The emerald ash borer may be a tiny insect, smaller than a penny, but it looms large in the lives of researchers such as Chris Williamson, an entomologist with UW-Extension in Madison.

For Williamson and his colleagues, the discovery of the ash borer in southeast Wisconsin earlier this month was not unexpected because the insect is already in Illinois and in Michigan, where it was discovered in 2002. Still, the nearby presence of the invasive and destructive insect in Wisconsin has put even more pressure on scientists who are racing to find a way to control the ash borer.

The news from the laboratories is not heartening, even after several years of research in the wake of the insect's arrival in Michigan.

"There are no known silver bullets," Williamson said.

At stake are 740 million ash trees in Wisconsin. Most of these are in the state's forests and are not only an important part of the ecology but also a valuable source of commercial wood. More than 5 million ash trees shade city neighborhoods and grace the lawns of homes. In Michigan, the pest has killed entire forests and experts estimate 25 million trees have been killed in the 10 states where it is now present.

"This is not an insect I was looking forward to seeing here,'' said Phil Pellitteri, a UW-Madison entomologist.

No natural predators

The problems are many and complicated, so much so that even experts expect the loss of millions of trees before a reliable way to stop the ash borer is found. The first and most frustrating difficulty is the nature of the enemy, Williamson said. The ash borer, as with most invasive species, has no natural predators in North America, nothing to stop its relentless chewing.

In Asia, where it is from, native ash trees have co-evolved along with the ash borer and, through the process of evolution, have developed natural defenses against the insect, according to Robert Haack, a researcher with the U.S. Forest Service in Michigan who has been studying the ash borer for several years. He added that other opportunistic insects in Asia, including a number of wasps, have evolved to become predators of the ash borer.

Another problem is that the ash borer is hard to find and, by the time it is discovered, has already done its damage or perhaps has been transported on firewood or other wood products to another location. Williamson tells how, while on vacation, he called and got directions to an ash borer infestation in Indiana. He arrived at a quiet residential street with ash trees lining the block.

"I looked at these ash trees and there was nothing wrong with them," Williamson recalled. "I called and asked if I was in the right place. But then I climbed up one of the trees and there were the d-shaped exit holes of the insects. It was just that at low population densities, it is almost undetectable."

The problem with this, Williamson and others said, is that the insect may actually have been doing damage for a number of years before it was discovered, putting trees beyond help.

Working near Evanston

Despite the difficulties, Williamson and other researchers are working on a number of fronts to find weapons that will slow or halt the spread of the ash borer. Much of the work centers on finding a pesticide that will kill it.

Williamson has been studying insecticides. Because there had been no known infestations in Wisconsin until this month, Williamson has been working on a research plot on a golf course near Evanston, Ill., just north of Chicago. He's studying how various insecticides work on trees that are near a known ash borer infestation. Much of his work is focused on finding an insecticide that will protect a healthy tree from a nearby infestation.

"If you have a tree and it doesn't have ash borers, what can you do to protect it before it gets it?'' Williamson asked.

Although some pesticides have proven effective against the ash borer, the problem is that those same pesticides have been found to fail when used again under similar circumstances.

"Sometimes a pesticide will work and sometimes it will fail," Williamson said. "The question is 'Why?' and we don't have a reason."

Studies have shown that the variations in effectiveness may have to do with how the pesticide is delivered, Williamson said. Beneath its bark a tree has different layers of tissue that deliver nutrients and carry away waste. The success of many pesticides can depend on which of those tissues take up and transport a particular pesticide and much current research is focusing specifically on that issue, Williamson said. Some ash borer pesticides are injected directly into the tree while others are put on the ground to be taken up by the roots.

While some pesticides, such as a compound called imidacloprid or a chemical called emamectin benzoate, have been shown to be effective, no one chemical is certain to work at this point, Williamson said. So a homeowner with a prized ash tree who is deliberating about whether to treat the tree will be given no guarantees. Some of the treatments can be expensive, Williamson added, so the decision may be difficult.

"If you have a greater than 40 percent dieback in the canopy, it's not in your best interest to try any kind of rescue treatment,'' Williamson said. "At that point, it's very difficult for that tree to recover.''

Hybrid trees

Although most attention has been focused on producing an effective pesticide, some longer-term research is also looking at ways to control the ash borer by introducing natural predators or even producing hybrid trees that are resistant to the insect.

Researchers from Michigan, for example, have already journeyed to China where they spent months in the field looking for natural predators of the ash borer. Haack, the U.S. Forest Service researcher in Michigan, said a special rearing laboratory is being built in Michigan to raise a species of parasitic wasp that was discovered in China and which kills the ash borer. Field research on the wasp is just getting under way and any promise of that approach is still years away, Haack warned.

Some thought is also being given to finding species of ash in Asia that are resistant to the ash borer and which could be cross-bred with American ash species to create a new, hardier tree that can survive attack by the insect.

And who knows, Haack speculated, over the years, American ash trees may evolve to become resistant on their own.

"I have ash trees in my backyard," Haack said. "And I still have hundreds of tiny little ash seedlings. Maybe over time, natural selection will occur and the new trees will have resistance."