On a day in early June, state forester Will Phifer carried a pill bottle-orange canister into a southeastern Oklahoma forest, tied it to a shaded tree trunk and left. The area was a confirmed spot for a growing population of tree-killing beetles called emerald ash borers, which likely seeped into the state from the east.
The container held what scientists hope is a solution to controlling the harmful pest: more than 100 minuscule parasitoid wasp eggs.
“These emerald ash borer eggs are laid on the outer bark of the tree,” Dieter Rudolph, forest health specialist for Oklahoma Forestry Services, said. “So, this wasp will go find them and basically inject an egg into the emerald ash borer egg.”
Instead of producing an emerald ash borer larva, the host egg will hatch a new wasp.
The emerald ash borer traveled to North America sometime before the early 2000s after it hitched a ride from northeastern Asia. The green beetle with a metallic sheen is responsible for killing hundreds of millions of native ash trees by boring into their bark and feeding on nutrients.
A thinning canopy, branch dieback and D-shaped exit holes are markers of the destructive beetle.
The bug was discovered in Oklahoma in 2016 but went undetected for several years. In 2023, forestry services confirmed a large, established population of emerald ash borer in the southeastern part of the state.
“ We weren't caught off guard, you know, we knew it was coming,” Rudolph, who leads the emerald ash borer program, said. “So, everyone has kind of been planning for this because it's always been a ‘not if but when’ type of situation.”
The state’s 2010 Forest Action Plan listed the emerald ash borer as a potential incoming threat to tree health and sustainability. Although ash species are a small fraction compared to the state’s majority of oak-hickory forest, Rudolph said the trees are a valuable piece of its tourism.
“If you think about where all of our state parks are, they're usually around a lake,” he said. “These lakes have a lot of ash, and there's the potential of, in the future – with a lot of trees dying all at once – it's not very pleasant to look at.”
The cost of removals and safety hazards related to tree die-off could also cause economic harm, he said.
Since its detection, researchers have looked for avenues to control the emerald ash borer’s spread. The bugs have no significant predators in the United States, aside from woodpeckers, which usually only reach the larvae once a population is high and the tree is dead. Insecticides work for individual trees, but spreading them would be labor-intensive and may exceed chemical limits in a forest.
Combating the beetles with their natural enemies, however, proved to be worth an attempt by scientists with the U.S. Department of Agriculture (USDA). About four species of wasps from the emerald ash borer’s native region were selected as biocontrol agents, and the first batch was released in 2007.
This is the first year Oklahoma is releasing one species of the chosen wasps, called Oobius agrili. They grow to be about the size of the sharp point of a pencil and don’t sting.
“They crawl out of that canister and they start searching,” Rudolph said. “I don't know how bugs do the things they do – bugs are pretty cool – but they know how to find emerald ash borer eggs.”
Whether or not the wasp control will save most of the state’s ash tree population is unclear. The wasps are useful in controlling the number of emerald ash borers, but they won’t be eradicated.
More than 30 emerald ash borer-confirmed states have released the wasps as of 2024, and some are reporting healthy ash tree groves. Kelly Oten, extension specialist and assistant professor at North Carolina State University, has been working in the field to control the emerald ash borer since its detection in her state in 2013.
“ It's still early on to determine the full efficacy of using biological control against emerald ash borer,” she said. “But the areas where they have been releasing the longest, there are areas where they've reported, ‘Hey, our trees are lasting longer, and if we were to pair this with maybe some chemical protection, some host plant resistance, then maybe we could have more full management.’”
Oten said about 0.1% of ash trees show resistance to the invasive beetle, which may be the result of a gene variation. Some scientists are considering developing a hybrid ash tree capable of surviving the effects of the emerald ash borer using that small percentage of trees. Species like the American chestnut have undergone similar programs, and Oten said advancements in gene editing could speed up the development.
In the meantime, states are continuing to release the parasitoids before ash tree populations drop. Each wasp averages out to about $3 once it has been deployed, Oten said. A majority of funding comes from the USDA’s Animal and Plant Health Inspection Service, which operates a lab in Michigan that raises the wasps before they’re sent to states like Oklahoma.
“The idea is that, ‘Hey, if this works, at some point we'll be able to put our hands up and walk away and let them do their thing,’ Oten said. “So, it's not something that we'll have to do year after year, after year.”
Individual ash trees, like the ones found along city sidewalks or in backyards, can get help from arborists to treat the emerald ash borer. Those who spot D-shaped exit holes on their ash trees can alert forestry services or contact specialists. Rudolph said moving wood for purposes like camping or branch trimming can cause an outbreak.
Wood movement is the primary mode of travel for the beetles, researchers say.
“ There’s always going to be another insect,” Rudolph said. “The world got a lot smaller with how much trade is going on, and stuff is able to move across the ocean a lot easier, so there's always going to be something else.”
“And if we're not moving the wood, we're just preventing one way it's being spread.”
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