Carabid beetle responses to herbicide application, shelterwood seed cut and insect defoliator outbreaks

Managing forests to promote biodiversity requires understanding the effects of silvicultural practices on a range of forest species and communities. We evaluated carabid beetle (Coleoptera; Carabidae) responses to operational herbicide and shelterwood seed cut treatments in northern hardwood stands on the Allegheny National Forest, Pennsylvania, USA, from 1992 to 2000. There was substantial defoliation by elm spanworms (Ennomos subsignarius Hübner) or cherry scallopshell moths (Hydria prunivorata Ferguson) during four of the 9 years of this study, so we also evaluated effects of these lepidopteran outbreaks on carabids. We found no differences in carabid species richness between herbicide-treated and no herbicide plots overall or in any year, but carabid abundance was higher in herbicide-treated plots in the year following application. Carabid community composition differed among years and increased in dissimilarity over the course of the study but did not differ between herbicide-treated and no herbicide plots. Shelterwood seed cuts had no effects on carabid species richness, abundance or community composition. The relatively few significant effects of experimental treatments on individual carabid species tended to be small and responses we did find differed somewhat from previous studies. In 1992, carabid abundance was significantly correlated with elm spanworm defoliation and in 1995 both species richness and abundance were significantly higher in areas defoliated by cherry scallopshell moth. These results support previous findings that forestry practices that have relatively minor and short-term effects on forest vegetation are unlikely to have substantial effects on carabids; however, natural resource variation resulting from forest lepidopteran outbreaks may have important cascading effects for carabid communities that have not been fully explored.

► We studied the effects of forest management on carabid beetles.
► Herbicide treatment and shelterwood seed cut did not affect carabid diversity.
► Lepidoptera outbreaks increased the abundance of several carabid species.
► These results contradict findings from some previous research on forest management.