Collaborative approach to management of hemlock woolly adelgid: Strategies and outcomes

Since 2002, eastern hemlock in the Great Smoky Mountains National Park (GRSM) has declined due to hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae). Many organizations and institutions are actively conducting research to reduce the spread and effect of this invasive pest and preserve hemlock in its native range.  Two well-developed management strategies against A. tsugae include release of introduced predators, i.e., Sasajiscymnus tsugae (Saasaji and McClure) and Laricobius nigrinus Fender, and application of insecticide. These two predators have been recovered from several areas of release, and the coexistence of these predators, which has been observed on eastern hemlock in multiple locations, may enhance their impact on adelgid populations by providing prolonged feeding. Several positive findings have been documented in areas where S. tsugae has been recovered. First, regression analysis indicated that recovery of S. tsugae was associated with elevation of the release sites and average maximum temperature seven days following release, and normalized difference vegetation index values were higher in S. tsugae recovery sites than non-recovery sites which indicates that hemlocks are healthier in  S. tsugae recovery sites than non recovery sites. Additionally, relatively higher hemlock mortality was observed in non-recovery sites than recovery sites, and percent crown transparency, percent live crown, and percent branch dieback was also significant with presence of S. tsugae. These findings, in conjunction with persistence of hemlocks in several areas of GRSM, indicate management efforts against A. tsugae can protect eastern hemlock.