Tolerance to herbicide drift in "weedy" species is context-dependent and driven by early vegetative growth

While plant communities that exist at the edges of agricultural fields are typically composed of 'weedy' species, they provide essential resources for insect pollinators and other wildlife. However, little is known about how sublethal exposures to herbicides via drift (off-target chemical movement by air) affects the fitness of weedy plant species across varying agro-ecological environments. If weed species vary in sensitivity to herbicide drift, then this could result in plant community shifts in favor of more tolerant species. Consequently, these changes in species composition could alter resource availability in agroecosystems. Here, we replicated a common garden experiment in three US states where agriculture is a major form of land use (Michigan, Pennsylvania, and Tennessee). We exposed nine weed species to a sublethal, drift-level rate of the herbicide dicamba and examined plant fitness. We found that fitness in the presence of dicamba drift depended on both the weed species and the environmental context. For some species, dicamba drift significantly reduced fitness. For others, it had no effect, indicating greater tolerance to the herbicide. However, more variation in tolerance to dicamba drift among species was observed in Michigan compared to Pennsylvania or Tennessee, a result that was partially explained by the severity of damage from insect herbivory that occurred at each location. In a follow-up study, we explored the relationships between plant developmental traits and fitness to determine the functional pathways that underlie tolerance to dicamba drift using structural equation modeling. This approach revealed that weedy plants that are better able to buffer the effects of dicamba drift on short-term growth are then better equipped to minimize effects on flower production and ultimately fitness (seed set). Overall, our work advances knowledge about how wild plant populations in different contexts are responding to unintentional, yet increasingly common occurrences of anthropogenic stress in the form of off-target herbicide exposures.