Post-release evaluation of Laricobius nigrinus for the biological control of hemlock woolly adelgid in the eastern United States

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Virginia Tech


Laricobius nigrinus is an insect predator introduced into the eastern United States for the biological control of hemlock woolly adelgid, Adelges tsugae. Laricobius nigrinus dispersal characteristics and predation on A. tsugae were evaluated by monitoring its immature life stages that were distinguished from larvae of the native, L. rubidus using a portion of the cytochrome oxidase subunit one gene. Conventional polymerase chain reaction (PCR) followed by restriction fragment length polymorphism produced DNA fragment sizes that were unique to each species. Real-time PCR used a TaqMan assay with two species-specific nucleotide sequence probes labeled with fluorescence molecule VIC or FAM that indicated the DNA template was L. nigrinus or L. rubidus, respectively. Laricobius nigrinus beetles released in the lower hemlock crown (< 7 m) dispersed to the upper crown (>15 m) for oviposition. Four years post-release, its rate of spread into the surrounding forest was 75 m/yr and the number of L. nigrinus detected on hemlock trees was positively correlated with the density of A. tsugae, regardless of proximity to the release area. Adelges tsugae survival rate was lower on release trees than on geographically separated control trees for three years at a L. nigrinus release site in each Pennsylvania and North Carolina. Yet, the density of A. tsugae remained greater on release trees than control trees. In the eastern United States, the average density of A. tsugae was 3.8 times greater than the upper 95% confidence interval in the native range of L. nigrinus and A. tsugae, as observed in Seattle, WA. This contributed to the average ratio of predator to prey remaining 4.6 times lower than the 95% confidence interval observed in WA. Five to seven years post-release, hemlock health of release and geographically separated control trees declined at similar rates, though the average A. tsugae infestation level was 35% greater on release trees at the time of L. nigrinus release. The full efficacy of L. nigrinus predation was likely underestimated because this study assessed experimental release sites. Interactive roles of biotic and abiotic pressures on A. tsugae should continue to be evaluated at L. nigrinus introduction sites.



biological control, predator impact, dispersal, hemlock, PCR, Laricobius nigrinus, Adelges tsugae