Hanging in the treetops: anin situexperiment in ancient hemlock assessing outbreak defoliator performance among crown levels

Abstract

1. Past studies have shown that both egg lay and larval feeding of a generalist defoliator, pale-winged gray (Iridopsis ephyraria Walker) (Lepidoptera: Geometridae), are concentrated in the mid-lower crown of eastern hemlock [Tsuga canadensis (L.) Carr.]. 2. Using tree-climbing techniques, we carried out in situ bioassays in large hemlock trees (∼25 m) to determine whether the observed foraging preferences are adaptive and how they reflect associated intra-tree variations in microhabitat quality associated with temperature and foliage nutritional quality. 3. In 1 of 2 years, larval survival was significantly higher in the shaded lower versus sunlit upper crown; however, in both years, groups that fed in the sunlit upper crown branches had larger male and female moths and more female-biased sex ratios. 4. Differences in pale-winged gray performance among crown levels were somewhat supported by trends in foliage nutritional chemistry but not well-correlated with variation in temperature. 5. The present study is one of only a few to carry out bioassays on large mature trees and results reinforce the idea that foraging patterns of herbivores are likely to reflect trade-offs among several factors that vary between sunlit and shaded branches within forest canopies.