/Contribution from the Inst. of Food and Agric. Sci. Florida Exp. Stn., Univ.
of Florida, and the North Florida Res. and Educ. Ctr., Quincy, FL 32351.
Research Report NF 87-9.
a/Assistant Professor of Entomology, Univ. of Florida and Professor of
Entomology, Auburn Univ. 36849-4201.
? Central Science
Bigeyed Bug Abundance and Dispersion Patterns of Geocons Scien
(Hemiptera: Lygaeidae) in Soybean Fields'
I SEP 23 1987
J. E. Funderburk and T. P. Mack!-/
ABSTRACT Universty of Florida
Seasonal abundance and dispersion characteristics of adult and nymphal
Geocoris spp. (bigeyed bug) populations were determined for soybean fields
located in Alabama and Florida. Three fields were sampled in 1985 and two
fields were sampled in 1986. Populations were present in each field from
middle or late vegetative stages of crop growth until crop senescence.
Numbers increased through the growing season and were greatest on the last or
near the last sample dates. At least three complete and additional partial
generations were typical, with populations of different generations broadly
overlapping. Population dynamics were similar to previously reported data in
other states in the southern U.S., indicating a similar phenology for bigeyed
bugs in soybean fields throughout the region. Dispersion statistics of
variance/mean ratio and Taylor's power law were calculated for adult and
nymphal sample estimates. Populations of nymphs were always aggregated.
Adults usually were randomly distributed, but sometimes aggregated.
Geocoris spp. (bigeyed bugs) are common, polyphagous insect predators in
many crops. Populations of Geocoris punctipes (Say) occur in large numbers in
soybean fields in the southern U.S. (Tugwell et al. 1973, Turnipseed 1972,
Shepard et al. 1974). The diet of arthropod prey is supplemented with some
feeding on plants, which improves survival and decreases developmental time
(Naranjo and Stimac 1985). The predator is an important biological control
agent of Anticarsia gemmatilis Hubner (Elvin et al. 1983) and Nezara viridula
(L.) (Crocker and Whitcomb 1980), which are the most economically important
pests of soybean. Bigeyed bugs may be important agents in the suppression of
populations of occasional pests of soybean, such as Heliothis zea Boddie
(Whitcomb and Bell 1964), H. virescens, (F.) (McDaniel and Sterling 1979),
Pseudoplusia includes (Walker) (Richman et al. 1980), and many others.
The population dynamics of bigeyed bugs in soybean fields has been found
to vary according to geographical location. Numbers were greatest in late
August to early September in Kentucky (Raney and Yeargan 1977), early August
or mid-September in North Carolina (Deitz et al. 1976), late September in
South Carolina (Shepard et al. 1974a), and between late June and early August
in Mississippi (Pitre et al. 1978). More than one generation occurred in each
case, as populations were present during most of the growing season. No
published information exists on the population dynamics of bigeyed bugs in
soybean in more southerly areas of the southern U.S. growing region.
The dispersion characteristics of bigeyed bug populations in South
Carolina soybean fields were investigated by Waddill et al. (1974).
Populations were randomly distributed on most sample dates, fitting the
Poisson distribution. Pieters and Sterling (1973) reported that their
populations in cotton were clumped on most occasions, fitting the negative
binomial. Dispersion characteristics of insect populations have been shown to
vary with population density and sample-unit size (Wilson and Room 1982,
1983). Therefore, additional information taken over a range of population
densities and at a different sample-unit size from the Waddill et al. (1974)
study would provide valuable information about the dispersion characteristics
of their populations in soybean.
The primary purpose of the present study was to determine the temporal
abundances of bigeyed bug populations in soybean in Alabama and Florida.
Their dispersion characteristics over a range of population densities also
were quantified. Such information will allow for implementation of pest
management practices that conserve these natural control agents in soybean
Materials and Methods
Bigeyed bug populations were monitored over two cropping seasons in
soybean fields located in Florida and Alabama. Each was a production field
that was grown in a manner typical for the agroecosystem, but none were
treated with an insecticide that would directly affect bigeyed bug
populations. Fields were disked before planting, with no subsoiling.
Planting dates ranged from late May to late June. Bigeyed bug nymphal and
adult populations were monitored in three fields in 1985. All fields were in
different counties of Alabama, which were Dallas, Elmore, and Henry counties
(Fig. 1). Nymphal and adult populations were monitored in two fields in 1986.
One field was located in Dallas Co., Alabama and the other in Gadsden Co.,
Florida (Fig. 2). All fields were at least 2 ha in area. -Soybean varieties
were 'Tracey M' for the Alabama fields and 'Braxton' for the Florida field.
Nymphal and adult population density was estimated ca. weekly within each
field. Sampling was begun at the early vegetative crop-growth stages and
continued to the late seed stages of crop growth. An exception was the Dallas
Co., Alabama field sampled in 1986. There, extreme drought resulted in
retardation of plant growth, and sampling was discontinued in mid-August.
Each field was divided into 12 parts of equal area, and four random samples
taken within each.
Sampling procedures were according to those established in previous
studies as being most appropriate for estimating nymphal and adult bigeyed bug
populations in soybean (Rudd and Jensen 1977, Shepard et al. 1974b, Turnipseed
1974). Descriptions of methods for sampling their populations in soybean are
contained in Irwin and Shepard (1980). The ground cloth method was employed.
Forty-eight, 1.8-m samples were taken in each field on each sample date when
their populations were in detectable numbers. This sample-unit size differed
from the 1.2-m size in the Waddill et al. (1974) study. For each sample, the
ground cloth was laid between two rows, the soybean plants from both sides
(0.9 m on each side) beaten onto the cloth, and the number of nymphs and
adults determined. Also, bases of the plants and adjacent soil were examined
for any bigeyed bugs.
The variance/mean ratio was calculated for adult and nymphal sample
counts on each sample date in each field (Southwood 1978). These analyses
were performed by using Myers' (1978) FORTRAN program. Log-transformed means
and variances of adult and nymphal sample counts then were calculated for each
sample date in each field, and Taylor's (1984) power law relationship for each
field determined. A Taylor's power law relationship also was determined for
each year by combining data from all fields sampled in that year. All
Taylor's relationships were determined by using SAS programs (SAS Institute
The mean numbers (+SE) of adult and nymphal bigeyed bugs on each sample
date in each soybean field sampled in 1985 and 1986 are shown in Figs. 1 and
2, respectively. Overall precision of the sampling program, as determined by
the SE values, was excellent. The SE was rarely over 25% of the mean except
when populations were very small. Populations of bigeyed bugs were present in
all soybean fields during most of the growing season. Nearly all of the
bigeyed bugs in all fields was G. punctipes, but G. uliginosis was noted in
very small numbers (i.e., < 5% of total).
Adults were first detected in the fields during middle to late vegetative
soybean growth stages. The size of adult populations at this time was never
great. Usually, nymphal populations appeared about a week or two later. Both
adult and nymphal populations then were present in substantial numbers during
the rest of the growing season in all but one of the fields. Adult and
nymphal populations did not occur in large numbers in that field until late in
the growing season. In nearly all fields, generational cycles were indicated
by peaks in nymphal populations ca. every 30 days. There was considerable
overlap between the generations in all fields, because the number of bigeyed
bugs rarely declined greatly in magnitude. Population size was small in the
1985 Henry Co. and 1986 Dallas Co., Alabama fields, compared to the other
fields sampled in this study. The drought may be the reason for the lower
populations in 1986 Dallas Co., Alabama field.
Population size varied among fields, but population trends were similar.
At least 3 complete generations occurred, with additional partial generations
also present. The number of bigeyed bugs increased through the growing
season, with numbers usually greatest on the last or near the last sample
8 HENRY CO., AL NYMPHS
7 .*.-" ELMORE CO., AL
---DALLAS CO., AL
0 ...... --
I I I ,
6/18 6/28 7/8 7/18 7/28 8/7 8/17 8/27 9/6 9/16 9/26
1985 SAMPLE DATE
Fig. 1. Mean numbers (+SE) of adult and nymphal Geocoris spp.
-populations in three soybean fields sampledin 1985
(Dallas, Elmore, and Henry Cos., Alabama).
(Dallas, Elmore, and Henry Cos., Alabama).
........ GADSDEN CO., FL
DALLAS CO., A
1*- .* II
... .I ............ ....... .A UL
: !-i---.1-- ....tI....1...--.i. .--tt...
l" I I ,I
9/1 9/21 10/11
1986 SAMPLE DATE
Fig. 2. Mean numbers (+SE) of adult and nymphal Geocoris spp.
populations in two soybean fields sampled in 1986
(Dallas Co., Alabama and Gadsden Co., Florida).
For each sample date in the 1985 and 1986 soybean fields, the
variance/mean ratio of adult and nymphal populations are presented in Table 1.
Variance/mean ratios of <1, 1, and >1 represent uniform, random, and clumped
distributions, respectively (Southwood 1978). According to this measure of
dispersion, nymphal populations were aggregated on 55.8% of the field/date
data sets when data were sufficient for analysis. Adult populations were
aggregated 47.1% of the time. Values were >1, but not significantly greater
than 1, on most other dates for both nymphs and adults.
Taylor's power law relates variance (s2) to mean density (m) by the
relationship, s2 = amb. Taylor et al. (1978) considered the slope (b) to be a
constant for a species (with values of bl indicating uniform,
random, and clumped distributions, respectively) and the intercept (a) to be
reflected by sample-unit size. Taylor's power law is a particularly useful
dispersion index, because it allows for a description of a species
distribution pattern as changing with density.
Regression statistics of Taylor's power law relationships for bigeyed bug
nymphal and adult sample estimates for each soybean field and for each year
are shown in Table 2. For all relationships involving nymphal populations, b
was statistically > 1(P < 0.05) according to t tests. These values of b and
the relatively high r2-values (mean = 0.94 and 0.94 for 1985 and 1986,
respectively) indicate that bigeyed bug nymphal populations were aggregated
over a wide range of population densities. Results were less definitive for
adult populations, with b statistically > 1 for 1 field. For 3 fields, b was
very near 1, strongly indicating a random distribution. The precision of the
regression relationships was good, with mean r2-values of 0.76 and 0.74 for
1985 and 1986, respectively.
Table 1. Variance/tean ratios of nymphal and adult Geocoris spp. populations
in Henry, Elmore, and Dallas Cos., AL and Gadi-en Co., FL.
SAMPLE s2/ SAMPLE s2 /
DATE NYMPHS ADULTS DATE NYMPHS ADULTS
1985 DALLAS CO., AL
22 Aug. 1.02
5 Sept. 1.69*
13 Sept. 1.34*
1985 ELMORE CO., AL
HENRY CO., AL
1986 GADSDEN CO., FL
1986 DALLAS CO.,
*Significantly different than l(P < 0.05) by X2 test.
Table 2. Regression statistics of Taylor's power law relationships for
nymphal and adult sample data in the 1985 and 1986 soybean fields
(Henry, Elmore, and Dallas Cos., Alabama and Gadsden Co., Florida).
(All relationships are significantly linear beyond the 0.01 level
and no intercept is significantly different from 0 according to a
students t test.)
FIELD AND YEAR INTERCEPT SLOPE r2 INTERCEPT SLOPE r
DALLAS CO., AL 1985 -0.02 1.59* 0.95 -0.01 1.63 0.74
ELMORE CO., AL 1985 -0.05 1.40* 0.98 0.05 0.95 0.59
HENRY CO., AL 1985 -0.02 1.25* 0.89 -0.01 1.22* 0.94
ALL FIELDS 1985 -0.02 1.33* 0.96 0.01 1.16* 0.96
DALLAS CO., AL 1986 0.00 1.16* 0.99 0.01 0.98 0.79
GADSDEN CO., FL 1986 -0.07 1.43* 0.88 0.09 0.89 0.68
ALL FIELDS 1986 -0.04 1.40* 0.93 0.03 1.07 0.86
*Significantly > 1 (P < 0.05) according to a t test
Bigeyed bugs typically were common throughout most of the growing season
in the soybean fields sampled in our study, with greatest population densities
occurring near the end of the growing season. These results from Alabama and
Florida are similar to previously published studies conducted in other states
in the southern U.S. (Raney and Yeargan 1977, Dietz et al. 1976, Shepard et
al. 1974a, Pitre et al. 1978). Bigeyed bugs apparently have a similar
phenology in soybean throughout the region. Bigeyed bugs obviously are
important, indigenous natural enemies of soybean pests in the southern U.S.
Enhancement and conservation of their populations should be a major priority
in soybean IPM programs.
Nymphal populations were clumped in the fields sampled in this study,
which contrasts with the conclusion drawn by Waddill et al. (1974) that
populations were random and best described by the poisson distribution.
Differences between the studies may indicate that dispersion characteristics
of bigeyed bug nymphs change as a function of population density and/or
sample-unit size. However, our results from a wide-range of densities in a
number of fields over two growing seasons would be a strong indication that
clumping is typical for nymphal populations. Adult populations usually were
randomly distributed, but sometimes clumped. These results corroborate the
conclusions drawn by Waddill et al. (1974) that their populations are best
described as randomly distributed.
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