Submitted to: Journal of Applied Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/28/2011
Publication Date: 9/6/2011
Citation: Zeilinger, A., Olson, D.M., Andow, D. 2011. Competition between stink bug and heliothine caterpillar pests on cotton at within-plant spatial scales. Journal of Applied Ecology. 141:59-70. DOI:10.1111/j.1570-7548.2011.01165x. Interpretive Summary: Outbreaks of secondary pests have become a serious ecological risk associated with transgenic Bt cotton in a number of cotton production areas. In the southeast United States, Bt cotton effectively manages the caterpillar pests Helicoverpa zea and Heliothis virescens. The stink bug species, Nezara viridula and Euschistus servus have recently become serious pest problems associated with Bt cotton, requiring continued insecticide use. Yet the causes of such outbreaks in Bt cotton remain unclear. We tested the hypothesis that reduced caterpillar populations in Bt cotton release stink bug populations from competition and contribute to stink bug outbreaks. We found that large corn earworm larvae reduced growth rates of both stink bug species by 60% when caged on a single cotton boll and reduced growth rates of only brown stink gugs by 36% when caged on a cotton branch with multiple bolls. Tobacco budworm had no effect on stink bug growth rates. Our results indicate that competition occurs between both stink bug species and corn earworm larvae with competitive effects more intense for the brown stink bug. This study is the first published empirical test of competitive release as a cause of secondary pest outbreaks, and may be used to improve predictions and management of current and future such pest outbreaks.
Technical Abstract: We investigated competition between Heliothine larvae and the secondary pests, southern green and brown stink bugs at the single boll and multiple boll scales; if competition does not occur with this very close association, it might be unlikely at larger scales with less close association. In both single and multiple boll experiment, we randomly selected equal-aged, morphologically similar bolls along a branch that is next to the main stem. We placed one young nymph of either southern green or brown stink bug in a fine-mesh netting cage with or without one caterpillar of either corn earworm or tobacco budworm, on known age bolls attached to the plant. Two different caterpillar stages were used to vary the competitive pressure on the cotton boll—either a 2nd instar (3 d old) or a 4th instar (7-8 d old). Caterpillar-only cages were also established as control. After three days, we collected the insects, and cotton bolls from the field, starved all insects for six hr, and measured final mass. Relative growth rates (RGRs) for each individual insect were calculated. We show that the stink bug species are inferior competitors, and that competition is species-specific, varying between caterpillar species and stink bug species. Competition was strongest in interactions between the brown stink bug and the corn earworm.