Insecticide alters the evolution of glyphosate resistance in Ipomoea purpurea

The evolution of plant resistance naturally occurs in complex, multifaceted environments that consist of multiple simultaneous stressors. Understanding how shifting environmental contexts may shape resistance evolution requires empirical studies that consider the combined effects of interacting stressors on fitness and selection. Here, we examined how exposure to an insecticide impacts the evolution of resistance to the herbicide glyphosate in Ipomoea purpurea (common morning glory). Through a factorial field experiment, we manipulated glyphosate and an insecticide to estimate selection on glyphosate and herbivory resistance. We found that glyphosate acted as the primary agent of selection, favoring higher levels of glyphosate resistance. In the presence of glyphosate alone, positive correlational selection favored a combination of higher glyphosate and herbivory resistance, supporting prior work that suggested these traits may be linked. Importantly, insecticide exposure modified both glyphosate resistance and the strength of selection acting upon the trait by increasing resistance and weakening selection. Together, our results indicate that the evolution of herbicide resistance is context-dependent and that secondary stressors like insecticide can alter the evolutionary trajectories of plant defense.