TRANSGENIC PLANTS AND INSECTS
Impact of Transgenic Bacillus thuringiensis Corn and Crop Phenology
on Five Nontarget Arthropods
CLINTON D. PILCHER,1, 2 MARLIN E. RICE,2, 3
AND
JOHN J. OBRYCKI2, 4
Environ. Entomol. 34(5): 1302Ð1316 (2005)
ABSTRACT Large-scale Þeld studies were conducted to determine if temporal plantings of Bacillus
thuringiensis (Berliner) (Bt) corn (event 176 and Bt11) would affect the seasonal abundance of
the following generalist predators: Coleomegilla maculata DeGeer and Cycloneda munda (Say)
(Coleoptera: Coccinellidae), Orius insidiosus (Say) (Heteroptera: Anthocoridae), Chrysoperla carnea
Stephens (Neuroptera: Chrysopidae), and one specialist parasitoid, Macrocentrus cingulum Brischke
(Hymenoptera: Braconidae). Adult populations were monitored using Pherocon AM yellow sticky
traps at three locations in Iowa (1996 Ð1998). At each location, a split-plot design was used with Bt and
non-Bt corn as main plots and three planting dates as the split plots. Few differences in abundance
were observed between Bt and non-Bt corn for the generalist predators studied. However, M. cingulum,
a specialist parasitoid of European corn borer, was signiÞcantly affected by the presence of Bt corn.
Densities of adult M. cingulum were 29 Ð 60% lower in Bt corn compared with non-Bt corn. Regression
analyses indicated M. cingulum adults were preferentially recruited to and subsequently increased over
time in the non-Bt corn treatments at each location within each year. SigniÞcant differences were
observed among planting dates for all Þve species. Abundance effects from Bt corn on these natural
enemies were not unexpected given the foraging and searching behaviors of different species and their
varying levels of dependence on the presence of European corn borer.
KEY WORDS generalists, specialists, nontarget effects, transgenic crops, Cry1Ab
FIELD CORN, Zea mays L., has been genetically engineered (i.e., transgenic) to be resistant to European
corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera:
Crambidae) (Koziel et al. 1993, Armstrong et al. 1995,
Jansens et al. 1997). The crystalline protein (cry) produced by the plant is derived from the bacterium
Bacillus thuringiensis (Berliner) (Bt) (Koziel et al.
1993). Transgenic Bt corn provides high levels of
European corn borer control (85Ð100%, depending on
genetic event), reduced insecticide use and control
costs, season-long expression throughout the whole
plant, reduced plant disease occurrence (Munkvold et
al. 1997), and improved yield protection (Ostlie et al.
1997, Rice and Pilcher 1998).
Natural enemies contribute signiÞcantly to European corn borer mortality (Mason et al. 1996), and
Phoofolo et al. (2001) observed larval parasitism of European corn borer by Macrocentrus cingulum Brischke
(Hymenoptera: Braconidae) (formerly Macrocentrus
grandii Giodanich) to range from 0 to 31%. Four common generalist predators observed attacking European
corn borer eggs and early instars include Coleomegilla
1 Current address: Corn States Hybrid Service LLC, 9820 McWilliams Dr., Johnston, IA 50131.
2 Department of Entomology, 103 Insectary, Iowa State University,
Ames, IA 50011.
3 Corresponding author, e-mail: [email protected].
4 Current address: Department of Entomology, S-255 Ag. Science
Center North, University of Kentucky, Lexington, KY 40546.
maculata DeGeer and Cycloneda munda Say (Coleoptera: Coccinellidae), Orius insidiosus Say (Heteroptera:
Anthorcoridae), and Chrysoperla carnea Stephens
(Neuroptera: Chrysopidae) (Sparks et al. 1966, Andow 1990, Mason et al. 1996, Phoofolo and Obrycki
1997). Previous studies have identiÞed these predators
as important sources of European corn borer egg mortality (Sparks et al. 1966, Frye 1972). However, the
level of control is not spatially or temporally consistent
(Sparks et al. 1966). European corn borer continues to
reach economically damaging population levels despite these natural mortality factors (Mason et al.
1996). Transgenic Bt corn provides the most viable
alternative for growers looking to manage this serious
corn pest (Pilcher et al. 2002).
Many scientists believe that large-scale growth of
transgenic products (e.g., Bt corn) may carry potential
ecological risks to natural enemies (Hokkanen and
Wearing 1994, Jepson et al. 1994, Gould 1998, Hoy et
al. 1998, Way and van Emden 2000, Obrycki et al. 2001,
2004). Potential negative effects include (1) signiÞcant reductions of populations of the target pest;
(2) direct effects of transgenic plantÐproduced toxins
on natural enemies; and (3) negative effects that are
mediated through insect herbivore hosts, e.g., altered
host suitability for growth and development of natural
enemies. Conversely, potential beneÞts of large-scale
transgenic product use could include (1) reduction in
insecticide use, which leads to decreased natural en-
0046-225X/05/1302Ð1316$04.00/0 䉷 2005 Entomological Society of America
October 2005
PILCHER ET AL.: Bt CORN AND NONTARGET ARTHROPODS
emy mortality; (2) increased secondary insect pest
prey/host availability; and (3) an indirect behavior or
physiological impact that would increase a herbivoreÕs
vulnerability to natural enemies. These beneÞts and/or
negative effects would variably apply depending on
whether the natural enemy was a generalist or specialist.
Tritrophic studies to date have indicated that natural enemy interactions with transgenic cultivars vary
from synergism to antagonism (Johnson and Gould
1992, Johnson 1997, Mascarenhas and Luttrell 1997,
Pilcher et al. 1997, Riddick and Barbosa 1998, Hilbeck
et al. 1998a, b, 1999, Schuler et al. 1999a, b, Zwahlen et
al. 2000, Wold et al. 2001, Bernal et al. 2002, Bourguet
et al. 2002, Al-Deeb et al. 2003, Romeis et al. 2004).
Pilcher et al. (1997) found no difference in the abundance of predators in small plots of transgenic Bt corn
compared with non-Bt corn. In addition, Zwahlen et
al. (2000) found no impact of transgenic Bt corn on the
development and survival of Orius majusculus after
feeding on Anaphothrips obscurus, a thysanopteran
pest of corn. Conversely, Wold et al. (2001) detected
a signiÞcant trend for higher densities of C. maculata
in non-Bt sweet corn compared with Bt sweet corn. In
addition, Hilbeck et al. (1999) discovered that C. carnea development and mortality may be impacted if the
Cry1Ab toxin concentration in their host diet exceeded 100 ␮g/g. Romeis et al. (2004) recently refuted
the Þndings of Hilbeck et al. (1999) when they concluded that transgenic maize expressing Cry1Ab
posed a negligible risk to C. carnea. All authors concur,
however, that additional tritrophic level studies are
needed to better understand the compatibility of
transgenic insecticidal plants and their associated natural enemies.
Macrocentrus cingulum was introduced to control
European corn borer in 1927 and continues to be one
of the most important natural enemies of this pest in
North America (Baker et al. 1949, Mason et al. 1996).
Phoofolo et al. (2001) showed variable levels of
parasitism by M. cingulum of European corn borer,
and Onstad et al. (1991) showed larval parasitism by
M. cingulum had no major effect on European corn
borer densities in Illinois. However, this parasitoid
successfully maintains its existence because of its host
synchrony with European corn borer, ability to use
various food resources, polyembryony, and ability to
exploit various distance cues (Udayagiri and Jones
1993, Sked and Calvin 2005). M. cingulum is highly
speciÞc and dependent on the availability of European
corn borer larvae (Mason et al. 1996). Udayagiri and
Jones (1993) determined that M. cingulum exhibited
positive ßight responses to isolated volatiles from corn
plants. Female parasitoids typically use herbivore-induced volatiles released from plants to locate their
insect hosts, a behavior that is critical for successful
parasitism (Turlings et al. 1990). Because of this
plantÐinsect interaction, Schuler et al. (1999a) stated
that the primary route of exposure of natural enemies
to engineered toxins will be through contact with
hosts or prey feeding on plant tissues. Given the dependence of M. cingulum on European corn borer
availability and their attraction to speciÞc herbivore-
1303
induced plant volatiles, it would be expected that this
parasitoidÕs abundance would decline with a decrease
in European corn borer populations.
Much speculation exists on the impact large-scale
planting of Bt corn may have on regional European
corn borer populations and on its natural enemies
(generalists and specialists). Scientists continue to
emphasize the importance of considering behavioral
as well as toxicological aspects when looking at possible effects of transgenic crops on nontarget organisms (Schuler et al. 1999a, Obrycki et al. 2001). Two
studies have evaluated the impact Bt corn and the
herbivore may have on parasitoid development and
behavior (Bernal et al. 2002, Bourguet et al. 2002).
Bernal et al. (2002) showed that a subtropical stem
borer, Eoreuma loftini, receiving sublethal doses from
Bt corn negatively affected some Þtness components
in Parallorhogas pyralophagus, a gregarious external
parasitoid. Bourguet et al. (2002), found increased
parasitism by Lydella thompsoni and Pseudoperichaeta
nigrolineata on non-Bt cornÐfed European corn borer
compared with Bt corn-fed larvae. Both of these studies identify certain indirect toxicological effects of
Bt corn on the natural enemy. In separate studies,
scientists concluded that M. cingulum parasitism rates
of European corn borer did not differ signiÞcantly
between Bt and non-Bt corn (Orr and Landis 1997,
Venditti and Steffey 2002).
Poppy and Sutherland (2004) discussed a threetiered structure for assessing risk of genetically modiÞed crops. The Þrst tier involves laboratory studies
followed by second-tier studies that were conceptually divided into two habitat types, giving the insect
a choice between transgenic or isogenic cultivars.
The Þnal tier involved longer-term large-scale Þeld
studies breaking the insect choices into three different
habitat options including transgenic crop, buffer zone,
and isogenic cultivar (Poppy and Sutherland 2004).
Pilcher et al. (1997) concluded that large-scale Þeld
experiments might improve a researcherÕs ability to
more accurately assess the population dynamics of
Þeld crop-associated insect species. Pilcher and Rice
(2001) observed that phenological growth differences
in Bt and non-Bt corn affected European corn borer
population dynamics. These plantÐpest interactions
may also affect natural enemy population dynamics
(Hogg 1986, Price 1986). The objective of this study
was to determine how large-scale Þeld plantings of Bt
and non-Bt corn planted at different dates might affect
the seasonal abundance of several generalist predators
(C. maculata, C. munda, O. insidiosus, and C. carnea)
and a specialist parasitoid (M. cingulum).
Materials and Methods
This study was conducted at three Iowa State University research farms: (1) Armstrong Research Farm,
near Lewis, in southwestern Iowa; (2) Bruner Farm,
near Ames, in central Iowa; and (3) Northeastern
Research Farm, near Nashua, in northeastern Iowa.
Locations were chosen to represent variations in European corn borer ßight periods during each Euro-
1304
Table 1.
Location
Lewis
Ames
Nashua
a
ENVIRONMENTAL ENTOMOLOGY
Vol. 34, no. 5
Planting dates at three experimental sites in Iowa, 1996 –1998
Planting category
1996
R1a
1997
R1
1998
R1
Early
Middle
Late
Early
Middle
Late
Early
Middle
Late
19 April
19 May
12 June
6 May
21 May
11 June
23 April
7 May
31 May
1 Aug.
4 Aug.
12 Aug.
1 Aug.
6 Aug.
13 Aug.
4 Aug.
4 Aug.
9 Aug.
29 April
12 May
21 May
29 April
14 May
23 May
23 April
7 May
21 May
27 July
28 July
28 July
28 July
28 July
29 July
27 July
28 July
3 Aug.
28 April
11 May
27 May
24 April
6 May
18 May
27 April
11 May
21 May
14 July
23 July
4 Aug.
13 July
20 July
28 July
13 July
24 July
4 Aug.
R1, approximate date that 50% of plot was in R1 stage of development (silking).
pean corn borer generation as well as different hybrid
maturity zones (Pilcher and Rice 2001). At each location, two separate Þeld studies evaluated different
Bt genetic events that varied in efÞcacy (Rice 1997).
One study evaluated event 176 (KnockOut; Syngenta
[Novartis, Ciba] Seeds, Greensboro, NC) using
MAX454 (Cry1Ab) (1996 Ð98) as the transgenic hybrid at each location. The non-Bt hybrids (near isogenic to the Bt hybrid) were 4490 (1996) or 4494
(1997Ð98). The second study evaluated event Bt11
(YieldGard; Syngenta [Novartis, Northrup King],
Minneapolis, MN) using the following hybrids:
N6800Bt (Cry1Ab) and N6800 (near isogenic hybrid)
(1996: all three locations), N7333Bt (Cry1Ab) and
N7333 (near isogenic hybrid) (1997Ð98: southwestern
and central Iowa), and N4640Bt (Cry1Ab) and 4640
(1997Ð98: northeastern Iowa). All hybrids were 110 Ð
112 d maturity hybrids, except N4640, which was a
102Ð106 d hybrid.
A split-plot design was used for each experiment
with corn type (Bt and non-Bt corn) as the main plot
and planting time as the split-plot. Three planting-time
treatments were used: early (20 Ð30 April), middle
(1Ð10 May), and late (11Ð20 May) (Table 1). Each of
the three plantings was sequentially planted at ⬇10-d
intervals and represented one-third of the plot being
planted. Planting dates were as close to a 10-d separation as practical given the differences in planting
conditions each year. Each hybrid by planting date
combination was replicated in four blocks, with planting times randomized within main plots and main plots
randomized within blocks. Main plots (corn type ⫽ Bt
or non-Bt corn) were 72 rows wide (55 m) and 21Ð
30 m long. Each individual plot (six plots per replicated block) represented a different planting date and
was 24 rows (18.3m) wide by 21Ð30 m in length (depending on location). Recently, Prasifka et al. (2005)
recommended plot widths stay in excess of 9 m for
nontarget studies on transgenic crops. Each study,
including alleys and border area, ranged from 1.3 to
1.6 ha. Plots were planted using 0.76-m rows at a seed
rate of 68,400 Ð71,100 seeds/ha. The seeding rate was
consistent across all planting dates within a given year
at each location. Silking and pollen shed (corn growth
stage ⫽ R1) dates varied from a 1-d separation up to
a 22-d difference between the early and late planting
dates (Table 1). Field maintenance including fertilizer, herbicide, and tillage was based on normal prac-
tices for all locations. No insecticides were applied to
any of the Þelds.
Adult natural enemies were sampled during each
European corn borer generation using Pherocon AM
nonbaited yellow sticky traps (Great Lakes Integrated Pest Management, Vestaburg, MI) (Boeve
1992, Udayagiri et al. 1997). Wooden stakes (2.5 by
5.1 by 244 cm), with one trap per stake, were placed
between rows 12 and 13 of each plot equidistant from
the edge and each other (e.g., 30-m plot; stakes placed
at 10 and 20 m). Two sticky traps per plot were used
to collect natural enemies in 1997 and 1998. In 1996,
three sticky traps per plot were used, but it was determined that the sample size could be lowered to two
to maintain a 10% level of sampling precision. Each
trap was folded around the stake and held in place
using two binder clips with the trap face transecting
corn rows at a 90⬚ angle. The height of the trap
matched the top of the plant whorl during vegetative
stages and just above ear height during reproductive
stages. Traps were replaced weekly during each European corn borer generation (3 wk during late June
and early July; 5 wk during late July through August)
and returned to laboratory for analysis.
At each location, the two studies evaluating each
genetic event (KnockOut, event 176 and YieldGard,
event Bt11) were analyzed separately. Within each
year, each location (Lewis, Ames, and Nashua) and
European corn borer generation (Þrst and second)
were analyzed separately because of differences in
European corn borer populations and corn phenological growth patterns at each location. The main plots
(corn types ⫽ Bt and non-Bt corn), split plot (planting
times ⫽ early, middle, and late), and subsplit plots
(sampling date) were subject to a split-split plot analysis of variance (ANOVA) using PROC GLM for unbalanced data sets (SAS Institute 1999).
Hypothesis tests were used to determine whether
the abundance of M. cingulum was signiÞcantly affected by each variable or if a signiÞcant interaction
between variables occurred. The variables included
corn type (Bt and non-Bt corn), planting time (early,
middle, late) and sampling date (weekly during each
European corn borer ßight period). Hypothesis tests
for the following interactions were tested: corn type
by planting date, sampling date by corn type, and
sampling date by planting time. In addition to the
above analyses, all years and locations were combined
October 2005
PILCHER ET AL.: Bt CORN AND NONTARGET ARTHROPODS
1305
Fig. 1. European corn borer mean egg masses per plant for each Bt event (176 and Bt11). Data are divided into planting
date (early, middle, late), European corn borer ßight (Þrst and second), and Bt or non-Bt corn. Data are summarized across
all years (1996Ð1998) and locations (Lewis, Ames, and Nashua).
and subject to an ANOVA to summarize Þndings.
Correlation analyses were used to evaluate relationships between each predatory species and European
corn borer egg mass counts and stalk tunneling
(Pilcher and Rice 2001). SigniÞcance for all tests was
set at P ⫽ 0.05.
In addition, the timing of movement (during the
second European corn borer generation only) into Bt
and non-Bt plots for each location within each year
was evaluated using regression analyses (Zar 1999).
The cumulative total number of adults captured over
time in the Bt (Event 176 and Bt11) and non-Bt plots
was summed across planting dates, and the average
number of adults per trap was graphed. A linear regression was Þtted to the data. If equal numbers of
natural enemy adults were recruited to both corn
types at the same time at a constant rate, the resulting
slope would be 1 with an intercept of 0. If recruitment
into non-Bt plots (y-axis) over Bt plots (x-axis) increased over time, the resulting slope would be ⬎1.0.
If the slope was ⬍1.0, the rate of recruitment into the
Bt plots would have increased as the season progressed. A positive y-intercept would indicate that
adult natural enemy recruitment began in the non-Bt
plots, whereas a negative y-intercept value would indicate that adults Þrst entered the Bt corn plots. Regression analyses were conducted to determine if the
regressions were signiÞcant, and StudentÕs t-test was
used to test the slope and intercept parameters to
determine signiÞcance from zero (both the slope of
the line and y-intercept; JMP 5.1; SAS Institute 2004).
Results
The planting dates and subsequent phenological
growth stages varied greatly among the different locations and years (Table 1). The greatest abundance
of European corn borer eggs was observed in the early
planting during the Þrst ßight and in the latest planting
during the second ßight (Fig. 1). The population dynamics of the four generalist predators reported
herein were highly variable in their relation to ovipositional patterns and stalk injury (Fig. 2) observed
by European corn borer. The specialist parasitoid,
M. cingulum, population dynamics were more consistent and more closely related to abundance of European corn borer in response to corn type (Bt or non-Bt
corn) and planting date.
Coleomegilla maculata. During the Þrst European
corn borer generation, C. maculata adults tended to
prefer the earliest-planted corn. Of 18 possible comparisons, signiÞcantly more beetles were observed in
Þve early-, one middle-, and one late-planted corn
treatment during the Þrst generation. When all years
and locations were combined, a signiÞcantly greater
number of C. maculata were observed in the earlyplanted corn (event Bt11; Table 2). Only one significant difference was observed between Bt and non-Bt
corn, where greater numbers were observed in Bt
corn. When all years and locations were combined for
the Þrst generation, no signiÞcant differences in corn
type (Bt or non-Bt corn) were observed (Table 2).
During the second generation, adult C. maculata
numbers were similar among all planting dates. A few
signiÞcant differences were observed at a few locations, but no trends were identiÞed. Only one significant difference was observed between Bt and non-Bt
corn, where greater numbers were observed in the Bt
corn. Most of the locations provided a signiÞcant sampling date and/or sampling date by planting date interaction; however, no conclusive results were observed. Similar to the Þrst generation, when all years
and locations were combined for the second generation, no signiÞcant differences were observed between Bt and non-Bt corn or planting dates (Table 2).
1306
ENVIRONMENTAL ENTOMOLOGY
Vol. 34, no. 5
Fig. 2. European corn borer mean centimeters tunneling per plant for each Bt event (176 and Bt11). Data are divided
into planting date (early, middle, late), European corn borer generation (Þrst and second), and Bt or non-Bt corn. Data are
summarized across all years (1996Ð1998) and locations (Lewis, Ames, and Nashua).
Table 2. Mean ⴞ SE adult predators per trap analyzed across years and locations for each event by corn type (Bt and non-Bt corn),
European corn borer generation (sampling period), and species
Generationa
First
Second
Planting date
Event 176
Bt
Non-Bt
Event Bt11
SigniÞcanceb (n)
Mean ⫾ SE no. C. maculata per trap
1.4 ⫾ 0.10
(1,904)
1.2 ⫾ 0.10
1.2 ⫾ 0.11
2.1 ⫾ 0.12
(3,025)
2.0 ⫾ 0.11
1.8 ⫾ 0.12
Early
Middle
Late
Early
Middle
Late
1.6 ⫾ 0.12
1.4 ⫾ 0.12
1.4 ⫾ 0.12
2.2 ⫾ 0.13
2.2 ⫾ 0.12
2.1 ⫾ 0.13
Early
Middle
Late
Early
Middle
Late
0.37 ⫾ 0.05
0.19 ⫾ 0.03
0.14 ⫾ 0.03
2.7 ⫾ 0.18
2.3 ⫾ 0.17
1.2 ⫾ 0.12
Early
Middle
Late
Early
Middle
Late
1.0 ⫾ 0.16
0.7 ⫾ 0.10
0.4 ⫾ 0.06
12.2 ⫾ 0.90
11.4 ⫾ 0.63
10.2 ⫾ 0.50
1.1 ⫾ 0.16
0.7 ⫾ 0.08
0.4 ⫾ 0.07
11.5 ⫾ 0.61
10.0 ⫾ 0.49
10.5 ⫾ 0.57
Early
Middle
Late
Early
Middle
Late
0.20 ⫾ 0.03
0.17 ⫾ 0.03
0.16 ⫾ 0.03
1.1 ⫾ 0.09
1.1 ⫾ 0.08
1.0 ⫾ 0.07
0.19 ⫾ 0.03
0.22 ⫾ 0.03
0.13 ⫾ 0.02
1.1 ⫾ 0.07
1.0 ⫾ 0.07
1.0 ⫾ 0.07
Bt
Non-Bt
SigniÞcanceb (n)
1.4 ⫾ 0.11
0.9 ⫾ 0.08
0.9 ⫾ 0.08
2.0 ⫾ 0.11
1.7 ⫾ 0.10
1.8 ⫾ 0.11
1.2 ⫾ 0.10
0.9 ⫾ 0.07
1.0 ⫾ 0.09
1.8 ⫾ 0.11
1.7 ⫾ 0.10
1.8 ⫾ 0.12
b (1,981)
0.19 ⫾ 0.03
0.15 ⫾ 0.02
0.14 ⫾ 0.02
2.2 ⫾ 0.14
2.0 ⫾ 0.14
1.4 ⫾ 0.13
0.24 ⫾ 0.04
0.21 ⫾ 0.03
0.14 ⫾ 0.02
2.1 ⫾ 0.15
1.8 ⫾ 0.14
1.2 ⫾ 0.12
(1,981)
0.5 ⫾ 0.08
0.4 ⫾ 0.08
0.3 ⫾ 0.05
8.6 ⫾ 0.50
8.8 ⫾ 0.47
9.0 ⫾ 0.45
0.5 ⫾ 0.07
0.4 ⫾ 0.05
0.3 ⫾ 0.04
8.8 ⫾ 0.48
9.0 ⫾ 0.55
11.0 ⫾ 0.57
b (1,981)
0.09 ⫾ 0.02
0.09 ⫾ 0.02
0.09 ⫾ 0.02
1.0 ⫾ 0.08
0.8 ⫾ 0.07
0.9 ⫾ 0.08
0.06 ⫾ 0.02
0.08 ⫾ 0.02
0.10 ⫾ 0.02
1.0 ⫾ 0.08
0.9 ⫾ 0.07
1.1 ⫾ 0.07
(1,981)
(3,037)
Mean ⫾ SE no. C. munda per trap
First
Second
0.24 ⫾ 0.04
0.14 ⫾ 0.02
0.17 ⫾ 0.03
2.0 ⫾ 0.13
1.5 ⫾ 0.12
1.2 ⫾ 0.12
b (1,904)
b,c (3,025)
b (3,037)
Mean ⫾ SE no. O. insidiosus per trap
First
Second
b (1,904)
(3,025)
(3,037)
Mean ⫾ SE no. C. carnea per trap
First
Second
a
(1,904)
(3,025)
(3,037)
The Þrst European corn borer generation occurs in June. The second generation occurs in late July through August.
Analyses completed using ANOVA, with signiÞcance set at P ⫽ 0.05. SigniÞcance for the following hypothesis tests are denoted by the
following letter: a ⫽ corn type; b ⫽ planting date; and c ⫽ corn type ⫻ planting date.
N, total no. sticky traps counted for that particular analysis.
b
October 2005
PILCHER ET AL.: Bt CORN AND NONTARGET ARTHROPODS
1307
Table 3. Regression analyses of generalist predator adults captured in Bt and non-Bt corn plots during the second generation on
consecutive sampling dates
Year
Location
Slopea
Slope P value
1996
Ames
Lewis
Nashua
Ames
Lewis
Nashua
Ames
Lewis
Nashua
0.95
0.95
0.77
0.47
0.72
0.34
1.04
1.12
1.28
⬍0.001
0.001
0.007
0.028
0.005
0.006
⬍0.001
0.002
⬍0.001
Ames
Lewis
Nashua
Ames
Lewis
Nashua
Ames
Lewis
Nashua
0.92
0.76
0.71
0.75
0.72
0.85
0.83
0.78
0.96
⬍0.001
0.008
0.003
⬍0.001
⬍0.001
0.002
⬍0.001
⬍0.001
⬍0.001
Ames
Lewis
Nashua
Ames
Lewis
Nashua
Ames
Lewis
Nashua
0.95
1.17
1.02
0.74
0.73
0.96
0.70
1.20
0.99
⬍0.001
0.001
0.009
⬍0.001
⬍0.001
0.003
0.007
⬍0.001
⬍0.001
Ames
Lewis
Nashua
Ames
Lewis
Nashua
Ames
Lewis
Nashua
0.85
0.94
1.28
1.84
1.32
0.61
0.49
1.09
0.90
0.001
0.043
0.007
⬍0.001
0.031
0.013
0.128
0.012
⬍0.001
y-interceptb
y-intercept
P value
R2 value
0.210
⫺0.116
⫺0.050
0.561
0.040
0.519
⫺0.084
⫺0.221
⫺0.148
0.501
0.360
0.804
0.151
0.851
0.005
0.802
0.412
0.554
0.969
0.989
0.773
0.651
0.820
0.876
0.975
0.925
0.966
⫺0.004
⫺0.031
⫺0.038
0.027
0.032
0.224
0.171
⫺0.072
⫺0.024
0.947
0.782
0.777
0.905
0.702
0.797
0.424
0.314
0.909
0.941
0.929
0.995
0.963
0.986
0.929
0.989
0.981
0.985
0.102
⫺0.270
0.560
1.391
0.903
⫺0.243
2.436
0.599
1.513
0.902
0.713
0.372
0.200
0.076
0.845
0.338
0.831
0.348
0.941
0.985
0.982
0.953
0.991
0.911
0.866
0.959
0.986
0.100
0.144
⫺0.294
0.505
⫺0.188
0.558
0.099
0.040
0.103
0.288
0.581
0.302
0.110
0.737
0.342
0.244
0.873
0.159
0.844
0.797
0.987
0.925
0.641
0.818
0.479
0.827
0.982
C. maculata
1997
1998
C. munda
1996
1997
1998
O. insidiosus
1996
1997
1998
C. carnea
1996
1997
1998
The slope indicates the ratio of the adult predators in non-Bt corn relative to Bt corn (non-Bt ⫽ Slope ⫻ Bt corn).
The positive y-intercept indicates that adult predators entered non-Bt corn plots before entering Bt corn plots and a negative no. indicates
an inverse relationship.
a
b
Coleomegilla maculata was not closely associated
with European corn borer egg density compared with
other natural enemies evaluated. Correlation coefÞcients between C. maculata beetle numbers and egg
density range from ⫺0.425 to 0.425 (seven of nine
correlations signiÞcant; P ⬍ 0.01). Across all 3 yr and
locations, the correlation coefÞcient was 0.18 (P ⬍
0.001). Regression analyses provided no clear differential recruitment into Bt or non-Bt corn over time
(Table 3).
Cycloneda munda. Cycloneda munda adults preferred the early-planted corn in 10 of the 36 comparisons from both Þrst- and second-generation comparisons. This observation was not dependent on event
(event 176 or Bt11) or location. Most of the differences occurred in 1998, however. SigniÞcant differences also occurred among sampling dates at 8 of 18
Þrst-generation comparisons and all 18 comparisons
from the second generation. However, as was observed with C. maculata, no conclusive results occurred because of adult ßuctuation through each sampling period. Although a few C. munda were collected
during the Þrst European corn borer generation, numbers were substantially higher during the second generation (Table 2). Transgenic Bt corn did not negatively affect C. munda (Table 2) because adult
populations were of similar densities or larger in Bt
corn. When all years and locations were combined,
C. munda had the greatest number of signiÞcant differences among the predators evaluated in this study
(Table 2). SigniÞcant differences among planting
dates were observed for event 176 during the Þrst and
second generation. In addition, a signiÞcant planting
date by corn type interaction was observed during the
second generation for event 176, which may explain
the regression analyses observations (Table 3). A sig-
1308
ENVIRONMENTAL ENTOMOLOGY
niÞcant planting date interaction also occurred with
Bt11 during the second generation. In all cases, higher
numbers of C. munda were observed in the earlyplanted corn.
When C. munda populations were correlated to
European corn borer egg density, correlation coefÞcients ranged from 0.084 to 0.676 (four of nine were
signiÞcant; P ⬍ 0.01). Across all 3 yr and locations, the
correlation coefÞcient was ⫺0.04 (P ⫽ 0.18). In addition, all nine regression lines for each location within
each year observed a slope ranging from 0.71 to 0.96,
indicating that recruitment rate of C. munda was
greater into Bt corn compared with non-Bt corn over
the second-generation time period (Table 3). The
y-intercept parameter indicating early recruitment
into Bt or non-Bt corn never differed signiÞcantly from
zero (P ⬎ 0.314 in all cases; Table 3).
Orius insidiosus. Mean O. insidiosus numbers were
considerably lower during the Þrst generation (0.5 per
trap) across all locations and years compared with the
second generation (9.5 per trap; Table 2). SigniÞcantly
higher numbers of adult O. insidiosus preferred the
early planting date with both events when analyses
were run across all locations and years during the Þrst
European corn borer generation (Table 2). Results
from the second generation reveal that 10 of the 18
comparisons were signiÞcant for planting date. No
distinct patterns were observed, however, because six
comparisons gave higher numbers in the late-planted
corn, whereas three gave higher numbers in the middle-planted plots. Bt corn did not affect O. insidiosus
when analyses were run across all years and locations
(Table 2).
SigniÞcant correlations occurred between O. insidiosus and European corn borer egg mass numbers
(0.211 ⬍ r ⬍ 0.869, all correlations were signiÞcant,
P ⬍ 0.01), indicating a strong relationship between this
natural enemy and European corn borer egg density.
Across all years and locations, the correlation coefÞcient was 0.18 (P ⬍ 0.001). Regression analyses provided no conclusive results (Table 3). In six of the nine
regressions, the slope was ⬍1.0, indicating preferential
recruitment to Bt corn over time (Table 3). In addition, seven of the nine y-intercept values were positive, indicating initial recruitment into the non-Bt
plots; however, the y-intercept values never differed
signiÞcantly from zero (P ⬎ 0.076 in all cases; Table 3).
Chrysoperla carnea. Chrysoperla carnea numbers
were fairly low during the Þrst generation (0.1 per
trap) compared with the second generation (1.0 per
trap) (Table 2). Few signiÞcant planting date differences were observed. Of 18 possible second-generation comparisons, C. carnea numbers were higher in
Þve early and one late-planted plots. No signiÞcant
differences in C. carnea adult numbers were observed
between Bt and non-Bt corn when all locations and
years were combined for both Þrst and second European corn borer generations (Table 2).
Correlation values between adult C. carnea and European corn borer egg mass numbers ranged from
0.028 to 0.749 (eight of nine were signiÞcant, P ⬍ 0.01).
Across all years and locations, the correlation coefÞ-
Vol. 34, no. 5
cient was 0.24 (P ⬍ 0.001). Regression analyses were
inconsistent in comparing recruitment into Bt and
non-Bt corn (Table 3). The R2 values were considerably lower compared with other predators in this
study, indicating a greater amount of variability and
less consistent movement pattern.
Macrocentrus cingulum. During the Þrst European
corn borer generation, M. cingulum numbers varied
signiÞcantly among planting dates. Eleven of 12 planting date comparisons showed signiÞcant differences
(Table 4). Ten of these comparisons showed signiÞcantly higher numbers in the early planting (Table 4).
Few differences were observed between Bt and
non-Bt corn during the Þrst generation (4 of 12 comparisons). However, M. cingulum adult numbers were
signiÞcantly higher in non-Bt corn in three of those
four comparisons. In 5 of the 12 Þrst-generation comparisons, there was a signiÞcant sampling date by corn
type interaction. In addition, in 10 of the 12 Þrstgeneration comparisons, there was a signiÞcant sampling date by planting date interaction. A decrease in
numbers and separation were evident between Bt
and non-Bt corn as planting dates were delayed (Fig. 3;
Tables 4 and 5). However, no corn type by planting date interaction was detected for the Þrst generation when all locations and years were combined
(Table 5). A signiÞcant decrease in M. cingulum numbers was evident in Bt corn event 176 when all locations and years were combined for the Þrst generation
(Table 5).
During the second generation, signiÞcant differences among planting dates were observed in 14 of 18
comparisons. Among all 14 comparisons, M. cingulum
numbers were highest in the late-planted corn
(Table 4). SigniÞcant differences also occurred in 10
of 18 comparisons made between Bt and non-Bt corn
(Table 4). Among all 10 comparisons, M. cingulum
numbers were signiÞcantly higher (29Ð60%) in non-Bt
corn. In 15 of 18 comparisons, a signiÞcant sampling
date by corn type interaction was observed (Table 4).
Similarly, in 16 of 18 comparisons, a signiÞcant sampling date by planting date interaction was observed.
Adult M. cingulum responded differently in the second
generation compared with the Þrst generation. During
the second generation, M. cingulum adult abundance
increased in the later-planted corn over time as the
separation in abundance between Bt and non-Bt
corn also increased (Fig. 3). When second-generation
M. cingulum adult numbers were combined across all
locations and years, there were signiÞcant differences
among planting dates and between Bt and non-Bt corn
for both events (Table 5). There was also a signiÞcant
planting date by corn type interaction for both events
(Table 5).
Second-generation European corn borer oviposition increased in the later planting dates, whereas
larval population and plant damage increased in the
non-Bt corn (Figs. 1 and 2). European corn borer
damage in Bt corn was nil in the Þrst generation, but
evident in the second generation in event 176 (Fig. 2).
Correlation coefÞcients comparing M. cingulum adults
to European corn borer egg mass density were highly
Second
First
Second
First
Second
ECBa
Second
First
Second
First
Second
96
97
97
98
98
Yr
96
97
97
98
98
Early
Mid
Late
Early
Mid
Late
Early
Mid
Late
Early
Mid
Late
Early
Mid
Late
DOPb
Early
Mid
Late
Early
Mid
Late
Early
Mid
Late
Early
Mid
Late
Early
Mid
Late
DOPb
Non-Bt
6.6 ⫾ 1.4
5.3 ⫾ 1.0
7.9 ⫾ 1.5
11.8 ⫾ 3.0
16.5 ⫾ 4.0
6.7 ⫾ 1.4
4.7 ⫾ 0.74
7.5 ⫾ 1.3
9.4 ⫾ 1.3
0.8 ⫾ 0.25
0.6 ⫾ 0.11
0.1 ⫾ 0.05
1.2 ⫾ 0.21
2.6 ⫾ 0.40
4.8 ⫾ 0.73
Bt
2.6 ⫾ 0.49
3.2 ⫾ 0.55
5.5 ⫾ 1.2
13.2 ⫾ 3.4
25.7 ⫾ 6.3
8.6 ⫾ 2.3
3.6 ⫾ 0.50
5.0 ⫾ 0.69
4.4 ⫾ 0.58
1.1 ⫾ 0.24
0.4 ⫾ 0.11
0.3 ⫾ 0.08
0.9 ⫾ 0.17
1.2 ⫾ 0.24
2.6 ⫾ 0.42
Lewis
4.2 ⫾ 0.64
5.7 ⫾ 1.0
15.3 ⫾ 2.6
19.3 ⫾ 4.4
14.6 ⫾ 3.4
5.0 ⫾ 0.76
7.6 ⫾ 0.98
7.3 ⫾ 0.98
9.3 ⫾ 1.3
1.0 ⫾ 0.26
0.5 ⫾ 0.18
0.4 ⫾ 0.09
1.7 ⫾ 0.29
3.2 ⫾ 0.51
5.4 ⫾ 0.81
3.1 ⫾ 0.04
4.1 ⫾ 0.71
10.3 ⫾ 1.8
11.4 ⫾ 3.0
11.4 ⫾ 2.6
4.4 ⫾ 0.82
3.4 ⫾ 0.46
4.0 ⫾ 0.66
3.7 ⫾ 0.53
0.8 ⫾ 0.17
0.4 ⫾ 0.13
0.1 ⫾ 0.06
1.1 ⫾ 0.22
1.7 ⫾ 0.24
2.5 ⫾ 0.49
Lewis
Non-Bt
Bt
a,b,d,e,f (285)
b,d,f (190)
a,b,c,d,e,f (332)
a,b,d,e,f (239)
d,f (360)
SigniÞcancec (n)
a,b,c,d,e,f (286)
b,d,f (189)
a,d,e,f (335)
b,d,e,f (238)
b,c,d,e,f (359)
SigniÞcancec (n)
0.7 ⫾ 0.14
0.9 ⫾ 0.18
1.3 ⫾ 0.24
1.1 ⫾ 0.27
0.6 ⫾ 0.15
0.6 ⫾ 0.25
9.4 ⫾ 1.2
9.3 ⫾ 1.3
10.4 ⫾ 1.4
1.0 ⫾ 0.23
0.4 ⫾ 0.13
0.3 ⫾ 0.13
2.0 ⫾ 0.34
3.8 ⫾ 0.57
4.3 ⫾ 0.87
Bt
Ames
Event Bt11
1.5 ⫾ 0.28
3.1 ⫾ 0.67
3.4 ⫾ 0.75
3.4 ⫾ 0.62
1.9 ⫾ 0.36
1.6 ⫾ 0.37
10.4 ⫾ 1.2
11.1 ⫾ 1.3
14.6 ⫾ 1.7
1.4 ⫾ 0.43
1.0 ⫾ 0.32
0.4 ⫾ 0.13
3.2 ⫾ 0.65
5.4 ⫾ 0.84
9.1 ⫾ 2.1
Non-Bt
a,b,d,e,f (288)
b,d,f (142)
a,b,d,e,f (334)
b,d,e,f (238)
b,d,e,f (572)
SigniÞcancec (n)
1.4 ⫾ 0.29
2.2 ⫾ 0.39
1.9 ⫾ 0.31
1.6 ⫾ 0.27
1.1 ⫾ 0.25
0.7 ⫾ 0.23
8.2 ⫾ 1.1
10.8 ⫾ 1.3
14.4 ⫾ 1.9
2.1 ⫾ 0.46
1.3 ⫾ 0.37
0.4 ⫾ 0.15
3.1 ⫾ 0.54
5.0 ⫾ 0.76
10.0 ⫾ 1.7
Non-Bt
Ames
b,c,d,e,f (288)
a,b,d,e (143)
b,c,d,f (333)
a,d,f (240)
d,e,f (576)
SigniÞcancec (n)
Mean ⫾ SE no. M. cingulum per trap
0.7 ⫾ 0.15
1.1 ⫾ 0.28
2.1 ⫾ 0.38
1.6 ⫾ 0.40
0.9 ⫾ 0.27
0.4 ⫾ 0.13
6.8 ⫾ 0.93
7.8 ⫾ 1.0
8.4 ⫾ 1.1
1.3 ⫾ 0.35
0.5 ⫾ 0.13
0.1 ⫾ 0.07
2.3 ⫾ 0.42
2.5 ⫾ 0.41
3.4 ⫾ 0.62
Bt
8.1 ⫾ 1.3
13.5 ⫾ 2.4
16.3 ⫾ 2.6
1.4 ⫾ 0.39
0.8 ⫾ 0.26
1.1 ⫾ 0.26
2.4 ⫾ 0.37
2.2 ⫾ 0.32
2.9 ⫾ 0.41
2.2 ⫾ 0.60
1.1 ⫾ 0.33
0.5 ⫾ 0.17
0.8 ⫾ 0.18
3.1 ⫾ 0.57
3.7 ⫾ 0.54
Bt
4.9 ⫾ 0.86
5.0 ⫾ 0.89
11.8 ⫾ 2.1
1.5 ⫾ 0.55
1.4 ⫾ 0.45
0.9 ⫾ 0.52
3.4 ⫾ 0.48
4.0 ⫾ 0.50
4.6 ⫾ 0.69
2.0 ⫾ 0.60
0.8 ⫾ 0.20
0.1 ⫾ 0.09
3.4 ⫾ 0.54
4.3 ⫾ 0.59
5.6 ⫾ 0.78
Bt
Nashua
11.4 ⫾ 1.9
18.3 ⫾ 2.7
29.4 ⫾ 5.5
2.1 ⫾ 0.44
1.1 ⫾ 0.28
0.9 ⫾ 0.27
2.6 ⫾ 0.38
3.6 ⫾ 0.45
4.4 ⫾ 0.56
7.3 ⫾ 1.7
1.3 ⫾ 0.29
0.2 ⫾ 0.10
2.6 ⫾ 0.41
5.3 ⫾ 0.70
6.1 ⫾ 0.42
Non-Bt
Nashua
3.6 ⫾ 0.70
5.1 ⫾ 0.85
15.4 ⫾ 3.3
2.7 ⫾ 0.81
2.2 ⫾ 0.57
1.2 ⫾ 0.38
5.8 ⫾ 0.74
5.7 ⫾ 0.69
6.1 ⫾ 0.78
4.1 ⫾ 0.89
1.5 ⫾ 0.36
0.3 ⫾ 0.09
3.8 ⫾ 0.56
6.2 ⫾ 0.83
9.3 ⫾ 1.5
Non-Bt
a,b,d,e,f (287)
b,c,d,e,f (143)
a,b,d,e (288)
b,d,f (190)
b,d,e,f (288)
SigniÞcancec (n)
a,b,d,e,f (288)
a,b,d,f (143)
a,d,e (284)
b,d (189)
b,d,f (279)
SigniÞcancec (n)
c
b
The Þrst European corn borer ßight occurs in June. The second ßight occurs in late July through August.
DOP, date of planting.
Analyses completed using a split-split plot ANOVA, with signiÞcance set at P ⫽ 0.05. Main plots were corn type (Bt or non-Bt corn); split plots were planting dates (early, middle, late); subsplit plots were
sampling times (adult numbers recorded on a weekly basis). SigniÞcance for the following hypothesis tests are denoted by the following letter: a ⫽ corn type; b ⫽ planting date; c ⫽ corn type X planting date;
d ⫽ sampling date; e ⫽ sampling date X corn type; and f ⫽ sampling date X planting date.
n, total no. sticky traps counted for that particular analysis.
a
ECBa
Yr
Mean ⫾ SE no. M. cingulum per trap
Event 176
Table 4. Mean ⴞ SE adult Macrocentrus cingulum per sticky trap divided into event by corn type (Bt and non-Bt) comparisons at each location by year and European corn borer flight period (sampling
period)
October 2005
PILCHER ET AL.: Bt CORN AND NONTARGET ARTHROPODS
1309
1310
ENVIRONMENTAL ENTOMOLOGY
Vol. 34, no. 5
Fig. 3. Mean M. cingulum per trap in Bt and non-Bt corn for each sampling date pooled across planting dates at (A) Lewis,
1997, (B) Ames, 1997, (C) Nashua, 1997, (D) Lewis, 1998, (E) Ames, 1998, and (F) Nashua, 1998. Breaks in date lines are
time periods in which samples were not taken.
signiÞcant (0.276 ⬍ r ⬍ 0.812; all P values ⬍0.001).
Across all years and locations, the correlation coefÞcient was 0.32 (P ⬍ 0.001).
All nine regression lines evaluating the parameter
estimates for M. cingulum abundance during the second generation for each year and location signiÞcantly
Þt the data,(R2 ⱖ 0.882; Table 6). The slope varied
from 1.19 to 2.05 and was signiÞcantly ⬎1.0 in all nine
cases (P ⱕ 0.0054), whereas the y-intercept was positive in seven of nine comparisons, although it did not
differ signiÞcantly from zero in any comparisons (P ⬎
0.100; Table 6).
Discussion
Price (1986) discussed that plants can have both
intrinsic (e.g., Bt toxin in corn) and extrinsic (e.g.,
natural enemies) resistance mechanisms that impact
the trophic system and that compatibility is dependent
on whether the intrinsic defense has a positive or
negative impact on each of the trophic levels. These
effects may be direct or indirect in nature, depending
on the interactions among the plant, its affected herbivores, and associated natural enemies. Bt corn has an
intrinsic defense against European corn borer and has
October 2005
PILCHER ET AL.: Bt CORN AND NONTARGET ARTHROPODS
1311
Table 5. Mean ⴞ SE M. cingulum per trap analyzed across years and locations for each event by corn type (Bt and non-Bt) and
European corn borer flight period (sampling period)
Mean ⫾ SE no. of M. cingulum per trap
Flighta
First
Second
Planting date
Early
Middle
Late
Early
Middle
Late
Event 176
Bt
Non-Bt
3.5 ⫾ 0.28
3.1 ⫾ 0.31
1.2 ⫾ 0.09
3.0 ⫾ 0.19
3.6 ⫾ 0.22
5.6 ⫾ 0.38
6.0 ⫾ 0.54
4.2 ⫾ 0.40
1.7 ⫾ 0.21
4.5 ⫾ 0.26
5.7 ⫾ 0.31
9.4 ⫾ 0.61
Event Bt11
SigniÞcanceb (n)
(1,139)a,b
(3,024)a,b,c
Bt
Non-Bt
3.8 ⫾ 0.39
5.9 ⫾ 0.78
2.2 ⫾ 0.24
3.2 ⫾ 0.24
4.4 ⫾ 0.34
5.4 ⫾ 0.39
4.4 ⫾ 0.31
4.3 ⫾ 0.27
1.8 ⫾ 0.18
4.4 ⫾ 0.33
6.3 ⫾ 0.42
9.1 ⫾ 0.72
SigniÞcanceb (n)
(1,145)b
(3,037)a,b,c
a
The Þrst European corn borer ßight occurs in June. The second ßight occurs in late July through August.
Analyses completed using ANOVA, with signiÞcance set at P ⫽ 0.05. SigniÞcance for the following hypothesis tests are denoted by the
following letter: a ⫽ corn type; b ⫽ planting date; and c ⫽ corn type X planting date.
n, total no. sticky traps counted for that particular analysis.
b
a direct negative effect that is toxic to this herbivore.
Bt corn kills 85Ð100% of the larvae within hours of
feeding on plant tissue (Ostlie et al. 1997). We did not
study the toxicological effects of Bt corn on these
natural enemies; however, depending on which Bt
corn event (event 176 or Bt11) is used (Fig. 2), we can
begin to understand the level of exposure to Bt corn
these natural enemies may experience. For example,
M. cingulum has the ability to search for surviving
European corn borer larvae, which occurs to a greater
degree on event 176 Bt corn compared with event Bt11
corn.
Decreasing the target pest populations to minimal
numbers (close to zero) can drastically change existing multitrophic interactions in Þeld corn as we observed with M. cingulum. Shelton et al. (2002) states
that regardless of whether one uses Bt plants, a biological control agent, a resistant plant, an insecticide,
a cultivation technique, or any other method to control a pest, if the pest population is reduced there will
be some impact on the overall biological community.
The primary factor affecting the success of a natural
enemy may be its ability to use multiple insect hosts
when one or more of the primary hosts are removed
or affected by a resistant host plant (i.e., Bt corn). The
primary difference between a specialist and generalist natural enemy is that specialists tend to be
monophagous (single, speciÞc host) and generalists
tend to be polyphagous (multiple food hosts) (Pedigo
2002). In addition, specialist natural enemies tend
to be highly synchronized with the life cycle of
their hosts and generalists are not (Pedigo 2002). The
four predators evaluated in this study (C. maculata,
C. munda, C. carnea, and O. insidiosus) are considered
to be generalists because they consume multiple insect hosts and tend to not be synchronized with the
life cycle of any individual host species. The observed
plantÐinsect interactions that occurred in this study
reßect these generalist behaviors. The combination of
an intrinsic defense mechanism (Bt corn) and differences in crop phenology affected European corn
borer population dynamics (Pilcher and Rice 2001).
The interaction of crop phenology, Bt corn, and
European corn borer populations were predicted to
inßuence natural enemy abundance (extrinsic defense mechanism).
Among the three variables evaluated, predator
abundance was affected to a greater degree by the
different phenological stages of corn development
rather than the presence or absence of European corn
borer and Bt or non-Bt corn. Phenological differences
were dependent on the planting dates, and greater
differences among planting dates were achieved in
1996 and 1998. Separation among planting dates is
usually less apparent during pollen-shed (R1; 3-d sep-
Table 6. Regression analyses of M. cingulum adults captured in Bt and non-Bt corn plots during the second generation on consecutive
sampling dates
Year
Location
Slopea
Slope P value
y-interceptb
y-intercept
P value
R2 value
1996
Ames
Lewis
Nashua
Ames
Lewis
Nashua
Ames
Lewis
Nashua
1.89
1.41
1.39
1.19
2.05
1.28
1.93
1.83
1.63
⬍0.0001
0.0003
0.0002
⬍0.0001
⬍0.0001
0.0023
0.0039
0.0054
0.0001
0.124
0.680
0.354
1.276
⫺0.625
0.516
0.074
0.116
⫺0.102
0.270
0.276
0.354
0.102
0.211
0.528
0.958
0.878
0.848
0.996
0.993
0.999
0.990
0.993
0.923
0.899
0.882
0.981
1997
1998
The slope indicates the ratio of M. cingulum adults in non-Bt corn relative to Bt corn (non-Bt ⫽ Slope ⫻ Bt corn).
The positive y-intercept indicates that adult M. cingulum entered non-Bt corn plots before entering Bt corn plots and a negative no. indicates
an inverse relationship.
a
b
1312
ENVIRONMENTAL ENTOMOLOGY
aration in planting date translates to 1-d separation in
pollen-shed) (Benson and Thompson 1977).
Coleomegilla maculata appeared to be the least affected by the treatment parameters in comparison
with other predators evaluated. Coccinellidae are
known to colonize Þeld corn and feed on aphids,
pollen, or lepidopteran prey including European corn
borer eggs and young larvae (Sparks et al. 1966,
Gordon 1985, Jarvis and Guthrie 1987, Andow 1990).
SpeciÞcally, C. maculata has three to four generations
per year (Obrycki and Tauber 1978), and previous
work has shown populations peaking in density when
second-generation European corn borer eggs and neonates were at their peak (Coll and Bottrell 1991).
However, in our study, this beetle was more closely
associated with differences in corn phenology and was
not greatly inßuenced by European corn borer egg
density or the presence of Bt corn. This suggests that
plant pollen and other plant characteristics may have
a greater inßuence on C. maculata behavior. In support of this, Godfrey et al. (1991) found higher numbers of coccinellids in drought-stressed corn compared with irrigated corn. Conversely, however, Wold
et al. (2001) reported fewer C. maculata adults in Bt
sweet corn compared with non-Bt sweet corn but
suggested further long-term Þeld testing be conducted
to further characterize what may have been subtle
population effects. Additional studies have concluded
no impact of Bacillus thuringiensis on C. maculata
development or Þeld abundance (Giroux et al. 1994a,
b, Pilcher et al. 1997, Riddick et al. 1998, Riddick and
Barbosa 1998). C. maculata populations preferred the
earliest planted or tallest corn in the area early in the
growing season. However, minimal differences were
observed during the second generation, and no direct
conclusions could be drawn. Mason et al. (1996) suggests that C. maculata consumption of European corn
borer egg masses is decreased with the presence of
pollen, which reafÞrms the observed abundance levels
being closely related to phenological growth differences in corn.
Cycloneda munda preferred the early-planted corn
during the Þrst and second European corn borer
generations, which does not follow what was observed by European corn borer oviposition (Pilcher
and Rice 2001). This was also slightly different from
the trends observed by C. maculata during the second
generation. Of the four predators reported in this
study, C. munda seemed to be the most impacted by
the phenological corn growth differences. C. munda
abundance was also 8 Ð10 times higher in the second
generation compared with the Þrst generation across
all locations and years. C. munda abundance was not
greatly affected by the presence of Bt corn. However,
trends were for greater recruitment over time to Bt
corn compared with non-Bt corn during the second
generation. This may be caused by plant health because Bt corn tends to dry down slower than its isogenic counterparts, which may impact aphid presence
or other secondary pests.
Orius insidiosus abundance was not affected by
the presence of Bt and non-Bt corn. However, signiÞcant
Vol. 34, no. 5
differences in numbers were observed among the
different planting dates. O. insidiosus populations
have been found to coincide with peak densities of
second-generation European corn borer egg and neonate populations during the tasseling and silking
stages of corn in Maryland (Coll and Bottrell 1991).
Our results were similar in Iowa. However, peak European corn borer egg density typically occurs in silking corn, and O. insidiosus was still attracted to Bt
silking corn in the absence of high European corn
borer populations. In 1998, European corn borer
populations were low across all three locations where
O. insidiosus populations were high, which may have
been one of the contributing factors to high European
corn borer mortality and low surviving populations.
Reid (1991) determined that as O. insidiosus density
increased, the total percentage of European corn
borer and corn earworm [Helicoverpa zea (Boddie)]
eggs destroyed also increased. Although high correlations with European corn borer egg density occurred, O. insidiosus population dynamics were highly
variable, not following any particular pattern during
the second generation. Other factors may be responsible for determining O. insidiosus preferences besides
European corn borer populations and differences in
phenological stages of corn growth, because pollenshedding corn did not always recruit the largest number of O. insidiosus adults.
Chrysoperla carnea is another generalist predator
that only feeds on European corn borer eggs and
larvae as an immature (Jarvis and Guthrie 1987).
Abundance during the Þrst European corn borer generation was low, but adult populations increased dramatically during the reproductive stages of growth
because adults are known to prefer shedding pollen
during the silking stage (R1) (Jarvis and Guthrie
1987). Pilcher et al. (1997) similarly observed more
chrysopid eggs during pollen shed. Once the R1 stage
was complete, preference seemed to shift back to the
more mature stages of corn growth. C. carnea seemed
to be the least impacted by both corn phenology and
Bt and non-Bt corn. Much attention has been given to
the effects of Bt on the development of C. carnea
(Pilcher et al. 1997, Hilbeck et al. 1998a, b, 1999, Lozzia
et al. 1998, Romeis et al. 2004). Variable results have
been observed. To validate these Þndings, C. carnea
Þeld exposure to Bt corn must be better understood,
especially with new and improved transgenic events
reaching the market place. With Bt corn providing
99 Ð100% control of European corn borer larvae, consumption of Bt-fed larvae will be nonexistent unless
European corn borer larval survival increases because of resistance or additional Bt corn events are
marketed that allow for greater European corn borer
survival. Further studies need to be conducted on
other secondary pests of corn that are not controlled
to the same degree of European corn borer. Differences in C. carnea abundance between Bt and non-Bt
corn were not observed in this Þeld study, which
concurs with observations by Pilcher et al. (1997).
The four generalist predators evaluated tended to
be impacted to a greater extent by differences in crop
October 2005
PILCHER ET AL.: Bt CORN AND NONTARGET ARTHROPODS
phenology than they did by differences between Bt
and non-Bt corn. Recent studies further validated the
minimal impact of Bt corn on generalist predators
(Daly and Buntin 2005, Dively 2005). The four generalist predators reported in this paper are polyphagous and can survive on a diversiÞed diet including
both plant and insect hosts. Our study suggests that Bt
corn will have little impact on the abundance of generalist natural enemies, despite the complete removal
of a signiÞcant host, European corn borer.
Macrocentrus cingulum responded to both crop phenology (planting date) and corn type (Bt or non-Bt
corn). The success of M. cingulum as a specialist
parasitoid of European corn borer is partially caused
by the synchronization of each of their life cycles.
Preferences by European corn borer for different
phenological growth stages of corn are evident in
previous studies (Pilcher and Rice 2001). In this study,
M. cingulum followed similar patterns (Fig. 3), as indicated by the high correlation coefÞcients between
European corn borer egg density and M. cingulum
adult numbers. M. cingulum was more abundant in the
early plantings of corn, similar to the newly emerged
European corn borer adults (Pilcher and Rice 2001).
Conversely, M. cingulum was more abundant in the
late-planted corn later in the growing season, similar
to what was observed with the second European corn
borer ßight (Pilcher and Rice 2001). Onstad et al.
(1991) showed a signiÞcant correlation between the
proportion of larvae parasitized by the braconid
M. grandii (renamed M. cingulum) and the frequency
of stalks infested with European corn borer. Studies by
Onstad et al. (1991) were concluded before the introduction of Bt corn and the possibility of complete
absence of European corn borer plant feeding. The
lack of European corn borer plant feeding would
likely decrease the amount of herbivore-induced plant
volatiles. In this study, the larger plot size and arrangement were likely inadequate to completely prevent
M. cingulum movement into neighboring Bt corn
plots. However, Udayagiri and Jones (1993) suggested
that corn plant volatiles provide searching cues for
M. cingulum in the absence of European corn borer
feeding.
Macrocentrus cingulum abundance was 29Ð60% lower
in Bt corn compared with non-Bt corn. Recruitment
into non-Bt corn during the second generation increased as the densities of European corn borer larvae
and tunneling increased. Egg densities in Bt and
non-Bt corn were similar, but differences in tunneling
between the two corn types were signiÞcant. Stalk
injury and plant damage were substantially lower in Bt
corn versus non-Bt corn. Increased M. cingulum
abundance in non-Bt corn was greatly inßuenced by
either the presence of larvae or plant damage or a
combination of both factors. Bt corn negatively affects
European corn borer larvae; larval survival is very low
and therefore little plant injury occurs. Because of the
lack of host survival in Bt corn, the level of exposure
of M. cingulum to Bt toxin is likely minimal. However,
the absence of larvae and plant injury in Bt corn may
indirectly affect M. cingulum by reducing the geo-
1313
graphical host-searching range that non-Bt corn has
historically provided.
Our data suggest that the lack of larval hosts and
plant injury will decrease the abundance of M. cingulum in Bt corn Þelds, which may lead to lower parasitism rates. Siegfried et al. (2001) observed a slight
decrease in parasitism rates in event 176 corn. However, other scientists have observed no signiÞcant effects on parasitism rates by M. cingulum (Orr and
Landis 1997; Venditti and Steffey 2002). Bourguet et
al. (2002) suggested that Bt corn likely affects tachinid
parasitism of European corn borer by decreasing the
density of its larval stage.
Based on the current level of European corn borer
efÞcacy (⬎99%) of the Bt corn events currently marketed, the likelihood of M. cingulum experiencing negative effects that are mediated through insect herbivore hosts (i.e., European corn borer) is minimal.
These levels of exposure to Bt corn may increase for
specialists, as well as generalists, if European corn
borer larval survival increases because of resistance or
additional Bt corn events are marketed that allow for
greater European corn borer survival.
The population dynamics of M. cingulum appear to
follow those of European corn borer. The likelihood
of European corn borer successfully completing development on commercialized Bt corn events is very
low. Sked (2003) completed a study very similar to
this study and achieved similar results under different
conditions in Pennsylvania. An increased prevalence
of M. cingulum in non-Bt corn Þelds was evident in
both studies. These observations may be useful in the
future design of refuge management strategies for
insect resistance management guidelines. Future studies are needed to determine how the population dynamics of M. cingulum are affected by different refuge
management alternatives and whether effective biological control contribution will increase, stay the
same, or decrease depending on population levels of
European corn borer within different refuge scenarios. If temporal refuge plantings were to be used as the
insect resistance management strategy, populations of
M. cingulum should be greater in the later-planted
non-Bt corn refuge plantings.
Acknowledgments
We thank K. Hadley, D. Davis, K. Sterling, S. Whittemore,
N. Jones, and R. Newman for Þeld and laboratory assistance,
and P. Hinz for statistical advice. We appreciate the help
and guidance from the following research farm managers:
K. Pecinovsky, B. Havlovic, D. Rueber, D. Sterret, and
J. Bahrenfus. This research was supported by Novartis Seeds.
C.D.P. was supported by a research/extension assistantship
from the Department of Entomology, Iowa State University.
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Received for publication 4 April 2005; accepted 29 June 2005.