Egg releases of the hemlock woolly adelgid, Adelges tsugae (Heteroptera: Adelgidae), predator Laricobius nigrinus (Coleoptera: Derodontidae)

In the southeastern U.S., the exotic hemlock woolly adelgid (HWA, Adelges tsugae Annand) is causing extensive mortality of eastern hemlock (Tsuga canadensis L. (Carriere)). Hemlock is a climax species and ecologically significant in mountainous riparian areas. In an attempt to prevent hemlock mortality, HWA natural enemies are being mass reared for classical biological control. However, extensive mortality (50-90%) of lab colonies commonly occurs when mass rearing the adelgid predator Laricobius nigrinus Fender. Our objective was to assess egg releases of L. nigrinus. A total of nine releases were conducted during the spring of 2007 and 2008. Each release date included 10 replicates. Releases were monitored in sleeve cages and compared to lab controls (N=110). Mean egg to mature larvae survival was 25% in sleeve cages; lab control survival was 62% (F_1,108=34.4, P<0.0001). L. nigrinus survival was higher during earlier release dates. Adverse temperatures and generalist predators may account for lower survival in field releases. Data suggests egg releases that occur earlier in the spring on pole-sized trees (5-10” DBH) with a high number of infested branches enhances larvae survival. Although survival was significantly less compared to lab colonies, releasing L. nigrinus eggs in the field is a viable method. Egg releases can bypass high levels of lab mortality in aestivating larvae, increase release numbers, and expand biocontrol efforts. Egg releases have also produced the first recovery of L. nigrinus one year post-release in Georgia.