FOREST ENTOMOLOGY Comparison of Numerical Response and Predation Effects of Two Coccinellid Species on Hemlock Woolly Adelgid (Homoptera: Adelgidae)

The hemlock woolly adelgid, Adelges tsugae Annand, is an introduced pest in North America that is native to Asia, and is causing extensive damage to eastern hemlock (Tsuga canadensis Carriere) and Carolina hemlock (Tsuga caroliniana Englemann) in the eastern United States. We compared two coccinellids imported for biological control of the adelgid: Scymnus ningshanensis Yu et Yao fromChina and Pseudoscymnus tsugae Sasaji andMcClure from Japan. In a laboratory study, we measured the numerical response of each beetle species to a range of prey densities, and in Þeld studies we examined the reproductive success and ability of the coccinellids to reduce populations of thehemlockwoollyadelgid. In the laboratory,S. ningshanensis showedapositivenumerical response as hemlock woolly adelgid density increased, and P. tsugae showed a density-independent response. In Þeld cages, the presence of S. ningshanensis resulted in negative hemlockwoolly adelgid population growth, in contrast to positive growth in both control cages and cages containing P. tsugae. Both our laboratory and Þeld experiments suggest that S. ningshanensishas good potential as a biological control agent of hemlock woolly adelgid.