Establishment and postrelease recovery of Laricobius nigrinus and Laricobius osakensis (Coleoptera: Derodontidae), released for biological control of Adelges tsugae (Hemiptera: Adelgidae), in the Northeastern United States.

Hemlock woolly adelgid (HWA), Adelges tsugae Annand, is a major forest pest in the eastern United States responsible for killing millions of eastern hemlock, Tsuga canadensis (L.) Carrière and Carolina hemlock, T. caroliniana Engelmann. The US biological control program for HWA has largely invested in the rearing and release of Laricobius nigrinus Fender and more recently L. osakensis Montgomery and Shiyake. Though the establishment of L. nigrinus has been well-documented in the southern, mid-Atlantic, and coastal portions of the northeastern United States, documentation in interior areas of the northeastern United States is limited. Establishment of L. osakensis in the northeastern United States has not yet been documented. Release locations in the northeastern United States were surveyed for L. nigrinus and L. osakensis establishment to examine the relationship between establishment success and winter temperatures, as winter minimum temperatures likely limit the northern range of introduced Laricobius species. Our results suggest that L. nigrinus establishment is limited by winter minimum temperatures and that the probability of establishment declines as absolute minimum temperature declines. We found L. nigrinus established at sites in Maine, New York, and Pennsylvania, but did not recover any L. nigrinus in Massachusetts, New Hampshire, or Vermont. Similarly, we found L. osakensis established at sites in New York and Pennsylvania and recovered individuals in Maine, though further sampling is necessary to confirm presence of the F3 generation. We also report the first field observation of reproduction of silver flies, Leucotaraxis argenticollis (Diptera: Chamaemyiidae), released predator of HWA, in the eastern United States.