Despite last winter's polar vortex, emerald ash borer continues to march across Wisconsin

Milwaukee Journal Sentinel

Emerald ash borers are shown in their insect stage. They survived briefly after they were removed as larvae from infected ash trees in Brookfield last winter.

If there was any doubt about the strength and resilience of the emerald ash borer, consider the experience of Bill McNee.

In February, he dug larvae out of the bark of ash trees in Endicott Park in Brookfield after the thermometer dipped to minus 26 degrees. Would last winter’s polar vortex deliver a knock-out punch?

A few days later, back at his office at the Department of Natural Resources in Oshkosh, most of the larvae, a native of east Asia, were alive and wriggling in the warmth. About 20 percent turned into adult beetles.

“It shows you how tough of an insect they are,” said McNee, a forest health specialist with the DNR. “They were designed to handle the temperatures we have in southern Wisconsin.”

Emerald ash borer, an invasive beetle discovered in Wisconsin 11 years ago, has devastated native ash populations, especially in southern Wisconsin. After the insects have burrowed into the tree, in their larval stage, they can kill ash trees by disrupting the flow of nutrients beneath the bark.

At the latest count, the insect has been found in 51 of the state’s 72 counties, according to state figures. That’s up from 48 counties this time a year ago.

Since it was first found in 2008 near the Milwaukee River on the Ozaukee and Washington county lines, emerald ash borer has become well established in southeastern Wisconsin and continues to kill trees.

Statewide, “we are not even close,” McNee said, to reaching peak infestation because the largest ash populations are in the north.

It’s one reason why Wisconsin continues to advise the public to obtain firewood locally to help reduce the accidental spread of insects to areas not yet infected.

In a sign of regeneration, a young ash tree grows in an area of Potawatomi State Park in Door County that has been infested with the emerald ash borer.

Across the Milwaukee area, property owners continue to grapple with dead and dying ash trees — their tell-tale skeletal branches have become an increasingly common sight.

McNee said the DNR has observed, and received many calls about, widespread woodpecker damage over the winter. That is one sign of a robust larval population beneath the bark. The agency also received many reports and photos this spring of larvae making it through the winter.

The University of Wisconsin Extension says insecticides can be effective, although not foolproof, by reducing the populations in individual trees, especially when dead branches compose less than 50% of the tree.

Extension specialists provide recommendations on the agency's website, either for the do-it-yourselfer or those who want to use an arborist.

McNee said because of cold spring temperatures, it’s not too late for an annual treatment of ash trees.

There had been hope that last winter’s brutal cold weather would kill ash borer populations. Although even then, experts said cold temperatures would have to plunge to minus 20 to minus 30 degrees for several days.

A U.S. Forest Service study in Minnesota estimated larvae mortality from a previous cold snap reached 5% at zero, climbed to 34% at 10 below and then to 79% at 20 below and 98% at 30 below.

The bark of ash trees serve as insulation, and before winter, larvae produce an antifreeze-like substance to withstand cold temperatures.

Last winter when McNee returned to his office from Brookfield, a few of the larvae were dark and appeared to have been affected by freezing temperatures. But the rest looked creamy white and seemed to be healthy. He incubated them in a sealed container. About a half-dozen survived.

(The insects were not released. After they die, McNee uses them for demonstrations.)

The first discovery of emerald ash borer in North America was near Detroit in 2002 and led to widespread devastation in vast stretches of Michigan. But based on a recent visit, McNee said, the region is rebounding as replacement trees take hold.

In Wisconsin, he said there is some evidence that infected trees continue to produce seed and some new ash trees are sprouting up in heavily infested areas.

“It doesn't mean that all of them will live, but it’s a good sign,” McNee said.