The hemlock woolly adelgid, Adelges tsugae annand, is an introduced insect pest that is causing severe decline and mortality of hemlocks (Tsuga spp.) in eastern North America. We used molecular methods to clarify the relationship among hemlock adelgids worldwide and thereby determine the geographic origin of the introduction to eastern North America. The source of the introduction to eastern North America is a population of adelgids in southern Japan in the natural range of Tsuga sieboldii. A separate mitochondrial lineage that is not present in eastern North America was found in the natural range of T. diversifolia, which occurs at higher elevations and more northern latitudes in Japan. Hemlock adelgids from China represent a separate lineage that is markedly diverged from insects in North American and Japan and should probably be described as a separate species. In addition, HWA in western North America, which was assumed to be exotic, appears to be endemic.
We are extending this work by using microsatellite markers to further elucidate the genetic structure of hemlock woolly adelgids to link population genetic processes to observed ecological and life history variation. Additionally, a PCR-based assay will be developed to detect and monitor introduction of invasive adelgid genotypes in areas where they are not already present. The results of this project will be used to predict the adaptive potential of HWA in its introduced range, to target exploration for biological controls, to aid development of resistant hemlock genotypes, and to clarify adelgid taxonomy.
We are developing molecular markers to distinguish cryptic immature stages of key adelgid natural enemies Laricobious, currently being released for biological control. This will provide an evolutionary and ecological context to predict their efficacy and help to monitor their impact after introduction. Laricobius nigrinus Fender is native to western North America and has been released in eastern North America as a biological control of the hemlock woolly adelgid (HWA). In eastern North America there is a congeneric native species, L. rubidus LeConte, that feeds on white pine adelgid, Pineus strobi (Hartig), and on HWA but without significant population impact. A phylogenetic analysis of the genus showed that L. nigrinus and L. rubidus are very closely related. Joanne Klein, Research Assistant, Yale Univeristy is currently developing the microsatellite markers for this study.
This work is a collaboration with Nathan Havill (joint Post Doc for EEB and US Forest Service), Dr. Michael Montgomery (US Forest Service).
Publication: Havill, N.P.,M. E. Montgomery, G.Yu, S. Shiyake, A. CACCONE. 2006. Mitochondrial DNA from hemlock woolly adelgids, Adelges tsugae (Hemiptera, Adelgidae), suggests cryptic speciation and pinpoints the source of introduction to eastern North America. Ann. of the Entomol. Soc. of America, 99(2): 195-203.