Microbial pesticide application during defoliator outbreaks may reduce loss of regional forest beetle richness

Abstract

To date, research on the effects of forest defoliator outbreaks and pesticide applications to suppress these outbreaks on non‐target forest arthropods has generally focused on non‐target Lepidoptera and has not examined effects on regional diversity. In this study, we assessed the effects of forest defoliation by gypsy moths and application of the microbial pesticide Bacillus thuringiensis var. kurstaki (Btk) to suppress gypsy moth populations on the species richness and (Shannon index) diversity of beetles in Shenandoah National Park, Virginia. In addition, we examined how these disturbances affected overall beetle abundance and abundances within different feeding guilds (herbivore, predator, fungivore, or xylophage). Three years after a gypsy moth outbreak, beetles were sampled four times throughout spring and summer at 15 sites belonging to one of three disturbance histories: undisturbed, defoliated, or sprayed with Btk. Beetles were captured using blacklight traps and then sorted to family and morphospecies. We did not detect significant effects of defoliation or Btk on local species richness (α richness) or local diversity (α diversity) of beetles. However, defoliated sites had lower β richness, i.e., lower heterogeneity in beetle species composition among sites, in early summer than sites that were undisturbed or sprayed with Btk. Overall beetle abundance was lower in spring at sites sprayed with Btk, but not significantly different among disturbance histories in other seasons. Further, effects of disturbance history on abundance were not significantly different among feeding guilds. Though the local (α) richness and diversity of beetles were not significantly influenced by gypsy moth defoliation or Btk, the reduced spatial heterogeneity of species composition (lower β richness) in defoliated areas suggests gypsy moth outbreaks could adversely affect regional species richness (γ richness). Our findings additionally suggest that Btk may protect against negative effects of gypsy moth defoliation on the regional species richness of beetles.