Aboveground effects of variation in the legume-Rhizobium mutualism on a generalist insect herbivore

Anthropogenic nitrogen deposition is a significant environmental danger that holds critical consequences for insect populations. One way that insects are threatened by N deposition is through alterations in plant-insect interactions that are mediated by N availability. For example, there is evidence that increased N deposition negatively affects an ecologically important resource mutualism, the legume-rhizobium mutualism, by inciting the evolution of less-cooperative strains of N-fixing rhizobia bacteria. The arising of less-cooperative rhizobia has the potential to influence insects at higher trophic levels, since plant and animal communities depend on the biologically-fixed N that this mutualism provides. This study aimed to evaluate the effect of the evolution of variable mutualist quality rhizobium strains on the nutrition of clover (Trifolium hybridum) tissues and the survival and development of generalist insect herbivore Spodoptera exigua on clover inoculated with high- and low-quality partner rhizobia. We found that inoculation with lower quality rhizobia generally decreased clover leaf tissue nutrition, and S. exigua fed tissues from these plants had decreased survival probabilities. S. exigua fed tissues from plants inoculated with higher quality rhizobia, conversely, had some improved developmental metrics. These results suggest that evolution of variable mutualist quality rhizobia has consequences for higher trophic levels, and that increased N deposition has the potential to disrupt not only plant populations but whole communities via alterations in the legume-rhizobium mutualism.