Sub-lethal doses of herbicide can increase susceptibility to aphid populations

Weed management tactics play an important role in shaping plant communities in agroecosystems, but the role of herbicides in influencing plant-insect interactions remains unclear. In response to the increasingly common development of glyphosate resistant weeds, the agricultural industry is planning to release soybeans resistant to dicamba, a vapor-drift-prone herbicide that could have substantial non-target effects. To understand the risk posed by dicamba drift to arthropod communities, we conducted experiments with soybeans, simulating dicamba drift events into adjacent susceptible soybean fields. Greenhouse and field studies were established in 2009 and 2010 to evaluate aphid and natural enemy response to plants stressed by a range of dicamba doses (0, 0.0056, 0.056, 0.56, 5.6, 56.1 and 177.4 g/ha). In the field, plants dosed with a 0.56 g/ha rate of dicamba experienced the highest level of aphid exposure, while ladybird beetle populations were highly variable across different dose rates. Soybean yield was similar in the drift level dose treatments (< 5.6 g/ha) despite higher aphid pressure. In the greenhouse, aphid populations developed best on plants dosed with 0.0056 g/ha. Plant biomass reduction due to aphid feeding on untreated plants was about 30% while on plants treated with drift level doses biomass reduction was only 12-18% despite the increased aphid exposure. We are also exploring similar experiments with other herbivores. Our results suggest that drift-level doses of dicamba have the potential to alter herbivore populations, and care should be taken to ensure that herbicide drift is minimized to avoid exacerbating pest problems.