Top-down regulation of hemlock woolly adelgid (Adelges tsugae) in its native range in the Pacific Northwest of North America.

The density of insect herbivores is regulated by top-down factors (e.g., natural enemies), bottom-up effects (e.g., plant defenses against herbivory), or a combination of both. As such, understanding the relative importance of these factors can have important implications for the establishment of effective management options for invasive species. Here, we compared the relative importance of top-down and bottom-up factors on the abundance of hemlock woolly adelgid (HWA), Adelges tsugae. HWA is invasive in eastern North America, but its native range includes the Pacific Northwest of North America where it has co-evolved with western hemlock, Tsuga heterophylla. Eastern hemlock, Tsuga canadensis, can also be found planted in city and park settings in the Pacific Northwest and the presence of both host species allowed us to directly compare the importance of predators (top-down) and host plant resistance (bottom-up) on HWA abundance by placing mesh exclusion bags on branches of both species and monitoring HWA abundance over two years. We found no evidence for bottom-up control of HWA on western hemlock (a native host). HWA established more readily on that species than on eastern hemlock on which it is a major pest in eastern North America. We found strong evidence for top-down control in that both summer and winter-active predators significantly reduced HWA densities on the branches of both tree species where predators were allowed access. These findings support the validity of the biological control program for HWA, the goal of which is to reduce outbreak populations of HWA in eastern North America.

Environmental adaptation in an asexual invasive insect.

Parthenogenetic reproduction is generally associated with low genetic variance and therefore reduced ability for environmental adaptation, and this could limit the potential invasiveness of introduced species that reproduce asexually. However, the hemlock woolly adelgid is an asexual invasive insect that has spread across a large geographic temperature gradient in its introduced range. Consequently, this insect has shown significant variation in cold hardiness among populations. We hypothesized that the increased cold hardiness of northern populations represents an adaptation to the colder temperatures. To test this, we collected individual adelgid from populations spanning their invaded range and inoculated them into a common thermal environment. We then experimentally sampled the supercooling point of the progeny of these adelgids and compared these results with tests of the supercooling point of adelgid sampled directly from their source populations. The results showed that the same significant differences in supercooling that was found among geographically distinct populations existed even when the adelgid was reared in a common environment, indicating a genetic basis for the variation in cold hardiness. These findings support the hypothesis that the adelgid has adapted to the colder environment as it has expanded its distribution in its invaded range.

Host use patterns by the European woodwasp, Sirex noctilio, in its native and invaded range.

Accelerating introductions of forest insects challenge decision-makers who might or might not respond with surveillance programs, quarantines, eradication efforts, or biological control programs. Comparing ecological controls on indigenous vs. introduced populations could inform responses to new introductions. We studied the European woodwasp, Sirex noctilio, which is not a pest in its native forests, is a serious invasive pest in the southern hemisphere, and now has an uncertain future in North America after its introduction there. Indigenous populations of S. noctilio (in Galicia, Spain) resembled those in New York in that S. noctilio were largely restricted to suppressed trees that were also dying for other reasons, and still only some dying trees showed evidence of S. noctilio: 20-40% and 35-51% in Galicia and New York, respectively. In both areas, P. sylvestris (native to Europe) was the species most likely to have attacks in non-suppressed trees. P. resinosa, native to North America, does not appear dangerously susceptible to S. noctilio. P. radiata, which sustains high damage in the southern hemisphere, is apparently not innately susceptible because in Galicia it was less often used by native S. noctilio than either native pine (P. pinaster and P. sylvestris). Silvicultural practices in Galicia that maintain basal area at 25-40 m(2)/ha limit S. noctilio abundance. More than 25 species of other xylophagous insects feed on pine in Galicia, but co-occurrences with S. noctilio were infrequent, so strong interspecific competition seemed unlikely. Evidently, S. noctilio in northeastern North America will be more similar to indigenous populations in Europe, where it is not a pest, than to introduced populations in the southern hemisphere, where it is. However, S. noctilio populations could behave differently when they reach forests of the southeastern U.S., where tree species, soils, climate, ecology, management, and landscape configurations of pine stands are different.