This study evaluated the effect of applying foliar insecticides at different timings in soybean fields on western corn rootworm beetle populations and subsequent root damage in rotated corn crops. The experimental design involved applying insecticide early when a nearby cornfield was at the tassel stage, applying late when nearby corn was at the brown silk stage, or leaving untreated control plots. Beetle counts showed no significant differences between early, late and control treatments before or after the insecticide applications. The authors conclude that applying insecticide to soybeans based on growth stages of nearby corn did not impact beetle populations, likely due to beetle movement between fields. This is the first year of a three-year study, with corn crops to

1. Evaluating the Effect of Foliar Insecticides on Populations of Suspected Bt-resistant
Western Corn Rootworm Beetles in Rotated Soybean
Alexandra L. McMillan, Nicholas A. Tinsley, Ronald E. Estes, Michael E. Gray and Joseph L. Spencer
University of Illinois at Urbana-Champaign, Department of Crop Sciences
INTRODUCTION
The western corn rootworm (WCR) is the most economically significant
insect pest of corn production in the U.S.1 and can inflict injury resulting
in substantial yield loss2. Historically, crop rotation was a successful
management strategy because oviposition occurred predominantly in
corn. However, beginning in 1995, severe injury to first-year corn was
observed in Illinois and Indiana3. Commercial Bt corn hybrids targeting
this pest were widely adopted and are an effective tool to manage
rotation-resistant WCR. However, field-evolved resistance to Bt traits has
been documented for WCR4, a phenomenon that has been associated
with the cultivation of continuous corn expressing the same Bt toxin. In
2013, severe injury to rotated Bt corn was documented in Illinois, adding
urgency to existing concerns about Bt resistance. Unexpected injury to Bt
corn hybrids in rotated cornfields and high beetle densities in corn and
soybeans have increased grower interest in adult management tactics.
OBJECTIVE
- To determine if the timing of an insecticide application to soybeans
affects larval damage to rotated corn the following season.
EXPERIMENTAL DESIGN AND METHODS
⁻ The experimental design was a split-plot, randomized complete
block with three replications.
⁻ Within each soybean field the trial area was 219 by 152 m.
⁻ Each trial area was divided into three plots. One plot received an
early insecticide treatment, one plot a late treatment, and one plot
served as the control.
⁻ Densities of WCR beetles were estimated weekly by taking three
100-sweep subsamples per plot.
ACKNOWLEDGEMENTS
We thank the cooperating producers for providing the trial areas to conduct this
experiment; Brian Johnston for coordinating the insecticide applications. This
project was funded by a 2013 Monsanto Corn Rootworm Knowledge Program
award to J.L. Spencer, M.E. Gray, R.E. Estes and N.A. Tinsley.
INSECTICIDE APPLICATION
Warrior II (lamda-cyhalothrin) was applied to the early treatment plots when the
adjacent cornfield began to tassel (VT stage of corn). Warrior II was applied to the
late treatment plots when the adjacent cornfield had brown silks (R3 stage of corn).
RESULTS
The data were analyzed using a square-root transformation and the actual means
are reported. Prior to the early spray when nearby corn was at the VT growth stage,
there was no significant effect of treatment on the mean number of beetles per
100 sweeps (𝐹2,4= 1.00, 𝑃 = 0.45). The densities were low and never exceeded
0.45 per 100 sweeps across all treatments. There was also no significant effect of
treatment during the time period between the early and late sprays on the mean
number of beetles per 100 sweeps (𝐹2,4= 0.25, 𝑃 = 0.79). Densities never
exceeded 29.9 per 100 sweeps across all treatments. During the period after the
late spray, when nearby cornfields were in the R3 stage, there was no significant
effect of treatment on the mean number of beetles per 100 sweeps (𝐹2,4=
3.73, 𝑃 = 0.12).
DISCUSSION
We observed that beetle densities were very low prior to the early application of
insecticide. Between the early and late spray, we expected the untreated check and
the late treatment to have similar densities and the early treatment to have fewer
beetles. We found that spraying early had no effect on the population buildup.
After the late spray, we expected populations in the late treatment to be lower than
the early treatment and the untreated check. However, no significant differences
were observed. This lack of significant differences in beetle density in the early and
late spray treatments is consistent with evidence for significant interfield
movement in WCR5. Intra- and interfield movement of WCR may cause
recolonization of Warrior-treated subplots because of daily interfield movement of
beetles in and out of soybean fields after the effects of Warrior are gone. We
conclude that applying insecticide to an adjacent soybean field based on the
growth stage of the nearby cornfield had no effect on the beetle populations.
FUTURE DIRECTION
This is the first year of a 3-year study. In 2015, corn plots will be placed directly
upon the soybean plots sprayed in 2014. These trials will be used to determine if
the application of a foliar insecticide in soybean has a significant effect on beetle
emergence and root injury in rotated corn.
REFERENCES
1) Gray ME, Sappington TW, Miller NJ, Moeser J, Bohn MO, 2009. Adaptation and invasiveness of western corn rootworm:
intensifying research on a worsening pest. Annu. Rev. Entomol. 54, 303–321.
2) Tinsley NA, Estes RE, Gray ME, 2013. Validation of a nested error component model to estimate damage caused by corn
rootworm larvae. J. Appl. Entomol. 137, 161–169.
3) Levine E, Spencer JL, Isard SA, Onstad DW, Gray ME, 2002. Adaptation of the western corn rootworm to crop rotation:
evolution of a new strain in response to a management practice. Am. Entomol. 48, 94–107.
4) Gassmann AJ, Petzold-Maxwell JL, Keweshan RS, Dunbar MW, 2011. Field-evolved resistance to Bt maize by western corn
rootworm. PLOS ONE 6, E22629.
5) Isard, S.A., J.L. Spencer, M.A. Nasser, and E. Levine. 2000. Aerial movement of western corn rootworm, Diabrotica virgifera
virgifera (Coleoptera: Chrysomelidae): Diel periodicity of flight activity in soybean fields. Environmental Entomology. 29:226-
234.
© Tom Murray