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Soybean Insect Pest Population Dynamics
in Relation to Preplant Tillage
J. E. Funderburk, D. L. Wright, and I. D. Teare
Central Science
Library
APR 27 1989
University of Florida
J. E. Funderburk, Entomology and Nematology Dep.; and D. L. Wright and I. D.
Teare, Agronomy Dep.; North Florida Research and Education Center. Contribu-
tion from the Institute of Food and Agricultural Sciences, University of
Florida, Route 3 Box 4370, Quincy, FL 32351. Research Report NF-89-5.
ABSTRACT
Tillage operations can modify soil and plant characteristics where many
insects reside during at least part of their life cycle. However, information
relating tillage and subsoiling to soybean insect pests is very scarce or
inconclusive. The objective of this study was to determine the effect of
tillage and subsoiling on population cycles and population densities of
soybean insect pests in soybean [Glycine max (L.) Merr.]. Soybean were grown
on a Norfolk sandy loam soil (fine-loamy siliceous, thermal Typic Paleudult)
and tillage treatments were disking to a depth of 0.15 m; disk plus subsoil to
a depth of 0.23 m; no till; and no till plus subsoil. Planting occurred
immediately after disking or in conjunction with no tillage with a 2-row cone
planter at a depth of 25 mm. Row width was 0.76 m.- Plots were sampled for
insects by beating the soybean plants on both sides of the row into a 0.9 m
square ground. cloth. Population densities of velvetbean caterpillar (VBC),
Anticarsia gemmatalis Hubner, increased at R4 and peaked at soybean growth
stage R5.5; green cloverworm (GCW), Plathypena scabra (Fabricius) in 1985 and
1986 were greatest at soybean growth stage R1 and R2.6, respectively, but
remained low in 1987. The population densities and cycles of southern green
stinkbug (SGSB), Nezara viridula (Linnaeus), were statistically similar for
each preplant tillage treatment in 1985, 1986, and 1987 for nymphs and adults.
Since preplant tillage treatments caused no gross effects on the population
dynamics of VBC, GCW, and SGSB; the modification of preplant tillage is a way
to increase insect predators and to optimize the benefits of biological
control without concomitant increase in these insect pests.
INTRODUCTION
Tillage and subsoiling effects upon soybean yield and soil resistance
(Brown et al., 1988) have been reported for the Southeast, but the outcome of
those cultural practices on soybean insect pests has only begun to be
documented. Tillage operations modify soil habitats where many insect pest
(Troxclair et. al., 1984) and natural enemies (beneficial insects; McPherson
et. al., 1982; and Funderburk et. al., 1988) reside during at least part of
their life cycle. These modifications can alter survival or development of
insects (Herzog and Funderburk, 1986). Troxclair et al., (1984) monitored
numerous pests and natural enemies by sweep nets for two years in conventional
and no till soybeans planted in narrow and wide row spacing at three locations
in Louisiana. At the three locations, the banded cucumber beetle, Diabrotica
balteata LeCante, was the only insect pest whose numbers consistently
benefitted by conventional tillage.' Southern green stink bugs (SGSB), Nezara
viridula (Linnaeus), were not affected by either tillage while velvetbean
caterpillar (VBC), Anticarsia gemmatalis Hubner, were inconsistently favored
by no tillage.
Funderburk et. al., (1988) measured the effects of tillage and subsoiling
on beneficial insects and found that disk tillage (conventional tillage) had
higher bigeyed bug nymphal and adult populations than no tillage treatments
for over 2 years. Disk tillage also significantly favored damsel bug
populations over no till without subsoiling in 1985.
Musick (1985) and Herzog and Funderburk (1986) concluded that each crop
and pest situation must be evaluated individually and control decisions made
for each specific geographical location. The primary purpose of this study
was to determine the effect of tillage and subsoiling on population cycles and
population densities of larval green cloverworm (GCW), Plathypena scabra
(Fabricius), larval VBC, and nymphal and adult SGSB in a subsequent soybean
crop following winter wheat and to determine if the tillage/subsoiling
practices that increased beneficial (Funderburk et al., 1988) would reduce or
not affect insect pests. Either finding should aid in implementation of
cultural control strategies in integrated pest management systems in soybean
fields in the southeastern U.S.
MATERIALS AND METHODS
Soybean [Glycine max (L.) Merr.] were grown on a Norfolk sandy loam soil
(fine-loamy siliceous, thermal Typic Paleudult) at Quincy, FL. Treatments
were (1) disk (gang disk in two directions, 0.15 m depth) and plant; (2) disk,
subsoil (chisel plow at a depth of 0.23 m) and plant; (3) no till plant; and
(4) no till, subsoil, and plant. A 2-row cone planter was used to plant the
soybean at 25 mm soil depth in all plots. In 1985, 'Cobb' soybean were
planted on 30 July in plots 7.6 x 30.4 m in size. In 1986, the cultivar was
changed to 'Kirby' soybean which was planted 11 June in plots 7.6 x 24.4 m in
size. In 1987, 'Cobb' cultivar soybean were planted in plots (7.6 x 24.4 m)
and fenced on 16 July to exclude the deer that were walking and feeding in
replication 4.
In all three years, the soybean planting rate was 45 kg ha- Irrigations
were scheduled with soil tensiometers placed at 0.15 m soil depth. No irri-
gation was needed or applied to the plots in 1985, but in 1986 and 1987 25 mm
water ha-l was applied preplant and at intervals during the growing season
when tensiometers reached 0.02 MPa.
Herbicide treatment in 1985 consisted of an initial broadcast post emer-
gence application (31 July) of Poast (2-[1-(ethoxyimino)-butyl]-5[2-(ethyl-
thio)-propyl]-3-hydroxy-2-cyclohexen-l-one) at 0.58 L ha-1 + Basagran {3-(1-
Methylethyl)-(1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) at 1.75 L ha~ +
Paraquat {1,1'-Dimethyl-4,4'bipyridinium ion); present as the dichloride salt
(ICI/Chevron/Crystal) or dimethyl sulfate salt at 1.17 L ha1 + crop oil at
2.38 L ha'-. A second post directed herbicide treatment of Paraquat at 0.58 L
ha- + 2,4-DB {4-(2,4-diclorophenoxy) butyric acid) at 1.17 L ha-1 + X-77 at
0.23 L per 380 L of solution was applied on 5 Sept.
The initial 1986 herbicide treatment (13 June) was a tank mix broadcast
application of Sencor DF {4-Amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-
triazin-5(4H)-one) at 0.56 kg ha-1 + Surflan {3,5-Dinitro-N4 ,N4-dipropylsul-
fanilamide) at 1.75 L ha- + Paraquat at 1.75 L ha- + X-77 at 0.23 L per 380
L of solution. A second herbicide treatment for grass control was a broadcast
application of Fusilade {butyl(R S)-2-[4-[5-trifluoromethyl)-2-pyridinyl]oxy]-
phenoxy]propanoate) at 3.51 L ha- + X-77 at 0.23 L per 400 L of solution
applied on 22 July.
In 1987, the first herbicide treatment was a broadcast spray preplant
application of Prowl {N-(l-ethylpropyl)-3,4-dimethyl-2,6-dinitro-benzenamine}
@ 1.75 L ha- + Sencor DF @ 0.42 kg ha- + Poast at 1.16 L ha~ + Paraquat @
0.87 L ha'- + Soydex oil @ 1.16 L ha~ with 197 L water. Paraquat was broad-
cast sprayed on 17 and 24 July @ 0.87 L ha-I with 189 L of water. Insecti-
cides were not applied at any time during the three years.
Nymphal and adult population densities were estimated on 6 dates during
1985 (8-23, 9-4, 9-16, 9-28, 10-9, 11-6), on 5 dates in 1986 (7-3, 7-15, 7-29,
8-12, 8-28), and on 5 dates in 1987 (8-6, 8-19, 8-31, 9-10). 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 (V4)
in all years and continued to the late pod fill stage (R7) of crop growth in
1985, until the middle podding stage (R4) in 1986, and middle plus podding
stage (R5) in 1986.
Insect sampling was carried out as described by Kogan and Pitre (1980).
All plots were sampled on each sampling date by beating the plants on both
sides of the row into a 0.9 m square ground cloth placed between the rows.
Three samples were taken per plot on each sampling date. Also, adjacent plant
bases and the soil surface were visually examined.
The influence of preplant tillage treatment on population densities and
population cycles of VBC larvae, GCW larvae, SGSB nymphs, and SGSB adults were
Evaluated by ANOVA. Data from each growing season were analyzed separately.
The design was a split plot with dates as whole plots and tillage treatment as
subplots. The main effect of tillage treatment compared the influence of
preplant tillage treatment on seasonal population density. Orthogonal
comparisons were used to define treatment differences. The interaction of
date treatment compared the influence of preplant tillage treatment on
seasonal population cycles. Conservative degrees of freedom were used in each
ANOVA as described by Weiner (1971), since the effect of date could not be
randomized.
RESULTS AND DISCUSSION
A description of plant growth in relation to planting date is given to
describe plant height, canopy closure, maturity, and expected yield between
the years of 1985, 1986, and 1987. Insecticides were not applied at any time
during the experiment to allow insect pests and beneficial to multiply
naturally. When this situation is allowed, one cannot expect to harvest a
crop for seed yield in our geographical location. 'Cobb' soybean were planted
on 30 July 1985 because it is a late maturing cultivar in Group VIII,
maximizes the growing season and produces a satisfactory yield when late
planted. 'Kirby' is an early cultivar of Group VIII, was selected and planted
on 11 June 1986 (the optimum time for planting soybean in North Florida). The
expected yield of 'Cobb' when planted on 30 July 85, 'Kirby' when planted on
11 June 86, and 'Cobb' when planted on 16 July 87 based on previous date of
planting research was 0.827, 2.554, and 2.016 Mg ha"-, respectively, in North
Florida (Herzog et al., 1988). The 'Cobb' soybean in 1985 obtained an average
height of 0.46 m and the canopy did not close; the 'Kirby' average height was
0.86 m in 1986 and the canopy closed at approximately R5.8; the 'Cobb' average
height was 0.71 m in 1987 and the canopy closed at approximately R6.0.
. Maturity dates for 'Cobb'-85, 'Kirby'-86, and 'Cobb'-87 were 21 Nov, 1 Nov,
and 21 Nov; respectively.
Seasonal population dynamics (population density and population cycles) of
larval VBC and GCW in the soybean plots are shown in Figure 1. Population
densities of VBC were very low each year until soybean growth stage R4.
Sample estimates were greatest in 1985 and 1987 during soybean growth stage
R5.5. In 1986, VBC density was greatest during soybean growth stage R4,
because sampling was discontinued after R4 for the remainder of the season due
to soybean lodging. Population density estimates of GCW were very low on all
sample dates during 1987. Sample estimates were greatest in 1985 at soybean
growth stage R1 and in 1986 during soybean growth stage R2.6.
Density estimates of VBC (Figure 1) sometimes exceeded economic thresholds
(12 larvae per meter of row) when control procedures are recommended in
production fields (Johnson et al. 1988). VBC estimates in all treatments were
greater than the economic threshold density on sample dates during soybean
growth stages R4 and R5 in 1985 and during soybean growth stages R3, R5, and
R6 in 1987. Population densities of VBC were not economically important on
any sample date in 1986. GCW population estimates in all treatments were
VELVETBEAN CATERPILLAR
V4 R1 R4 R5 R8.6
230 248 260 276 290 306
V4 V10 VI R2. R4
140
1986
120
100-
80
eo
40
.20 :
Cs'
180 190 200 210 220
V4 R2 R3 R5
GREEN CLOVERWORM
R1 R4 R5 R5.8
230 240 180 190 200
R6 V4 R2
210 220 230 240 260 260 270 210
210 220 230 240
R3 R5 R6
220 230 240 250 260 270
DAY OF YEAR
Figure 1. Population density of larval velvetbean caterpillars and green
cloverworms in relation to day of year (Days Julian, 1985, 1986, and 1987) and
physiological stage of soybean development (O represents no till, subsoil;
represents no till, no subsoil; O represents disk, subsoil; and represents
disk, no subsoil).
H.
-J
0U
0
07
1987
8
a6
4
'2
ap
below economic threshold levels (same as VBC) on all sample dates in 1985,
1986, and 1987.
The population densities of VBC (Figure 1) differed between preplant
tillage treatments in 1985 (F, = 14.1; P < 0.001). Orthogonal treatment
comparisons showed that density was significantly less in the no till without
subsoiling treatment than in the other three treatments (F1, = 13.5; P <
0.001). Other orthogonal comparisons were not significantly different (F,1g
.= 2.7). Population cycles differed between preplant tillage treatments in
1985 (Fs,,1 = 10.5; P < 0.001), because estimates were similar on sample dates
when densities were very low and not similar on other sample dates when densi-
ties were greater. The density of VBC was similar in all preplant tillage
treatments in 1986 and 1987 (F1,1 = 0.6 and Fi = 0.2, respectively). Popu-
lation cycles also were similar in 1986 and 1987 (F4,i5 = 0.8 and F4, = 1.2;
respectively). GCW population densities were similar in all preplant tillage
treatments in 1985, 1986, and 1987 (F1,18 = 0.9, F1 1 = 0.2, and F1 = 0.6;
respectively)'. Likewise, population cycles were similar in 1985, 1986, and
1987 (F5,18 = 1.5, F 4,s = 1.8, and F4, = 0.6; respectively).
Seasonal population dynamics of nymphal and adult SGSB in the soybean
plots are shown in Figure 2. Adults typically invade soybean fields and begin
reproducing during soybean growth stages R3 and R4 (Schumann and Todd 1982).
The soybean growth stage during each growing season when adults invaded the
tillage plots ranged from Vll in 1986 to R5.5 in 1985. Nymphs then were
present each year. Population density of nymphs was greatest on the last
sample date in 1985 and 1986. In 1987, population density of nymphs was
greatest during soybean growth stage R4, but population density was less on
the next sample date during R6, because most nymphs developed to adults. SGSB
estimates (nymphs & adults) exceeded the economic threshold (Johnson et al.
NYMPHS
R1 R4 R5 F
d
z
>-
O
z
0
a.
210 220 230 240 260 260
DAY
270 210 220
OF YEAR
230 240 250 260 270
Figure 2. Population density of nymphal and adult southern green stink bugs
in relation to day of year (Days Julian, 1985, 1986, and 1987) and physio-
logical stage of soybean development ( 0 represents no till, subsoil;
Represents no till, no subsoil; [represents disk, subsoil; and represents
disk, no subsoil).
ADULTS
1988) in at least some treatments during R7 in 1985, R2 and R4 in .1986, and R3
and R5 in 1987.
Populations of SGSB adult and early nymphal instars exhibit a clumped
dispersion (Todd and Herzog 1980). Sampling precision when estimating SGSB
density in the experimental plots (Figure 2) therefore was poor on some
individual sample dates, and any differences in estimates on these dates
apparently were the result of random sampling error, rather than preplant
S tillage treatment differences. Overall, the population densities of SGSB were
statistically similar in each preplant tillage treatment in 1985, 1986, and
1987 for nymphs (F ,8 = 0.2, F1,1 = 0.3, and F,8 = 0.8; respectively) and
for adults (F18 = 0.5, F,5 = 0.7, and F,8 = 0.8; respectively). Popu-
lation cycles also were similar in 1985, 1986, and 1987 for adults (fs,18 =
0.9, F4,1 = 0.7, and F48 = 1.0; respectively) and nymphs (Fs,1 0.4, F4,15
= 0.5, and FI = 1.5; respectively).
Preplant tillage practices of soybean in our study had little effect on
population densities or population cycles of VBC, GCW, and SGSB. The signifi-
cant effect on population dynamics of VBC in 1985 may relate to the very late
planting of the soybeans in that year. Plant growth was visibly retarded in
the no-till without subsoiling plots which had the lower population densities
of VBC compared to the other treatments. In 1986 and 1987, no differences in
VBC population density were noted between preplant tillage treatments.
Bigeyed bugs and damsel bugs are important insect predators on VBC, GCW,
and SGSB in soybean in the southeastern U.S. The amount of predation of
insect pests (viz., VBC, GCW, and SGSB) is a function of predator population
density (O'Neil 1984). Population densities of bigeyed bugs and damsel bugs
are enhanced by preplant disk tillage practices (Funderburk et al. 1988,
McPherson et al. 1982, Troxclair and Boethel 1984). Funderburk et al. (1988)
drew their conclusions from data collected in 1985 and 1986 in the same plots
as the present study.
Consequently, our results showing a lack of preplant tillage effects on
pest population dynamics are unexpected, but useful. Since preplant tillage
treatments caused no gross effects on the population dynamics of VBC, GCW, and
SGSB in our study, the modification of preplant tillage practices is useful as
a way to increase insect predators for biological control and should remain an
important consideration for integrated pest management programs of soybean.
Biological control is a desirable control tactic, and integrated pest manage-
ment programs are designed to optimize the benefits of natural biological con-
trol. Enhanced predator populations may provide unknown economic benefits to
pests that reach outbreak population levels less frequently than VBC or SGSB.
Additionally, it may be possible to combine other production modifications
(e.g., resistant cultivars) with preplant tillage practices to achieve reduced
pest injury.
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