Direct and indirect effects of the herbicide dicamba on two lepidopteran species

Herbicides are the most commonly applied pesticides in agroecosystems, and therefore pose potentially significant ecotoxicological risks to plants and insects.  Glyphosate is the most commonly used herbicide, and currently 23 weed species worldwide are resistant to glyphosate.  Because of the increasing prevalence of glyphosate-resistant weeds, crops resistant to dicamba or 2,4-D are in development and are expected to be quickly adopted by farmers, altering herbicide-use patterns.  Currently, dicamba and 2,4-D cause substantial herbicide-drift damage to non-target plants despite limited agricultural usage, but the effects of these herbicides on insects are poorly explored.  To understand the influence of dicamba on insects, we applied several drift-level rates of dicamba to soybean, Glycine max L., and Carduus thistle and evaluated growth and survival of Helicoverpa zea (Boddie) and Vanessa cardui (L.) larvae, respectively.  For thistle, we measured percent nitrogen content before and after dicamba application.  We also performed direct toxicity bioassays for each caterpillar species with several rates of dicamba.  Dicamba was not directly toxic to either species, and H. zea showed no negative effects when feeding on soybeans dosed with dicamba.  We did, however, detect negative, indirect effects of higher rates of dicamba on V. cardui larval and pupal mass, total nitrogen of thistles post application, and thistle biomass in the presence of V. cardui larvae.  Notably, thistle biomass was not related to dicamba dose in absence of larvae.  Our results indicate that dicamba can influence the performance of some caterpillar species, and suggest that dicamba may influence insect communities in agroecosystems.