Although pine trees hold their foliage all year, the European pine sawfly, a destructive insect pest of pine, only feeds during a narrow “window” of time in early spring. We tested the hypothesis that the life cycle of European pine sawfly is constrained to a narrow temporal window in early spring by phenological changes in plant quality that results in a limited period of host suitability. Consistent with the predictions of the phenological window hypothesis, larval growth and survival decreased as host-insect synchronicity was modified. Corresponding phenological changes in host chemistry are also being quantified.