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S. galinae has other advantages, too. Its offspring have a 3:1 female-to-male ratio, and its lifecycle is only about 30 days. This means it can yield two to three female-predominant generations of wasps during just one growing season. "This is a tremendous advantage for biocontrol of emerald ash borer, which has a 1- to 2-year life cycle in the United States," says Duan. Scientists released S. galinae and T. planipennisi in Connecticut, Massachusetts, and New York in 2016. By 2018, S. galinae was established and attacking EAB larvae in pole-size trees (6 to 12 centime- ters/2.4 to 4.7 inches diameter at breast height) at all release sites. Researchers think the wasp is effec- tive in trees up to 50 cm (~20 inches) in diameter. S. galinae's parasitism rate was 33 to 49 percent at two sites (a nearly 30-fold increase within just two years) and 13 to 15 percent at two more sites (a 13-fold increase). Rates stayed under 1.5 percent at the remaining two sites. In contrast, T. planipennisi showed parasitism at four of six sites, and those never reached 6 percent. These rates are lower than those in S. galinae's native Russia, where the wasp parasitizes about 62 percent of late-instar EAB larvae. Still, the strong results just two years after principal release are promising. "Natural enemy introduction normally takes more time for the introduced agent to firmly establish an appreciably high abundance that can cause high mor- talities of the targeted pest," Duan says. "We did not observe the strong recovery and sharp increase in densities and parasitism rate of the previously introduced EAB parasitoid, T. planipennisi, until approximately five years after the major releases." What's Next? Some questions remain about S. galinae, especially regarding dispersal, spread, and overwintering. Duan says he and his coauthors are already working on this. "Initially, we have documented spread of at least a mile from the release sites, and further sam- pling is likely to demonstrate spread over greater distances," says Duan. This makes sense because T. planipennisi can spread just shy of 2 miles (3.2 km) per year, and S. galinae has a larger body and wings. Figuring out S. galinae's cold tolerance is also on tap, as this data will guide future deploy- ments of the biocontrol agent. The parasitoid wasp Spathius galinae has been examined as a new candidate for biological control of emerald ash borer. Research in three northeastern states in the U.S. found wild populations established two years after release, and parasitization rates of emerald ash borers as high as 49 percent. Photo by Jian Duan, Ph.D., USDA-ARS 32 CityTREES S. galinae's parasitism rate was 33 to 49% at two sites