Impacts of gypsy moth defoliation and Bacillus thuringiensis application on beetle diversity

Background/Question/Methods

Gypsy moth (Lymantria dispar) defoliations have damaged millions of acres of forest habitat and caused important changes to forest composition. To lessen the adverse impacts of defoliation, a variety of pest control strategies have been employed with varying effects on non-target taxa. In this study, we assessed the effects of gypsy moth defoliation and suppression of gypsy moth outbreaks with pesticides on the species richness, abundance, and diversity of beetles in Shenandoah National Park. Beetles were sampled across 15 sites belonging to one of three defoliation histories: defoliated, defoliated and treated with the microbial pesticide Bacillus Thuringiensis var. kurstaki (Btk), and undefoliated and untreated (control). Beetles were captured using blacklight traps and were sorted to family and morphospecies. Understory plant and tree species richness, abundance, and diversity were measured at each site, along with the abundance of dead wood and mean tree size. 

Results/Conclusions

Beetle species richness and abundance were negatively affected by defoliation. These effects were evident even two years since the last defoliation. Control and Btk-treated sites had a significantly higher abundance and species richness than defoliated sites, suggesting that Btk was somewhat effective at reducing the impacts of defoliation on non-target insects. We found that understory plant and tree species richness were positively correlated with beetle species richness and a non-significant trend for lower understory plant richness at defoliated sites, suggesting that low plant richness was partially responsible for low beetle species richness at defoliated sites. The abundances of beetles in dominant feeding guilds (herbivore, predator, fungivore, and xylophage) did not significantly vary among defoliation histories, with the exception of xylophages which were less abundant at Btk-treated locations. The effect of Btk on xylophage abundance may be explained by indirect interactions between forest Lepidoptera and Coleopteran xylophages. Overall, gypsy moth defoliation had a strongly adverse effect on forest Coleoptera and should be considered a serious threat to forest biodiversity.