Competition among agricultural pest insects and its role in pest outbreaks associated with transgenic Bt cotton

Background/Question/Methods

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.

First, we tested if the presence of caterpillars had an effect on stink bug growth rates confined to small scales—a single cotton boll or a single branch with multiple bolls—on non-Bt cotton plants.  Second, we tested the effect of systemic plant compounds induced by caterpillar feeding on stink bug residence and oviposition choice at the plant scale, by spatially separating the caterpillars from the stink bugs.

Results/Conclusions

In the growth rate 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 stink bug species.  In the residence choice experiment, N. viridula nymphs showed no preference between damaged or undamaged plants but E. servus nymphs preferred undamaged plants.  In the oviposition choice experiment, females of both species oviposited more often on undamaged plants than on damaged plants.

Our results indicate that plant-mediated interference competition occurs between stink bugs and caterpillars in non-Bt cotton.  Stink bug oviposition behavior may be more sensitive to caterpillar feeding than other measures of competition.  Competitive release is 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.