Abstract: Host quality can have dramatic effects on performance of biological control agents but its importance is understudied. We used a combination of field measurements and laboratory experiments to determine the range of foliar nitrogen (FN) that larvae of the alligatorweed flea beetle (Agasicles hygrophila) are exposed to in the field and its importance to larval development and dispersal. Seasonal variability in FN was assessed at field sites spanning southern to northern Louisiana every 2–3 weeks during the growing season for four years. In a series of laboratory experiments, alligatorweed FN was manipulated to examine its influence on larval development and survival (under different temperature regimes), adult biomass, and dispersal of the biological control agent, A. hygrophila. Foliar nitrogen and rearing temperature had strong independent effects on larval development rate. We demonstrated that increasing nitrogen in leaf tissues shortens larval A. hygrophila developmental time and increases survival to adulthood, regardless of exposure temperature during development. It also suggests that foliar nitrogen may have important effects on biological control of alligatorweed, particularly as a result of seasonal variation in temperature and plant nutrition at field sites, and could contribute to observed variation in A. hygrophila efficacy in the field.