Is Pseudoscymnus tsugae the Solution to the Hemlock Woolly Adelgid Problem ? : An Early Perspective

Hemlock woolly adelgid, Adelges tsugae Annand (Homoptera: Adelgidae) is native to Japan where it is an innocuous inhabitant of Tsuga diversifolia Masters and T. sieboldii Carriere. Native populations of this insect are regulated by host resistance and natural enemies. However, introduced populations in eastern North America attain damaging levels on T. canadensis (L.) Carriere and T. caroliniana Engelmann and are regulated mainly by weather and negative density-dependent feedback mechanisms related to host deterioration. The current hope for suppressing introduced populations of hemlock woolly adelgid in eastern North America lies with the exotic predator, Pseudoscymnus tsugae Sasaji and McClure (Coleoptera: Coccinellidae). Extensive laboratory and field studies of the biology and predatory ability of P. tsugae revealed that it has great potential for biological control. Nearly 120,000 adults of P. tsugae were released in hemlock forests in Connecticut, New Jersey and Virginia from 1995 through mid-June 1999. P. tsugae reproduced, dispersed, overwintered, and showed remarkable short-term impact on A. tsugae by reducing adelgid densities 47 to 88% in only five months on release branches at the early sites. Spiders, the most important natural enemies of P. tsugae, reduced efficacy at some sites. In addition, the recent string of relatively mild winters has been conducive to the survival of A. tsugae and of the elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae) another introduced pest from Japan. Consequently, adelgid and scale populations at some sites have grown and trees have continued to decline despite the presence of P. tsugae. Larvae and adults of Harmonia axyridis Pallas (Coleoptera: Coccinellidae), a polyphagous predator from Japan, were observed in high numbers from April through September at several study sites, especially those where trees were heavily infested with A. tsugae. Considering how rapidly hemlock trees are injured following adelgid attack, P. tsugae must establish, reproduce and disperse quickly following a release of relatively few beetles. To become a permanent solution to the hemlock woolly adelgid problem in North America, P. tsugae must also be able to consistently maintain adelgid populations below injurious levels. Studies to evaluate the long-term efficacy of releasing 10,000 adults of P. tsugae in 5-10 acre infested hemlock forests were initiated in spring 1998 in Connecticut, Virginia and New Jersey and were expanded in 1999 to include additional sites there and others