Direct and indirect effects of the synthetic-auxin 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 common herbicide worldwide, and glyphosate-resistant weeds are quickly becoming serious challenges in some agroecosystems. Because of this resistance epidemic and the recent development of crops with resistance to dicamba or 2,4-D, herbicide-use patterns are likely to change. Presently, dicamba and 2,4-D cause most herbicide-drift damage to nontarget plants despite limited agricultural usage, but the effects of these synthetic auxin herbicides on insects have been poorly explored. To understand the influence of dicamba on insects, we applied several sublethal, drift-level rates of dicamba to soybean, Glycine max L., and Carduus thistle, and measured 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 on the two caterpillar species with several rates of dicamba. Dicamba was not directly toxic to larvae of either species, and H. zea showed no negative effects when feeding on soybeans dosed with dicamba. We did, however, detect significant 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 indirectly influence the performance of some caterpillar species, possibly by altering plant nutritional content.