After causing millions of dollars in damage to ash trees and other trees in the Midwest, the emerald ash borer (EAB) is spreading to the East. The good news is that efforts to combat the EAB have borne some fruit.
Researchers have found promising natural predators that could help limit future EAB outbreaks. Additionally, a new chemical insecticide called Tree-age should be available this summer for treating individual trees.
Wasps Prey on EAB
Researchers from the University of Michigan and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) are testing new strategies to combat EAB infestation.
Dr. Deborah McCullough, a forest entomology professor at Michigan State, described how a natural enemy of the EAG, a newly discovered species of wasp indigenous to Michigan, attacks the insect.
“It’s kind of like the movie ‘Aliens,” she said. The wasps lay their eggs in EAB larvae, where they “feed inside (EAB larvae) and burst out. Sigourney Weaver would be proud,” Deborah said, laughing.
The findings are preliminary, but the yet-to-be-identified wasps are creating quite a buzz among EAB researchers. Unfortunately, no one can identify the wasps. Entomologists are working to confirm the species, said Deborah, and also are trying to learn why the wasps have only recently begun targeting the EAB.
Although researchers found them in several areas across Michigan, there seems to be a higher concentration in one specific place. “We don’t know if that’s something unusual about that particular area or if the wasps have changed in some way and are recognizing ash trees as a good place to look for a host insect,” said Deborah. “We don’t really know what’s driving them.”
Researchers are also interested in learning how to raise the wasps in captivity so they can be unleashed on EAB infestations.
“If you can rear large numbers of a native insect like this, releasing it for EAB control would be simpler, and there would be less of a downside compared to bringing in any other kind of exotic parasitoid,” said Deborah.
Introducing invasive species to prey on the EAB may achieve the Agriculture Department’s goal of controlling the insect, but exotic species have a potential to alter ecosystems in ways that scientists do not anticipate.
Three Asian Wasps
In addition, three exotic species of Asian wasps, Spathius agrili, Tetrastichus planipennisi, and Oobius agrili have proven to be highly effective predators against the EAB.
The Asian wasps are naturally drawn to EAB larvae, noted Dr. Julie Gould, a supervisory entomologist with the APHIS Center for Plant Health Science and Technology Laboratory in Massachusetts.
The Asian wasps “crawl around in the tree, and they listen for the chewing sounds that the larvae make when they feed,” said Julie. “When they find an EAB larva, they drill into the tree and either lay their eggs in or on the EAB larva.” The Asian wasps, which can produce two or three generations per summer, hatch and then feed on the EAB larvae until they mature.
The Asian wasps pose no threats to humans. “None of these have stingers,” Julie said. “One wasp attacks emerald ash borer eggs, and it’s the size of a pin. The other two species are maybe the size of small ants. They only attack EAB larvae.”
While Asian wasps also appear to be a suitable predator to counter the EAB, scientists must take other variables into account before releasing large quantities of them. U.S. researchers only have a year’s worth of information on how the Asian wasps interact with the ecosystem.
“We had to do a lot of safety testing,” Julie said. “We tested these (wasps) against native boring insects in other trees that are closely related to EAB.” After the initial testing period, Julie’s research team received a permit to release them in Michigan.
According to Julie, rearing capabilities are beginning to expand, and researchers are authorized to release the Asian wasps in Ohio this summer.
After eight months of research, scientists have yet to see any damage caused by the Asian wasps, aside from preying on the EAB. Julie does not expect them to adversely affect native plants or animals in areas plagued by the EAB.
Julie’s team was pleased to find wasp cocoons under bark on EAB larvae this spring, which demonstrates the Asian wasps can survive a bitter Michigan winter.
“We’re still in research mode,” Julie said. Raising large numbers of Asian wasps and releasing them to combat the EAB “will still be years away.”
New Insecticide
Tree-age, a new chemical insecticide, has shown to be effective in controlling the EAB, too. Its use may be limited, however, to saving individual trees.
Tree-age, developed jointly by Swiss agrochemical company Syngenta and Massachusetts-based Arborjet, is regulated and was expected to be available for use in mid-May. Only certified arborists will be permitted to apply it.
The chemical kills the emerald ash borer and also helps prevent infestation. It has had unparalleled success in exterminating the EAB, according to Deborah. “We completely debarked seven trees that had been treated with this product, and there were a total of eight live larvae on them,” she said. “If we hadn’t treated those trees, we should have seen between 200 and 600 larvae. It seems to be working really well.”
Tree-age also only has limited effects on other plants and animals. Since it is injected directly into the tree, the chemical does not drift onto neighboring vegetation or wildlife. “If an insect lands on the tree, a butterfly for example, it’s not going to be affected,” said Deborah.
Insecticides that are injected into trees generally are safer than those that are sprayed on trees, she noted.
While both chemical and biological strategies may be useful in combating the EAB, their applications may vary, Julie noted. “The insecticides can be very useful in saving individual valuable trees, but I think biological control is going to be a better strategy. You can’t inject every tree in a wood lot with pesticides.” Both strategies will play complementary roles in diminishing the EAB threat, she added.
EAB Outbreak Overview
Michigan, Illinois, Indiana, and Ohio implemented strict quarantines in efforts to control the spread of the wood-eating insect, but new cases of EAB have been verified in Pennsylvania, West Virginia and Maryland.
The emerald ash borer destroys ash trees and similar species. It bores through the bark to lay eggs. When the larvae hatch, they tunnel through the wood, which destabilizes the trunks and branches and cuts off nutrient pathways that fuel growth.
Concern about the EAB and other wood-eating insects has prompted consideration of a federal regulation that would require treating all wood pallets, not just pallets for export. Federal officials are gathering information for a potential regulatory approach.
