Climate change is predicted to cause continued increases in global temperatures, greater variability in precipitation and in some cases, more frequent insect pest outbreaks. Here we seek to understand how abiotic and biotic stresses associated with climate change can affect plant-herbivore interactions in a model crop species (soybean, Glycine max (L.) Merr.) by answering three questions: (1) Do the combined effects of abiotic and biotic stresses associated with climate change cause synergistic negative effects on plant biomass? (2) Can abiotic stress affect resistance of plants to insect herbivores? (3) Does genetic variation in plant traits modify a plant's response to stress? We performed three experiments in controlled growth environments using up to 51 soybean genotypes selected to vary in numerous traits associated with drought and resistance against pests (e.g., insect herbivores, nematodes, and pathogenic fungi), and up to 3 generalistfeeding herbivorous noctuid moth species (Helicoverpa zea, Heliothis virescens, and Spodoptera exigua) that commonly feed on soybean in North America. Drought and herbivory had the largest and the most consistent negative effects on plant performance, reducing the above-and below-ground biomass by 10-45 %, whereas increased temperature had little to no effect on plants. Drought also increased susceptibility to generalist noctuid herbivores, but these results varied dramatically in magnitude and direction among plant genotypes. Our experiments show that the effects of abiotic and biotic stress on soybean biomass were largely due to the additive effects of these stresses, and there exists substantial genetic variation in the soybean germplasm pool we studied that could be used as a source of parental stock in breeding new crops that can more effectively tolerate and resist the combined negative effects of insect herbivory and drought.

As the Earth's climate continues to change, drought and insect population outbreaks are predicted to increase in many parts of the world. It is therefore important to understand how changes in such abiotic and biotic stressors might impact agroecosystems. The plant stress hypothesis predicts that, owing to physiological and biochemical changes, plants experiencing drought will be more susceptible to insect herbivory, which could have synergistic negative effects on plant performance. By contrast, the plant vigor hypothesis predicts that insects will preferentially feed on fast‐growing vigorous plants. These hypotheses were tested in a field experiment using 16 soybean (Glycine max (L.) Merr.) genotypes to determine: (i) the combined effects of drought and herbivory on plant performance; (ii) the impact of drought on soybean resistance to herbivores; and (iii) how genetically variable phenotypic traits in soybean correlate with these responses. It was found that drought had a greater effect on soybean performance than herbivory, and drought and herbivory did not interact to impact on any measure of plant performance. Drought caused decreased insect herbivory on average, suggesting that the plant vigor hypothesis is consistent with the effects of drought stress on soybean resistance to leaf‐chewing insect herbivores. This conclusion is further supported by genotypic correlations which show that plant growth rate is positively correlated with the amount of herbivory plants received. These results suggest that, although the effects of climate‐associated changes in drought and herbivory will have negative effects on soybean, these potential effects are quantifiable with simple experiments and can be mitigated through continued breeding of varieties that are tolerant and resistant to these abiotic and biotic stressors.