Host phenological changes in foliar nutrients and chemical defenses constrain the performance of European pine sawfly larvae

Although pine trees hold their foliage all year, the European pine sawfly, a key defoliator of pine, feeds only on mature foliage during a narrow "window" of time in early spring. We manipulated host and larval phenology experimentally to test the hypothesis that the life cycle of European pine sawfly is constrained 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. Strong correlates between the concentrations of free amino acids, soluble sugars, mono, sesqui and di-terpenes and larval growth suggested that this insect can tolerate plant defenses only if nutrients are relatively high with respect to chemical defenses. As nutrient concentrations in mature foliage decreased as immature foliage elongated after budbreak, insects ate more in an attempt to compensate but were less efficient at converting consumed foliage to body mass. Eventually, host quality declined to the point that larval survival decreased dramatically.