Pseudoscymnus tsugae for Biological Control of the Hemlock Woolly Adelgid in Suburban Settings Richard

In 1999 we released Pseudoscymnus tsugae on 30 adelgid-infested hemlock trees in landscapes at four locations in Rhode Island in an attempt to control the hemlock woolly adelgid, Adelges tsugae. Small, medium, and large trees at these locations received varying numbers of predators (ranging from 0 to 500 per tree) from mid-May to early June. Beat samples for P. tsugae taken on July 1 found 263 of the 4,920 beetles that were released 5 to 6 weeks earlier. However, we were unable to recover any P. tsugae in samples taken in October, 1999 or in April or August, 2000. There was no evidence of A. tsugae control from these releases. In 2000, we standardized many variables by transplanting healthy, noninfested hemlocks into spaced plantings and then infesting them with A. tsugae before releasing P. tsugae on them. Predators were released at densities of 0, 50, 100, 200, and 300 per 1.5 to 2 m trees on June 8. Samples taken on June 26 showed that predators remained on trees in densities proportional to release numbers. Releases of 50 or more P. tsugae per tree significantly reduced densities of A. tsugae for the 2000 season. However, as in 1999, we found no eggs or larvae of P. tsugae in any of our samples, and adult P. tsugae largely disappeared a few months after release. None were recovered in samples taken in 2001. The lack of reproduction by P. tsugae may be due to late releases that denied the predators access to the A. tsugae eggs, which are important to P. tsugae egg production. Predation by Harmonia axyridis and lacewings also appears to be important in determining P. tsugae survival.