Difference in leaf quality between canopy trees and seedlings affects migration and survival of spring-feeding moth larvae

The migratory behavior of moth larvae feeding on oak (Quercus crispula) leaves in spring was investigated with reference to seasonal changes in the quality of the canopy and current-year seedling leaves. After budbreak, the leaf toughness and tannin content of canopy trees increased, while the nitrogen and water contents decreased. These changes in leaf quality appeared to force the moth larvae to migrate from the canopy to the forest floor. A rearing experiment showed that moth (Conistra unimacula (Noctuidae)) larvae could not grow up to the sixth (mature) instar rearing only on canopy leaves. However, when the food material was changed from canopy leaves to seedling leaves after the fourth instar, 72% of the larvae could survive through the fifth instar, and 30% pupated. Seedling leaves were softer, fresher, and more nutritious than canopy leaves. These differences in leaf quality improved the performance of larvae reared with seedling leaves. Our findings suggest that moth larvae migrate from oak canopies to the forest floor in late spring not only to pupate in the ground but also to seek alternative food resources on the forest floor. The latter may have a great impact on the survival of seedlings under the crowns of mother trees.