There is (some) good news in the fight against emerald ash borer, which has devastated hundreds of millions of ash trees in the U.S. and Canada since its detection near Detroit in 2002. Soon after the discovery, researchers from the USDA’s Agricultural Research Service (ARS) and the USDA’s Animal and Plant Health Inspection Service (APHIS) began searching abroad for EAB-specific natural enemies, concentrating on the invader’s native range in northeast China. Three tiny parasitic wasps (or parasitoids) were determined to hold promise in the fight against the voracious EAB: Oobius agrili attacks emerald ash borer eggs, and Tetrastichus planipennisi and Spathius agrili parasitize EAB larvae.
Biocontrol efforts began in the U.S. in 2007, following APHIS’s approval of their release. Since then, the parasitoids have been released in 22 states, where successful establishment has been documented in Illinois, Indiana, Kentucky, Maryland, Michigan, Minnesota, New York, Pennsylvania, Ohio and Wisconsin.
Tetrastichus planipennisi, a miniscule wasp released between 2007 and 2010, has proved to be remarkably effective in the fight against emerald ash borer, according to a seven-year field study conducted in central Michigan. Scientists from the ARS, the U.S. Forest Service and the University of Massachusetts studied the impact of the wasps, which were released in the state where EAB was first detected. According to the ARS, the parasitic insects “spread quickly and contributed to a significant reduction in EAB population growth. T. planipennisi parasitizes EAB larvae by drilling through the bark and laying eggs on its host.”
Jian Duan, research scientist with the ARS, stated: “We observed a 90 percent decline in live EAB larvae in infested ash trees at both parasitoid-release plots and non-parasitoid release (control) plots from 2009 to 2014. This decline was due to significant increases in EAB larval parasitism, first by native parasitoids, then by T. planipennisi.”
The ARS reports that EAB egg parasitism by O. agrili increased from less than 5 percent in the early phases of the project (2008 to 2010) to nearly 30 percent by 2013, at the same study sites.
The effects of O. agrili on EAB population reduction are still being analyzed.
On the heels of this success, the USDA is ready to release another parasitoid from Russia, Spathius galinae. Following a study conducted by the ARS and the University of Delaware, the ARS has approved the tiny wasp’s use in the EAB fight.
Timothy Watt, lead author of the study’s paper (recently published in the journal Biological Control), explained that the scientists studied the effects of temperature on the wasp’s development in reproductive biology, as well as made certain that S. galinae was, in fact, host specific to emerald ash borer.
Co-author Jian Duan (see above) stated, “There’s a lot of behavior and ecological mechanisms to prevent this wasp from attacking other insects. Prior to the regulatory approval, we conducted extensive host specificity testing against 14 different non-target beetle species in the quarantine laboratory. Only one of the 14 non-target beetles was impacted, and that was the gold spotted oak borer, which itself is a serious invasive pest of oak trees in California. But that’s under laboratory conditions. In general, this is one of the most host-specific wasp species of emerald ash borer natural enemies.”
Following the extensive test protocols, APHIS approved the release as of May 2015, but building a sufficient population takes time. The original colony is being mass-reared in an APHIS laboratory in Michigan; the plan is to produce tens of thousands of the wasps. Once the numbers are deemed adequate, they’ll be released first in the Northeastern states.
According to the USDA’s Forest Service, “the response to EAB infestation between 2009 and 2019 could cost up to $10.7 billion. This estimate includes treatment, removal, and replacement of more than 17 million ash trees.”