/-6 3& /-7
6 7- /"y
Population Dynamics of Insect Predators In Relation to
Tillage and Subsoiling Prior to Planting Soybeans/
J. E. Funderburk, D. L. Wright, and I. D. Teare2/
Bigeyed bugs (Geocoris spp.) and damsel bugs (Nabis and Reduviolus spp.)
are common polyphagous beneficial insect predators in many crops. The object-
ive of this research was to measure the effects of tillage and subsoiling
regimes on population dynamics and population size of beneficial predators to
aid in development of cultural IPM strategies for biological control of insect
pests in a wheat/soybean sequential cropping system. Bigeyed bug nymphal and
adult population dynamics were similar in each tillage/subsoiling treatment
except that there was considerable overlap of generations in 1986 which was
not observed in 1985. Disk tillage had greater bigeyed bug nymphal and adult
population sizes than no tillage in 1985 and 1986, while subsoiling did not
influence population size. Damsel bug population dynamics were very similar
in each tillage/subsoiling treatment in 1985 and 1986. Population sizes of
adult and nymphal damsel bugs in 1985 were less in no tillage without subsoil-
ing than in disk tillage without subsoiling, disk tillage with subsoiling and
no tillage with subsoiling, which had similar population sizes. Population
sizes were similar in all treatments in 1986.
Additional index words: Bigeyed bug, Geocoris spp., damsel bug, Nabis spp.,
Reduviolus spp., disk tillage, no tillage, subsoiling.
!/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 87-14.
!/Assistant Professor of Entomology, Professor of Agronomy, and Research
Scholar/Scientist of Agronomy, Univ. of Florida. Central Science
SEP 29 1987
University of Florida
Geocoris spp. (bigeyed bugs) and Nabis and Reduviolus spp. (damsel bugs)
are common, polyphagous insect predators in many crops. In the Southeast,
populations of Geocoris punctipes (Say) occur in large numbers in soybean
fields in the southern U.S. (Shepard et al. 1974). R. rosepiennis Reuter is
the most common species of damsel bug comprising over 90% of all individuals
occurring in a soybean field (Dietz et al. 1976). Other Nabidae that occur in
soybean in the region are N. alternatus Parshley, N. americoferus Carayon, N.
capsiformis Germar, and N. deceptivus Harris. Adults and nymphs both genus
feed on eggs and almost any soft-bodied arthropod. The arthropod diet is
supplemented by some feeding on plants which improves survival and decreases
developmental time (Naranjo and Stimac, 1985), but the plants suffer no damage
(Ridgway and Jones, 1968). Damsel bugs and bigeyed bugs are important
predators of many economically important soybean pests. Bigeyed bugs are
predators of Anticarsia gemmatalis Hubner (Elvin et al., 1983), Nezara viri-
dula (L.) (Crocker and Whitcomb 1980), Heliothis zea Boddie (Whitcomb and Bell
1964), H. virescens, (F.) (McDaniel and Sterling 1979), and Pseudoplusia
includes (Walker) (Richman et al. 1980). Damsel bugs are important predators
of Anticarsia gemmatalis Hubner (Buschman et al. 1977), Heliothis spp.
(McCarty et al. 1980), and Plathypena scabra (F.) (Sloderbeck and Yeargan
The population dynamics of bigeyed bugs and damsel bugs in conventionally
tilled soybean fields has been elucidated for the southern U.S. and other
growing regions. Bigeyed bug 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). In Florida and Alabama bigeyed bugs were present during
most of the growing season, with three complete and additional partial gener-
ations being typical (Funderburk and Mack 1987). The number of bigeyed bugs
increased through the growing season, with numbers usually greatest near the
end of the growing season. Damsel bug numbers were greatest near mid-season
in Kentucky (Raney and Yeargan, 1977), Mississippi (Pitre et al., 1978), and
Brazil (Correa et al., 1977) and late in the season in South Carolina (Shepard
et al., 1974b) and North Carolina (Deitz et al., 1976). Two to four complete
and additional partial generations appear typical in soybean, with the number
of generations developing in individual fields dependant on the date of first
appearance of adults (Mack and Funderburk 1987).
Much research obviously has demonstrated that bigeyed bugs and damsel bugs
are abundant, indigenous naturalenemies, with their numbers increasing along
with the pests and reaching greatest numbers during mid- or late-season.
Enhancement and conservation of beneficial predators is a major priority in
soybean IPM programs. For example, insecticides used to control the pests may
also kill the beneficial predators. If this happens the pest frequently rein-
vades the field at a faster rate than the beneficial. Pest numbers then
build up rapidly because there are few beneficial predators left to hold down
the pest populations. IPM strategies in soybean therefore are designed to use
insecticides only when pest populations reach economic numbers and to select-
ively use insecticides in ways that reduce pest populations, but have the
least negative impact on populations of these predators.
Tillage operations modify soil habitats where many pest and natural
enemies reside during at least part of their life cycle. These modifications
can alter survival or development. Herzog and Funderburk (1986) discussed the
ways that tillage influences the biology of specific pests and natural
enemies. They concluded from a review of the literature that most soybean
pests and their natural enemies whose populations have been quantified in
tillage studies are affected by soil tillage practices. Numerous agronomic-
ally acceptable tillage and subsoiling regimens have been developed for
soybean production systems in the southeastern U.S. A knowledge of the
effects of these tillage and subsoiling regimes on pest and natural enemy
biology would aid development of cultural control strategies in soybean IPM
programs. It may be possible to select tillage/subsoiling production systems
that optimize the benefits of biological control by bigeyed bugs and damsel
bugs, thereby minimizing the need for insecticidal control.
Damsel bugs overwinter in grassy ground covers, and during the summer
prefer shady habitats such as those found in soybean fields. The bigeyed bugs
overwinter in low-growing grasses and ground trash (Sprenkel, 1983). Conser-
vation tillage systems have an effect on ground cover and ground trash, and
may affect damsel bug and bigeyed bug biology through modification of ground
litter and other ways. In Virginia, McPherson et al. (1982) reported that
Nabis spp. and Geocoris spp. were more numerous in conventional and drill-
planted soybean fields than in the double-cropped, late-planted soybeans
during the bean filling period when most insect-related crop damage occurs.
Troxclair and Boethel (1984) compared damsel bug and bigeyed bug populations
in preplant-disked and no-till soybean tillage regimes, but results were
inconclusive. Poor precision was undoubtedly a problem due to the choice of
sweep-netting as the sampling method (e.g., Irwin and Shepard, 1980). They
emphasized the importance of understanding the effects of tillage on bigeyed
bug and damsel bug populations and urged further research. No other research
has been conducted to elucidate the effects of tillage and subsoiling on
bigeyed bug and damsel bug population dynamics.
Musick (1985) and Herzog and Funderburk (1986) have concluded that each
crop and pest situation must be evaluated individually and independent judg-
ments made for each specific geographical location. Therefore, the primary
purpose of the present study was to determine the effect of tillage and sub-
soiling on the population dynamics and population sizes of bigeyed bugs and
damsel bugs in the subsequent soybean crop. Such information will allow for
implementation of pest management practices that conserve the natural enemies
in soybean fields produced in a wheat/soybean sequential-cropping system in
the southeastern U.S.
MATERIALS AND METHODS
Cobb soybean [Glycine max (L.) Merr.] was planted July 3, 1985 and Kirby
soybean was planted June 12, 1986 following wheat harvest each year on a
Norfolk sandy loam (fine-loamy siliceous, thermal Typic Paleudult) at Quincy,
FL. The divergent planting dates were used to determine if population numbers
of bigeyed bugs and damsel bugs were affected by time of planting in Florida
as described by McPherson, et al. (1982). Plot size for each treatment was
7.6 x 15.2 x in 1985 and 7.6 x 24.4 m in 1986. Tillage and subsoil treatments
are described in Table 1.
Table 1. Description of tillage and subsoil treatment and equipment used.
Code Name Equipment Used
DT Disk Tillage* Disk + Cone planter**
DTSS DT plus in subsoilt(SS) Disk + Cone planter
NT No Till Cone Planter
NTSS NT plus SS Cone planter + Subsoiler
*Gang disk in two directions.
**Almaco 2-row cone planter.
tSubsoiling with chisel plow at depth of 0.23 m.
Soybean planting rate was 45 kg ha1. Rainfall in 1985 required no
soybean irrigation. In 1986, the field received a preplant irrigation of 10
mm water ha Subsequent irrigations were administered when tensiometers
placed at 0.15 m depth reached 0.2 Mg Pa.
Herbicide treatment in 1985 consisted of broadcast sprayed Poast 2-[ethox-
yimino)butyl]-5-[2-ethylth-3-hydroxy-2-cyclohexen-l-one at 0.584 1 ha~- + Bas-
agran3-( 1-Methyl-ethyl)-(H-2,1,3-benzothiadiazin-4 (3H)-one2,2-dioxide at 1.753
1 ha-1 + Paraquat 1,1'-Dimethyl-4,4'- bipyridiniumion; present as the dichlor-
ide salt (ICI/Chevron/Crystal) or dimethyl sulfate salt at 1.169 1 ha- + crop
oil at 2.378 1 ha- on July 31. A second herbicide treatment of Paraquat at
0.584 1 ha-1 + 2,4-DB 4-(2,4-Dichlorophenoxy)butyric acid at 1.169 1 ha~1 + X-
77 at 0.227 1 per 379 1 of solution was applied on Sept. 5.
The initial 1986 herbicide treatment was a tank mix broadcast sprayed
application of Sencor DF 4-Amino-6-(l,l- dimethylethyl)-3-(methylthio)-1,2,4-
triazin-5(4H)-one at 0.56 kg ha1 + Surflan 3,5- Dinitro-N4 ,N4-dipropylsul-
fail-amide at 1.753 1 ha~- + paraquat at 1.753 1 ha-1 + X-77 at 0.227 1 per
379 1 of solution. A second herbicide treatment was a directed spray of Fusi-
lade butyl(RS)-2-[ 4-[ 5-( tri-fluoromethyl) -2-pyuridinyl] oxy]phenoxy]propanoate
at 3.507 1 ha1 + X-77 at 0.229 1 per 400 1 of solution applied on July 22.
The fields were not treated with an insecticide.
Nymphal and adult population density was estimated at 6 dates during the
1985 season (8-23, 9-4, 9-16, 9-28, 10-9, 11-6) and at 5 dates during the 1986
season (7-3, 7-15, 7-29, 8-12, 8-28). The sixth sampling date in 1986 was
discontinued because of excessive lodging caused by heavy rains and high
winds. Sampling was begun at early vegetative stage and continued to the late
seed stage of crop growth.
Sampling procedures were according to those established in a review of
previous studies, Irwin and Shepard (1980), as being most appropriate for
estimating their nymphal and adult populations in soybean. The ground cloth
method was employed, 1.8-m samples were taken in each plot on each sample
date. 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 and damsel bugs.
The experimental field design was a completely randomized block with four
replications. The statistical analysis was a split plot with date as the
whole plot and treatments (tillage and subsoiling) as the subplots. Conserva-
tive degrees of freedom were used in the analyses as described by Weiner
(1971), because the effect of date was a repeated measure.
RESULTS AND DISCUSSION
Population dynamics of bigeyed bug are shown for each tillage/subsoiling
treatment in 1985 and 1986 (Fig. 1). Bigeyed bug populations of a partial
generation and then two complete generations occurred in the soybean plots in
1985 and populations of different generations did not overlap greatly. A few
adults of a partial generation were detected on the first sample date of 1985
during soybean growth stage V4 indicating that adult populations undoubtedly
had been greater earlier in the season. Nymphal populations declined and
adults of this generation were greatest during soybean growth stage R1.
Nymphal populations of the next bigeyed bug generation then occurred in the
soybean plots with sample estimates greatest during soybean growth stage R5.5.
This generation had sufficient time to complete development, because nymphal
population estimates became very low and adult population estimates were
greatest during soybean growth stage R7.
Population dynamics in the 1986 disk tillage with no subsoiling (DT), disk
tillage with subsoiling (DTSS), and no-till with no subsoiling (NT) treatments
were similar to the 1985 population dynamics of bigeyed bugs in all treatments
(Fig. 1) with considerable overlap between bigeyed bug generations. However,
bigeyed bug populations appeared later in the season in the no tillage with
subsoiling (NTSS) treatment, and population estimates were much lower (Fig.
1). Only one, and possibly two, partial generations occurred. Adults of a
partial generation were evident during soybean growth stages V10 to V11.
Nymphal populations soon developed, with population estimates increasing
greatly from soybean growth stages V10 to V11. Sample estimates of adult
populations declined somewhat from V10 to Vll and R2, but not greatly. Adults
from the next generation then occurred, and sample estimates of adults
increased in two of the treatments (DT and DTSS) during soybean growth stage
R4. Nymphal populations of the next generation also were present in the
soybean plots during soybean growth stage R4. This last generation undoubted-
ly completed development in the soybean plots over the last part of the
growing season when sample estimates were not obtained.
Population size was affected by tillage/subsoiling treatment. Treatment
differences were significant in 1985 and 1986 for nymphs (F1,1, = 3.92 and
F,15 = 6.71, respectively; P < 0.05 and 0.01, respectively). The orthogonal
treatment comparisons revealed that the treatment differences were mostly due
to the tillage practice employed. The DT and DTSS plots had greater bigeyed
bug numbers than the NT and NTSS plots in 1985 and 1986 (F,18 = 7.27 and
F1,1s = 17.2, respectively; P < 0.05 and 0.01, respectively). Subsoiling did
not substantially influence bigeyed bug nymphal populations. Numbers were
statistically similar in the NTSS and NT plots in 1985 (F ,1s = 2.10), but the
difference in 1986 approached the 0.05 level of significance (F1,18 = 4.38).
Numbers were very similar in the DTSS and the DT plots in 1985 and 1986 (F
= 0.30 and F1, s = 0.86, respectively). There was no date*treatment inter-
action in 1985 or 1986 (F,8 = 1.43 and F = 1.55, respectively).
Adult population estimates in 1985 usually were greater in the DT and DTSS
plots than in the NT and NTSS plots. However, there were no significant
treatment differences (F1,18 = 1.34). Treatment differences of adult popula-
tion size just failed to be significant in 1986 (F 1 = 2.59; P = 0.06).
Orthogonal comparisons revealed that adult treatment differences, as with
nymphal populations, were mostly attributable to tillage practice. The DT
plots had greater numbers bigeyed bug nymphs than the NT plots (F1, 1 = 4.25;
approaches 0.05 level of significance). Subsoiling did not influence adult
populations of bigeyed bugs. Numbers were similar in the NTSS and NT plots
(F 1,s = 0.03) and in the DTSS and the DT plots (Fi,15 = 3.55). There was no
date*treatment interaction for adults in 1985 or 1986 (F 0.60 and F
= 1.05, respectively).
Population dynamics of damsel bugs were very similar in each tillage/sub-
soiling treatment in 1985 and 1986 (Fig. 2). One complete and a partial
generation developed in the soybean plots in 1985. There was little overlap
in populations of different generations. Adults of a partial generation were
present between soybean growth stages V4 to R4, with population estimates
greatest during soybean growth stage R1. Nymphal populations developed and
were greatest during soybean growth stage R5.5. This generation completed
development, and nymphal population estimates were very small and adult popu-
lation estimates large during soybean growth stage R7. In 1986, one complete
and a partial generation developed in each tillage/subsoiling treatment.
Adults of a partial generation were present between soybean growth stages V4
and V11. Nymphal population estimates were substantial during soybean growth
stage R2 and became greater during soybean growth stage R4. This generation
probably completed development during the remainder of the growing season when
sampling was discontinued. Adults of this generation were present on samples
during soybean growth stage R4. It is probable that sufficient time was
present for another partial damsel bug generation to be present in the soybean
plots during the growing season, but no data was collected.
Population size of adult damsel bugs was significantly affected by
tillage/subsoiling treatment in 1985 (F1,1 = 6.13, P < 0.05). Orthogonal
treatment comparisons revealed that the NT plots had fewer adults than plots
of all other treatments (FI18 = 10.3, P < 0.01). Adult numbers in the NTSS
plots were similar to numbers in plots of DT and DTSS treatments (F1,1 =
1.79), which also had similar population sizes (F Ig1 = 0.34). There were no
significant treatment differences in nymphal populations in 1985 (F,18 =
2.00), but population size was much less in the NT plots than in plots of the
other treatments during soybean growth stage R5.5 when nymphal population
estimates during the growing season were greatest. Nymphal population esti-
mates in all treatments were very low on nearly all other sample dates. Adult
and nymphal population estimates in 1986 were similar in each tillage/subsoil-
ing treatment (FI, 1 = 0.42 and 0.28, respectively). The date*treatment
interaction was not significant for nymphs or adults in 1985 (Fs,18 = 1.79 and
0.82, respectively) and 1986 (F 4,1 = 0.76 and 0.46, respectively).
Tillage and subsoiling practices in our study did not greatly affect popu-
lation dynamics of bigeyed bugs and damsel bugs. Population trends were
always similar for damsel bugs. One complete and an additional partial gener-
ation occurred in each tillage/subsoiling treatment in both years. Two com-
plete and additional partial generations of bigeyed bugs were typical in the
soybean plots, although only one complete generation apparently occurred in
one treatment in one year. Two to four complete and additional partial gener-
ations of damsel bugs and three to four complete and additional partial gener-
ations of bigeyed bugs are typical for full-season soybean in this region
(Mack and Funderburk 1987, Funderburk and Mack 1987). Our data would indicate
that fewer generations of both occur in soybean doublecropped with winter
wheat. The growing season is shorter, leaving less time for more generations
Tillage affected the number of bigeyed bugs in soybean in our studies.
Population size through the growing season was less in no tillage than in disk
tillage soybean. Tillage had less affect on damsel bug populations. No
tillage soybeans subsoiling in one year had lower population size than DT,
DTSS and NTSS soybean, but in the other year population size was similar in
all treatments. These findings for bigeyed bugs and damsel bugs corroborate
previous investigations by McPherson et al. (1982) and Troxclair and Boethel
(1984) and provide strong evidence that tillage practices affect populations
of these important indigenous predators in soybean. McPherson et al. (1982)
hypothesized that differences in bigeyed bug numbers between disk tillage and
no tillage soybean in their studies were related to differences in planting
dates. Planting date was the same for each treatment in our experiments. The
planting date between years was very different. Consequently, tillage, and
not planting date, was responsible for the treatment differences in our
studies. Subsoiling did not affect bigeyed bug numbers. Subsoiling did
affect damsel bug populations, with subsoiling increasing population size in
instances where their populations were reduced in no tillage soybean.
Our studies show that tillage is a production practice which can be modi-
fied in integrated pest management programs of soybean to enhance populations
of the most common above-ground predators of foliar pests. It should be safe
to assume that the amount of predation is a function of predator population
size. If so, biological control by these most important above-ground
predators would be greater in soybean produced by disk tillage than by no
tillage. There is not an adequate understanding of the effects of tillage and
subsoiling on pest populations. These production practices could affect the
biologies of pests in ways unrelated to effects on bigeyed bugs and damsel
bugs. Such an understanding is especially needed for soybean produced in the
southeastern U.S., where pest losses are great. Modification of tillage and
other production practices should prove an efficient way of increasing the
amount of biological control to soybean pests and of alleviating the need for
other, less desirable control tactics.
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V4 RI R4 R5 R5,5 R7 V4 VIOVII R2 R4
2_5`.` '--.. __ I I i-- "-'- ...U
255 275 295 310 184 204 224 240
1985- DAY OF YEAR -1986
Number of bigeyed bugs in relation to day of year (Days Julian for
1985 and 1986) and physiological stage of development. Solid line
- disc tillage, dashed line no tillage, O subsoiled, A not
V4 RI R4 R5R5.5
1985 DAY OF Y
Number of damsel bugs in relation to day of year and physiological
stage of development.
V4 VIO VII