Will stingless wasps work against the emerald ash borer?

One of the stingless wasps used to combat emerald ash borer infestations. (image via Stephen Ausmus, USDA)
One of the stingless wasps used to combat emerald ash borer infestations. (image via Stephen Ausmus, USDA)

DES MOINES — State and federal insect experts say it likely will be next year before they will be able to tell whether a swarm of stingless wasps introduced in Iowa are helping suppress a destructive pest that has attacked ash trees in 53 of Iowa’s 99 counties.

Three species of parasitic wasps were released in 2016 at two biocontrol sites — 11,000 in the Whitham Woods in Jefferson County and 14,000 at the Mount Hosmer park in the Allamakee County town of Lansing — as a natural predator of the emerald ash borer, a devastating beetle whose larvae eats away ash trees from within, said Mike Kintner, emerald ash borer and gypsy moth coordinator for the Iowa Department of Agriculture and Land Stewardship.

The program was expanded this year to three more locations where stingless wasps (both egg and larval parasitoids) were released — Lake Iowa Park in Iowa County, West Lake Park in Scott County and Millrace Flats in Louisa County — to target the ash borer’s larval and egg stages, Kintner added.

Kintner said researchers want a full two years of experience under U.S. Department of Agriculture guidance to assess the release and recovery efforts so it likely will be the fall/winter of 2018 at the earliest before they evaluate parasitoids at the two Jefferson and Allamakee sites by peeling trees and/or placing insect traps to get indications of wasp-related activity.

“We’ll just have to see how well they worked,” said Kintner, by eventually determining not just that the parasitic wasps are killing Emerald Ash Borer but also that they actually are reproducing natural areas.

“There are study plots in Michigan that show hope and success in what they’re doing there,” he added, with Iowa being one of 19 state where one or more wasp species have been released since 2007. “Parasitic wasps are not going to totally eradicate Emerald Ash Borers or put a complete stop to them by any means, but it offers hope for the future and that’s what we’re hoping for.”

Tetrastichus planipennisi female wasps, which are about the size of a grain of rice, lay eggs inside EAB larvae, terminating their development into adult beetles. Oobius agrili female wasps, which are the size of a gnat, lay eggs inside EAB eggs, parasitizing them before given the opportunity to hatch. Both species are harmless to people. Emerald Ash Borer is native to Asia and was introduced to the United States in 2002. It was first discovered in Iowa in 2010 in Allamakee County and spread to Linn County in 2015 and Johnson County in 2016.

With the arrival of fall weather, Kintner said the destructive insects have moved into a resting phase but he added it was “pretty safe to say” they likely will expand to more Iowa counties once the dormant period ends next spring. “We’re on a track record here from basically 2013 where we’ve been seeing a lot of counties added every year and I would see no reason why that wouldn’t continue next year as well,” he noted.

Another natural predator in Iowa is the woodpecker that currently are offering some local control, he said, “but unfortunately there aren’t enough of them and there are just too many Emerald Ash Borer. But that does offer some level of control.”

Also, efforts are underway to develop ash trees that are more resistant to borer attack but Kintner said that offers future hope but “unfortunately, the big, large ash trees that are already there on the landscape aren’t going to be safe.”

According to the USDA Forest Service, Iowa has an estimated 52 million rural ash trees and about 3.1 million more ash trees in urban areas.

Emerald Ash Borer-infested trees can include branch dieback in the upper crown, water sprouts along the trunk and main branches, vertical bark splits, D-shaped emergence holes, S-shaped tunneling under loose bark, as well as woodpecker damage.

All ash tree species are susceptible to attack by in invasive bug. The larval stage of this insect kills ash trees by tunneling under the bark and feeding on the part of the tree that moves water and nutrients throughout the tree.

The beetle is most frequently spread by the transport of infested firewood. Officials recommend residents buy local firewood and avoid moving it outside the area to prevent transportation of the destructive pest.

More information about USDA’s Emerald Ash Borer Biocontrol Program can be found at


More information about EAB and other pests that are threatening Iowa’s tree population, go to www.IowaTreePests.com



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