A Promising New Parasitoid Drills Down on Emerald Ash Borers
By Melissa Mayer
The wasp flies through the forest, heavy with eggs, following the scent of beetle infestation. She locates a promising tree and lands, using sensory organs on her legs to detect beetle larvae feeding below the surface. She drills through the bark and deposits her clutch. Within a few days, emerging wasp larvae will feast on the soft body of the unlucky beetle larvae.
It reads like a horror story, but for researchers managing the invasive emerald ash borer (EAB) beetle (Agrilis planipennis), it’s a biocontrol solution for a problem plaguing ash forests in 35 states.
A report published in June in the Journal of Economic Entomology offers data on the newest parasitoid wasp released against the EAB beetle: Spathius galinae. One of the authors, Jian Duan, Ph.D., of the Beneficial Insect Introduction Unit at the U.S. Department of Agriculture’s Agricultural Research Service, says including this species in the biocontrol lineup could be a gamechanger. “The addition of S. galinae to the current biocontrol arsenal will provide a whole spectrum of protection for surviving ash trees,” he says.
The Asia-native EAB beetle hitched a ride on U.S.-bound cargo sometime prior to its first detection in the U.S. in 2002 and began feeding on the phloem of local ash trees. The results have been devastating; infested trees lose most of their canopy within two years and die within three or four years.
Scientists decided to import the beetle’s natural enemies: parasitoid wasps. Between 2007 and 2012, they released three such species as biocontrol agents: Tetrastichus planipennisi, Spathius agrili, and Oobius agrili. The results were mixed. S. agrili‘s parasitism rate never reached 1 percent. The other two species did better: O. agrili clocked in at 1 to 32 percent, depending on the release area, and T. planipannisi achieved 30 to 85 percent parasitism—but only in saplings.
Unfortunately, T. planipannisi‘s effectiveness doesn’t extend to larger trees. Its 1.5- to 2.5-millimeter ovipositor is simply too short to reach larvae under bark thicker than 3.5 mm, which is the case for more mature trees.
Enter Spathius galinae
Researchers then turned to S. galinae, which boasts a substantial ovipositor, twice as long as that of T. planipennisi. This enables the wasp to reach larvae infesting much larger trees.
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 centimeters in diameter at breast height) at all release sites. Researchers think the wasp is effective in trees up to 50 cm 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 mortalities 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 5 years after the major releases.”
Some questions remain about S. galinae, especially regarding dispersal and 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 sampling is likely to demonstrate spread over greater distances,” says Duan. This makes sense because T. planipennisi can spread just shy of 2 miles 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 deployments of the biocontrol agent.
Journal of Economic Entomology
Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: firstname.lastname@example.org.