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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 and precludes an apparent opportunity for a second generation. The phenology of the European pine sawfly was experimentally 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 neonate larvae fed on earlier or more advanced needle ages. No neonate survived on previous-year needles in mid June nor in mid August. In September, host suitability slightly increased, but it was still very low, as indicated by highly reduced larval growth and survival. The suitability of new foliage subsequently increased in late winter to become highly suitable the following spring. Monoterpene level progressively increased in current –year needles, and remained constant during the next year. Chemical analyses detected a phenolic-based compound that was present at a higher concentration on the younger foliage.