Dicamba drift alters patterns of chewing herbivory in three common agricultural weeds

How agricultural regimes, such as novel herbicide exposure, may influence plant-herbivore interactions and specifically patterns of plant herbivory has come under increased interest in recent years due to rapidly changing herbicide use in agroecosystems. This paper examines patterns of plant herbivory using three common agricultural weeds exposed to low doses of dicamba, a synthetic auxin herbicide that is exponentially increasing in use given the adoption of dicamba tolerant crops. We used a replicated field study to examine how the amount and type of chewing herbivory may be altered in Ipomoea purpurea (common morning glory, Convolvulaceae), Datura stramonium (jimsonweed, Solanaceae), and Abutilon theophrasti (velvetleaf, Malvaceae) exposed to dicamba drift (i.e., 1% of the field dose). We found an increase in chewing herbivory damage when plants were exposed to dicamba and changes in the type of herbivory following exposure. Chewing herbivory differed among species in the presence of dicamba drift: A. theophrasti and D. stramonium showed more total leaf-chewing herbivory than controls, but I. purpurea showed no difference in the overall amount of herbivory. We also found that the type of herbivory was significantly altered in drift. A. theophrasti and I. purpurea both exhibited declines in hole feeding but increases in margin feeding, whereas D. stramonium showed no such changes. Overall, our results show that herbicide drift can induce shifts in plant-herbivore interactions, highlighting the need for mechanistic studies to uncover the cause underlying the shifts and comparative studies on weed communities to understand long-term consequences.