Changes in Tree Leaf Quality and Growth Performance of Lepidopteran Larvae

Despite the apparent low nutritional value of mature tree leaves, most temperature forest macrolepidopteran larvae feed on them during the summer or fall. This may occur in part because late—feeding larvae are nutritionally best adapted to exploit the nutrients in mature leaves. To test this hypothesis, larvae of the early—feeding species Malacosoma americanum and the late—feeding species Hyalophora cecropia were reared on spring—, summer—, or fall—collected leaves of Prunus serotina. Changes in leaf nutrient quality and larval growth of performance with leaf maturity were monitored. Chemical analyses confirmed that, as expected, leaf nitrogen, water, sugar, and cyanide content decreased while lignin, cellulose, and energy content increased with leaf maturity. There was no evidence for nutritional factors that produce proximate barriers to summer feeding by M. americanum or to spring feeding by H. cecropia. Larvae of both H. cecropia and M. americanum grew more rapidly and/to attained larger larval size when fed immature leaves than when fed mature leaves of P. serotina. However, the decrease in growth rate and/or larval size with leaf maturity was much greater for M. americanum than for H. cecropia, suggesting that H. ceropia is better adapted, relative to M. americanum, for feeding on mature leaves. The adaptations for using mature leaves by H. cecropia larvae were both nutritional (higher net growth efficiency and nitrogen conversion efficiency) and developmental (nearly complete compensation for decreased growth rate by increased duration of the larval period).