Spatiotemporal distribution of the hemlock woolly adelgid predator Laricobius nigrinus after release in eastern hemlock forests

1 Post-release distributions of Laricobius nigrinus, a biological control predator of hemlock woolly adelgid Adelges tsugae Annand, were evaluated at eight hemlock forests in the eastern U.S.A. 2 Vertical dispersal of F1 and F2L. nigrinus were assessed from within three crown strata (<7, 715 and >15 m) at four release sites. 3 Horizontal distributions of L. nigrinus within the forest surrounding central release areas were observed in two separate studies, which included (i) release and monitor to capture parent and F1 movement by sampling the immature life stages of the offspring, and (ii) assessment of F3 to F6 generations where beetles were previously determined to be established. 4 Laricobius nigrinus, released on lower crown branches, oviposited within the upper crown stratum and were slow to disperse from release trees. Monitoring L. nigrinus only from the lower crown would likely underestimate its presence because 86% of the F2 generation were detected above 15 m. 5 By the fifth generation, the frequency distributions of larvae increased at increasing distance from release areas; larvae were recovered at a maximum distance of approximately 400 m and the spread rate was approximately 39 m/year. 6 Slow dispersal of L. nigrinus and uninterrupted recovery of six generations in the presence of fluctuating prey density support its continued release as part of the A. tsugae biological control programme. These data contribute toward improved release strategies and monitoring for this biological control agent.