Empirical studies on the ecological causes of agricultural pest outbreaks have focused primarily on two biotic factors—release from natural enemies and changes in host plant quality. Release from competition, on the other hand, has been theorized as a potential cause but never tested.
With the expansion of transgenic Bt cotton cultivation in the southeast US, stink bugs, particularly Nezara viridula and Euschistus servus [Hemiptera: Pentatomidae], have become increasingly serious cotton pests, resulting in continued high insecticide use in cotton. Whereas Bt cotton provides effective control of the target caterpillar pests Helicoverpa zea and Heliothis virescens [Lepidoptera: Noctuidae], it is not toxic to stink bugs. Understanding the ecological 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. Yet hypotheses on the causes remain untested. We aim to test the hypothesis that release from competition with the target pests contributes to stink bug outbreaks.
We first tested for competition between the caterpillars and stink bugs confined to small scales—a single cotton boll or a single branch with multiple bolls—on non-Bt cotton plants. In these experiments, H. zea reduced the average relative growth rates of N. viridula and E. servus, with a stronger effect on E. servus. H. virescens had no effect on growth rates of either species.
We next tested the effect of systemic plant compounds induced by caterpillar feeding on stink bug oviposition choice at the plant scale. We presented a gravid N. viridula female with a cotton plant damaged by an H. zea caterpillar and an undamaged plant; but we restricted her access to only one undamaged leaf from each plant, thereby preventing direct interactions between insects. N. viridula females oviposited significantly more often on undamaged plants than on damaged plants.
Our results suggest that, at small scales, H. zea interferes with stink bug access to cotton bolls and E. servus is more severely affected than N. viridula. Also, N. viridula females are deterred from ovipositing on caterpillar-damaged plants—suggesting that stink bugs may respond negatively to systemic caterpillar-induced plant compounds. Further research will be necessary to determine if competitive release causes stink bug outbreaks in Bt cotton, however, our data indicate that it may be involved.