A Tiny Pest Is Leveling Mighty Trees

The wasps listen for sounds of their prey, then drill through bark to reach them. Either they paralyze the juvenile victim and glue eggs to its back, or pierce it to lay the eggs inside. When the eggs hatch, the wormy wasp young munch away at leisure.

For anyone who loves a day in a shady yard, a walk in the woods or the crack of a baseball bat, the gore is justified.

The target is the larvae of the emerald ash borer, an Asian beetle that has been 100% fatal to North American ash trees since its arrival about 10 years ago, likely in a shipping pallet. The beetle, first noticed in 2002, has blanketed most of lower Michigan and appeared in Ohio, Indiana and parts of Canada.

Worried that the bug cannot be stopped, researchers are trying to figure out how to help the ash tree survive an infestation. Scientists are studying borer-killing wasps, insecticides, crossbreeding and the possibility of breeding a tree that makes its own insecticide.

“What we need to do is contain this for as long as we can to give research a chance to catch up,” said Vic Mastro, director of the Department of Agriculture lab that detects and finds ways to eliminate exotic pests. “Ultimately, it would be good to eliminate this pest, but we don’t have the tools to do this right now.”

The scientists have a role model. The Asian ash tree lives alongside the beetle, but scientists there haven’t studied why, so researchers here are starting from scratch.

Ash trees are found throughout the eastern United States and along the West Coast. Many cities planted the trees -- which have rounded crowns and vibrant gold fall color -- along streets that were lined with elms before Dutch elm disease nearly wiped them out. Ash trees are also valued for wildlife food and their strong wood used for furniture and baseball bats.

The beetle larvae feed on the cell layers beneath the bark that the tree needs to transport water and nutrients, killing it in about four years.

The U.S. and Canadian governments are sticking with a strategy of cutting down swaths of trees to keep the beetle from spreading, but in the last year an agreement has been reached that the approach will at best slow the insect. The beetle spreads about half a mile a year, but in laboratory conditions has been shown to fly six miles without stopping.

More and more researchers say that flying ability, plus the impossibility of stopping campers from moving infested firewood, mean the spread will probably continue, devastating dense stands of ash in forests from the Dakotas to Maine. Already it has killed about 15 million of some 700 million ash trees in Michigan. Ohio has fared better, with about 250,000 trees cut and chipped to try to stop the spread.

Unlike the elm, in which a few trees survived because of genetic resistance, there’s no sign in ash trees of any resistance to the ash borer, said Jennifer Koch, research biologist with the U.S. Forest Service lab in the central Ohio city of Delaware. “That doesn’t mean it doesn’t exist, but we haven’t found it.”

The solutions being studied include insecticides, which work immediately but are practical only for yard or golf course application, and crossbreeding, which could take decades to develop a resistant tree if it works at all.

Three species of stingless Asian wasps are in quarantine in government labs in Michigan and Massachusetts. Necessary tests include how to rear them in the lab and whether they attack native species of ash borer that aren’t a problem, said Juli Gould, a researcher in the lab Mastro runs on Cape Cod in Massachusetts.

The wasps can produce three to four generations in a summer.

Even faster at reproducing are fungal infections and diseases, said Leah Bauer, a U.S. Forest Service researcher in East Lansing, Mich. Researchers hope to find some that attack ash borers in Asia.

At Purdue University, Rick Meilan is exploring ways for the trees to make their own pesticide.

Organic farmers often control pests with a bacterial toxin that can target a specific insect. Meilan has identified at least two toxins specific to emerald ash borer, which cause ulcers or paralyze their chewing parts.

The next step is getting the toxin-making gene into the tree, taking advantage of a natural bacteria that inserts genes in plant cells, then getting a few cells to grow into a tree.

The approach has been used before with corn and other plants but is met with resistance by environmental groups and a skeptical public that doesn’t like mixing creatures that couldn’t breed naturally.

“It’s just DNA,” Meilan said. “There are mechanisms by which genes move around in nature all the time.”

An insecticide-making tree is at least two years away, and the government would have to write regulations for it, which probably would include some requirement that the tree be sterile, he said.

In Ohio, the Forest Service is taking a much slower but more traditional approach.

Chest-high seedlings as thick as a thumb give Koch hope that she has succeeded in crossing Asian and American species of ash, but it will take genetic testing to make sure.

Even if the cross works, there’s no guarantee that it’s the tree’s genes that make it resist the borer. If they do, it will take decades of more crossbreeding to get an essentially American tree with Asian resistance.

It’s impossible to predict how natural controls such as the wasps will work until they’re released in the environment, Gould said.

“This will be another tool in the toolbox to slow the spread of this thing,” Gould said. “They need to throw everything they have at this pest.”