Wisconsin Department of Natural Resources

Experimental release of stingless parasitic wasps
may help Wisconsin control emerald ash borer

News Release Published: May 5, 2011 by the Central Office

Contact(s): Dr. Ken Raffa, UW-Madison (608) 262-1125 or Andrea Diss-Torrance, DNR (608) 264-9247

MADISON - Tiny stingless wasps the size of a grain of rice may someday play a role in controlling the emerald ash borer (EAB) infestation in Wisconsin. The invasive pest has killed millions of ash trees since its discovery in Michigan in 2002.

EAB was accidentally introduced from China within shipments of imported products. It is not a pest in Asia, where it only lives in dying trees, because ash trees there evolved with it and so healthy Asian ash have effective defenses. Unfortunately, there is no evidence of resistance in our ash species.

This spring, scientists with the University of Wisconsin - Madison, the DNR, and USDA-APHIS will introduce three species of stingless parasitic wasps that prey almost exclusively on the ash-killing beetle in a cooperative project. Emerald ash borer was first discovered in Wisconsin near the Village of Newburg in 2008. Since that time, EAB has shown up in Cudahy, Oak Creek, Franklin, West Bend and Victory, and has been trapped in Green Bay and Kenosha.

A Wisconsin Department of Natural Resources grant will help fund the research for the next three years.

Researchers plan to release two species of stingless wasps sometime in May and the third species later in the summer. The releases are timed according to specific weather conditions and will all be done at the Riveredge Nature Center near Newburg. All three stingless wasp species are native to China and are natural predators of EAB in the beetle's native range.

The two species that will be released in May parasitize the larval stage of EAB - when the insect lives under the ash bark. They insert their egg-laying appendage (ovipositor) through the outside of the tree and then lay their eggs on the surface or inside the EAB larva. The stingless wasp species that is scheduled for release later this summer parasitizes EAB by depositing its egg inside the egg of the emerald ash borer.

"This experiment will help our understanding of the potential these wasps that may have as part of our attempts to reduce the spread and impacts of the emerald ash borer in Wisconsin," said Dr. Ken Raffa, UW Madison professor of entomology. "Biological controls, such as these stingless wasps, can be an important component in an overall management strategy that also includes quarantines, education, and chemical treatments. These wasps have undergone intense scrutiny by the USDA to safeguard against harm to native species. So far, 180,000 wasps have been released in 9 states, including Michigan, Illinois, and Minnesota."

"These tiny, stingless wasps pose no threat to people," said Andrea Diss-Torrance, DNR forest entomologist. "They do not seek shelter in homes or other buildings as they all overwinter inside their host, EAB, under the bark of ash trees. As adults, these wasps are focused on finding EAB to host their offspring and so stay close to EAB-infested ash trees. Because they are so tiny and at work in the canopy of ash trees, most people will never see them. In fact, it is very hard even for trained entomologists to find the adult wasps in the field. To verify if our introductions are successful, we will have to peel the bark off of EAB infested ash and look for parasitized EAB larvae"

"The effectiveness of these stingless wasps against EAB is uncertain, yet it is clear that reducing EAB populations has a better chance of success when many approaches are employed," said Jane Cummings-Carlson, forest health specialist with the DNR. "Extensive research conducted by the USDA shows that they are highly selective in going after EAB, so there is some hope that they may eventually be used on a wide scale to help reduce EAB populations, especially in Wisconsin's forests where there are more than 700 million ash trees. It is very important that we explore every tool available to reduce the ecological and financial impact EAB will have on our urban and rural forests," said Cummings Carlson.

"It will take some time, roughly five years, before we can assess the potential of these stingless wasps for affecting the population of EAB," continued Cummings-Carlson. "If it does prove useful, it would be most effective in areas of known infestations since the stingless wasp needs the beetle to complete its life cycle. Cold-hardiness tests indicate the stingless wasps are winter hardy enough to survive from season to season in our climate."

Nature is already lending a hand when it comes to reducing EAB populations. Native woodpeckers are often attracted to the abundant source of larvae in infested trees. One of our native ground-dwelling stingless wasps feeds exclusively on beetles within the same family as EAB. And in Michigan, scientists recently discovered an entirely different species of native non stinging wasp that preys upon EAB larvae. However, it is clear from the devastation that these agents are not adequate. As with tree defense, the natural enemies that evolved with EAB in China are more effective.

"Key to helping reduce the population of EAB in Wisconsin and in North America as a whole will be finding ways to combat the beetle across large tracts of land," Cummings-Carlson said. "Currently, our insecticides work well to protect urban trees but are not practical in the forest setting. We really need an effective biological control to help us preserve our native North American ash forests."

"Riveredge looks forward to this exciting partnership with the DNR and researchers from UW-Madison and we're honored that they chose Riveredge as the release area," said Patrick Boyle, executive director of the Riveredge Nature Center. "We all greatly hope that this project yields some positive results that we can share with the community."

The emerald ash borer is a metallic green beetle about the size of a cooked grain of rice. It was introduced to North America in early 1990s in the Detroit area and has since been responsible for the destruction of millions of trees across 15 states and Canada. It attacks only ash trees.

For more information about EAB in Wisconsin, visit www.emeraldashborer.wi.gov or call 1-800-462-2803.