Release from interference competition as a mechanism for pest outbreaks associated with transgenic Bt cotton

With the expansion of transgenic Bt cotton cultivation in the southeast US, stink bugs, particularly Nezara viridula and Euschistus servus [Hemiptera: Pentatomidae], have become serious cotton pests, resulting in continued high insecticide use. Whereas Bt cotton provides effective control of the caterpillar pests Helicoverpa zea and Heliothis virescens [Lepidoptera: Noctuidae], it is not toxic to stink bugs. Understanding the causes driving stink bug outbreaks can improve future ecological risk assessments of Bt cotton through more accurate predictions of non-target pest outbreaks and expected changes in insecticide use. However, the causes remain poorly understood. We hypothesized that stink bug populations in Bt cotton experience competitive release from reduced populations of caterpillar pests. We tested the hypothesis with a series of choice and no-choice competition experiments on non-Bt cotton at multiple intra- and inter-plant spatial scales. Our results indicate that plant-mediated interference competition occurs between stink bugs and caterpillars in non-Bt cotton, and that this competition is species-specific. Helicoverpa zea had a strong competitive effect on stink bugs, whereas H. virescens did not. Competition was stronger for E. servus than N. viridula. Stink bug oviposition behavior may be more sensitive to caterpillar feeding than other measures of competition. Competitive release appears to be involved in stink bug outbreaks associated with Bt cotton and it should play a larger role in outbreaks of E. servus than N. viridula. Further research will be necessary to determine to what degree competitive release contributes to stink bug outbreaks.