Effectively managing outbreaks of defoliating insects requires an understanding of the factors regulating outbreaks, which in turn requires mechanistic models. Classical models of insect defoliators include only density-dependent attacks by natural insect enemies, but variation in host plant quality can also strongly affect defoliator survival. I have combined model fitting with field experiments and observational data collection to identify the mechanisms driving the complex dynamics of the forest defoliator jack pine budworm (Choristoneura pinus). I collected data in outbreaking budworm populations from 2012-2015, recording budworm density, rates of parasitism, tree quality, and climate measurements, as well as conducting parasitoid exclusion experiments. ,I fit mechanistic models to my observational and experimental data on jack pine budworm outbreaks, using non-linear differential equations. The results show that drivers of budworm population dynamics include direct density-dependence, density-dependent parasitoid attacks, and the effects of plant quality. I then embedded my model of larval mortality into an annual model of insect population dynamics and their interactions with jack pine trees and forest fires. This model shows that the influence of plant quality directly affects the period of insect outbreaks, which in turn changes the interval between forest fire events. My results indicate that before we can make predictions about the effects of climate change on fire frequency and forest health, we must understand effects of defoliating insects.