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Hemlock woolly adelgid, Adelges tsugae, is an invasive pest of hemlock trees (Tsuga) in eastern North America. We used 14 microsatellites and mitochondrial COI sequences to assess its worldwide genetic structure and reconstruct its colonization history. The resulting information about its life cycle, biogeography and host specialization could help predict invasion by insect herbivores. We identified eight endemic lineages of hemlock adelgids in central China, western China, Ulleung Island (South Korea), western North America, and two each in Taiwan and Japan, with the Japanese lineages specializing on different Tsuga species. Adelgid life cycles varied at local and continental scales with different sexual, obligately asexual and facultatively asexual lineages. Adelgids in western North America exhibited very high microsatellite heterozygosity, which suggests ancient asexuality. The earliest lineages diverged in Asia during Pleistocene glacial periods, as estimated using approximate Bayesian computation. Colonization of western North America was estimated to have occurred prior to the last glacial period by adelgids directly ancestral to those in southern Japan, perhaps carried by birds. The modern invasion from southern Japan to eastern North America caused an extreme genetic bottleneck with just two closely related clones detected throughout the introduced range. Both colonization events to North America involved host shifts to unrelated hemlock species. These results suggest that genetic diversity, host specialization and host phylogeny are not predictive of adelgid invasion. Monitoring non-native sentinel host trees and focusing on invasion pathways might be more effective methods of preventing invasion than making predictions using species traits or evolutionary history.

Laricobius osakensis Montgomery and Shiyake sp. nov., collected from Adelges tsugae Annand on hemlock [Tsuga sieboldii Carr. and Tsuga diversifolia (Maxim.) Mast.] in Japan, is described and illustrated. The new species was collected from several localities on Honshu, Shikokou, and Kyushu Islands. The genus has not been reported previously from Japan. Morphological features, a molecular phylogeny, and diagnostic DNA sites are provided to distinguish this new species from previously described species of the genus. Because of plans to release L. osakensis for the biological control of A. tsugae in eastern North America, a key and discussion are provided to differentiate it from the native North American species, Laricobius nigrinus Fender, Laricobius laticollis Fall, and Laricobius rubidus LeConte, and from two previously imported species—Laricobius erichsonii Rosenhauer, which is endemic in Europe, and Laricobius kangdingensis Zilahi-Balogh & Jelinek, which is endemic in China.

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Laricobius osakensis Montgomery & Shiyake, native to Japan, is being measured as a potential biological control agent of the hemlock woolly adelgid, Adelges tsugae Annand, in the eastern United States. Adelges tsugae, a destructive pest threatening the hemlock ecosystems in the eastern United States, was introduced from Japan. This is the first detailed study of the life history of L. osakensis, A. tsugae, and their interaction in Japan. The seasonal abundance of L. osakensis and A. tsugae was assessed in a field study conducted from December of 2007 to November of 2008 in Hyogo, Nara, and Osaka Prefectures (Japan). The sistentes generation of A. tsugae in Japan matured 2 mo later than that reported in British Columbia, Canada, but 1 and 2 mo earlier than in Virginia and in Connecticut, respectively. Timing of the sexuparae generation occurrence in Japan was similar to that observed in Connecticut and Virginia, but this generation has not been observed in British Columbia. Phenological differences among the various A. tsugae population in the eastern U.S. states and Japan can be explained by temperature differences at the locations. Aestival diapause of the predator L. osakensis coincided with diapausing first-instar A. tsugae sistentes. The start of L. osakensis adult activity, oviposition, and larval development was synchronized closely with sistentes development resumption, sistentes adults close to oviposition, and sistentes adults with eggs, respectively. These results indicate good synchrony between L. osakensis and suitable prey stages of A. tsugae in the native habitat of both species.