Competition between stink bug and heliothine caterpillar pests on cotton at within-plant spatial scales

Abstract

Outbreaks of non-target pests associated with transgenic Bt cotton threaten the economic and ecological benefits of the technology in cotton-producing countries. In the southeastern USA, stink bug pests, namely Nezara viridula L. and Euschistus servus Say (both Heteroptera: Pentatomidae), have recently become severe problems associated with Bt cotton, requiring continued insecticide use. However, the causes of non-target pest outbreaks remain unclear. Release from competition with Bt-susceptible pest species, in addition to other mechanisms, may contribute to increased stink bug populations in Bt cotton. We investigated the competitive interactions between the two stink bug species and the Bt-susceptible pests Helicoverpa zea (Boddie) and Heliothis virescens Fabricius (both Lepidoptera: Noctuidae) on non-Bt cotton. We tested for competition effects on stink bug growth rates in no-choice experiments at two spatial scales: a single cotton boll and a branch with multiple developing bolls. Although caterpillars of the two species had equivalent effects on resource availability, they had distinct effects on stink bug growth rates. Fourth instar H. zea reduced growth rates of both stink bug species by 60% when caged on a single cotton boll and reduced growth rates of only E. servus by 36% when caged on a cotton branch. In contrast, H. virescens had no effect on stink bug growth rates. Resource competition was apparent in the interactions between H. zea and E. servus, but interference competition may have contributed to the interactions as well. Competitive release of stink bug populations in Bt cotton is possible, and should be more likely for E. servus than for N. viridula. Understanding the causes of non-target pest outbreaks in Bt cotton will contribute to improved environmental risk assessments of future releases of Bt cotton and related transgenic crops.