Direct and Indirect Effects of the Synthetic-Auxin Herbicide Dicamba on Two Lepidopteran Species

This experiment was designed to evaluate the effects of dicamba on two lepidopteran species (Helicoverpa zea and Vanessa cardui) in the laboratory and greenhouse. Dicamba applications are expected to continue to increase, especially during the growing season, as new crops that are genetically modified to resist the herbicide are released. The authors applied sublethal levels of dicamba to soybean (Glycine max for H. zea – note that the soybean seed was also treated with fungicides and imidacloprid, which could have impacted results) and thistle (Carduus nutans for V. cardui) to approximate the effects of herbicide drift, then measured growth and survival of the two insect species. They also performed direct toxicity bioassays with several rates of dicamba via topical application to the larvae.

H. zea did not show any negative effects from feeding on soybeans treated with dicamba. There were significant negative indirect effects of higher rates of dicamba on V. cardui larval and pupal mass, total nitrogen in thistles post-application, and thistle biomass in the presence of V. cardui larvae for the experiments with smaller thistles which could be explained partly by the inability of individual plants to recover from herbicide damage. Thistle biomass did not change in relation to dicamba dose in the absence of larvae, but at higher doses single caterpillars could consume the majority of the plant. This suggests that some young, slow-growing perennials may not be able to regrow fast enough to compensate for herbivore feeding after dicamba damage. The authors note that many native Asteraceae plant species could be harmed by a combination of dicamba exposure and herbivore pressure. The smaller V. cardui caterpillars and pupae observed on the treated plants could translate into smaller adults and thus have negative consequences for mote competition and butterfly fecundity. However, it is possible that in the field caterpillars would move to another host plant of higher quality, potentially mitigating the effects of sublethal dicamba doses. On larger thistle plants, there was no difference in larval or adult size, possibly because the larger plants satisfied the nutritional requirements of the larvae. This result shows how dependent plant-insect interactions are on the growth stage of the plant. 

The study demonstrated that dicamba can indirectly influence some caterpillar species, possibly by influencing plant nutritional content and food availability. Specialist herbivores that rely on specific plant species may thus be more susceptible to plant-mediated effects than generalists.