Stink bug pests are released from competition with Helicoverpa zea larvae in transgenic Bt cotton

Background/Question/Methods

Transgenic Bt cotton provides effective management of the caterpillar pests, Helicoverpa zea and Heliothis virescens, in the southeast United States.  However, Bt cotton has not eliminated all pest problems.  The stink bug species, Nezara viridula and Euschistus servus are not susceptible to Bt toxins and have become the most serious pest group of cotton in the southeast US.  Their increase in pest status has been associated with the adoption of Bt cotton in the region.  Understanding the causes of outbreaks of stink bugs will contribute to improved ecological risk assessments of future releases of Bt crops through more accurate predictions of future secondary pest outbreaks.  Yet the causes of such outbreaks remain unclear.

Our goal was to investigate the causes of stink bug outbreaks associated with Bt cotton.  Here, we present results from experiments to test the hypothesis that reduced populations of Bt-susceptible caterpillar populations in Bt cotton causes competitive release of stink bug populations, thereby contributing to outbreaks.  We tested for competitive interactions between the stink bug and caterpillar species on non-Bt cotton, and measured the effects of caterpillar herbivory on stink bug juvenile growth rate, juvenile feeding behavior, and oviposition behavior.

Results/Conclusions

We found strong evidence for competitive interactions, with stink bugs being inferior competitors.  Competition was not associated with resource availability, and depended on the stink bug species, the caterpillar species, and spatial scale.  Our results indicate that induced responses in cotton plants from caterpillar herbivory mediated the competitive interactions found in our experiments.  Competition was strongest for interactions between E. servus and H. zea, with no apparent negative effects from H. virescens herbivory on stink bug performance or behavior.  Our results indicate that stink bug populations experience competitive release in Bt cotton fields, and that release should be greater for E. servus populations than N. viridula populations.  Further research will be valuable in understanding how the competitive interactions described in our study affect stink bug population dynamics, and the importance of competitive release in explaining stink bug outbreaks relative to other agronomic and ecological changes in Bt cotton fields.