Canadian Forest Service Publications
Hanging in the treetops: an in situ experiment in ancient hemlock assessing outbreak defoliator performance among crown levels. 2016. Hervieux, M.; Maguire, D.Y.; Johns, R.C.; Bauce, E.; Buddle, C.M.; Quiring, D.T. Ecological Entomology 41: 413–420.
Issued by: Atlantic Forestry Centre
Catalog ID: 39559
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- 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.].
- Using tree-climbing techniques, we carried out in situ bioassays in large hemlock trees (∼25 m) to determine whether the observed foraging references are adaptive and how they reflect associated intra-tree variations in microhabitat quality associated with temperature and foliage nutritional quality.
- 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.
- 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.
- 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.
Plain Language Summary
In this field study, we examined different hypotheses to explain why defoliation by an insect caterpillar pest (also known as the pale-winged grey, Iridopsis ephyraria) tended to be concentrated in the lower canopy of mature hemlock. Our results from bioassays indicated that the caterpillars likely fed in lower levels because the foliage was more nutritious. Analyses of foliage chemistry partially supported this conclusion.