1. Intraspecific variation in dietary breadth can influence important ecological and evolutionary processes, yet the mechanisms generating this variation are usually unknown. Maternally-transmitted bacterial symbionts frequently infect insect herbivores, and many have been shown to mediate key ecological interactions. For polyphagous herbivores, infection with particular symbionts is often strongly correlated with feeding on particular plant species, suggesting that facultative symbionts might directly determine herbivore food plant specificity. However, previous tests of this hypothesis have returned...