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. There was strong seasonality of FN in field sites with peak levels (4–8% dry weight nitrogen; DW N) recorded early during each year, declining during summer, and slightly increasing again in the fall which coincides with flea beetle activity. Foliar nitrogen and rearing temperature had strong independent effects on larval development rate. High FN increased survival by 40%, decreased developmental time by 15%, and resulted in 11% larger adults. Increasing temperature reduced survival by 43%, shortened developmental time by 28%, and led to 15% smaller adults. In the dispersal experiment we were unable to detect an interaction between FN and conspecific density on larval dispersal, though results suggested that FN may lessen effects at moderate densities. Mean dispersal was more than double in the low versus high FN treatment (51% vs. 23%). Larval density and nitrogen both affected larval weight; high nitrogen plants produced 33% larger larvae over the duration of the experiment and larval fresh weight decreased by 38% from low (one larva plant−1) to high (twenty larvae plant−1) density treatments. 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.