghum did not
affect the spider mite populations densities, nor did sig-
nificant changes in total leaf sugars occur as a result of
the treatments.

LITERATURE CITED

Cate, J. R., Jr. and D. G. Bottrell. 1971. Evaluation of foliar sprays for
controlling the Banks grass mite on grain sorghum in the Texas
High Plains, 1969. Tex. Agri. Exp. Stn. PR-2870z22-3.

Davis, D. W. 1952. Some eifectsfof DDT on spider mites. J.‘ Econ.
Entomol. 45:1011-9.

Ehler, L. E. 1974. A review of the spider mite problem on grain
sorghum and corn in West Texas. Tex. Agri. Exp. Stn. B-1144.
15 p.

Eilrich, G. L., R. C. Long, F. C. Stricklen and A. W. Pauliaw. 1964.
Stage of maturity plant population and row width as factors affect-

ing yield and chemical composition of atlas forage sorghums. Kan-
sas Agri. Exp. Stn. Bul. 138.

Pate, T. L. and G. W. Neeb. 1971. The Banks grass mite problem in
the Trans-Pecos area of Texas. Tex. Agri. Exp. Stn. PR-2871.
pp.23-5.

Sunderwirth, S. G., G. L. Olson and G. Johnson. 1964. Paper
chromatography-anthrone determination of sugars. J. Chromatog.
16:176-180.

Van de Vrie, M., J. A. McMurtry and C. B. Huffaker. 1972. Ecology
of Tetranychid Mites and their natural enemies. A Review. III
Biology, ecology and pest status, and host plant relations of Tet-
ranychids. Hilgardia. 41:343-432.

Ward, C. R. 1975. Status of control of the Banks grass mite in sor-
ghum. Proc. 29th Biennial Sorghum Res. And Util. Conf., Lub-
bock, Texas. pp. 96-105.

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