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Increasing temperatures can drive changes in the distribution and abundance of insects. The time of year when warming occurs (e.g., spring vs. autumn) may disparately influence the phenology of herbivorous insects and their host plants. We investigated the role of changing phenology in recent outbreaks of larch casebearer, an invasive defoliator of eastern larch in North America. We quantified degree-days required for eastern larch to break bud and larch casebearer to develop through each life stage from the onset of development in spring to autumnal dormancy. We developed degree-day models to reconstruct (1) spring phenological synchrony and (2) cumulative proportion of larvae reaching the overwintering stage based on historical climate data. The consequences of warmer autumns and winters (i.e., pre-spring warming) for the incidence and timing of spring activation of larvae were also investigated. Our results suggested that no significant changes have occurred in spring phenological synchrony, but the estimated proportions of larvae reaching the overwintering stage have significantly increased through time. Autumnal warming resulted in delayed spring activation, suggesting that warmer temperatures may act antagonistically on casebearer development, depending on time of year and intensity of warming. Our results provide evidence that increases in annual degree-day accumulation may have helped facilitate recent outbreaks of this invasive defoliator in North America.