Phenological window hypothesis proposes that phenological changes in plant quality result in a limited period of host suitability. European pine sawfly feeds only during early spring, foregoing an apparent opportunity for a second generation. In this study, the phenology of the European pine sawfly was advanced, delayed, or not manipulated relative to phenological stages of its host, Scots pine. Consistent with the predictions of the phenological window hypothesis, larval growth and survival progressively decreased as egg hatch was delayed. Corresponding phenological changes in host chemistry are also being quantified.