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Emerald ash borers are metallic green when mature. Their larvae damage and kill ash trees. (Photo courtesy of the North Dakota Department of Agriculture.)

Bitter cold won't stop emerald ash borers

It was tempting to hope that recent bitter cold in much of the Upper Midwest would end the growing threat of emerald ash borer. But though the extended cold wave may cut into emerald ash borer numbers, it won't destroy the pest altogether, an expert says.

"This (cold weather) will reduce the population, maybe even a lot. But it won't eliminate them," said Joseph Zeleznik, North Dakota State University Extension forester.

A native of Asia, the insects hadn't been found in North America until 2002. It's believed they arrived here in ash wood in packing crates. As of Feb. 1, the insects — sometimes known as EAB — have been found in 35 states, including Minnesota and South Dakota, and in Manitoba in Canada, according to the Emerald Ash Borer Information Network.

In 2017, they were found in Winnipeg, Manitoba, just 65 miles north of the Canadian border. That was especially concerning in North Dakota, to the south, where green ash is common in forest, windbreaks, conservation planting and communities in the relatively treeless state.

Emerald ash borers attack all types of ash in North America, and already has killed hundreds of millions of trees. The adult insect is dark metallic green, ½-inch long and ⅛-inch wide. The larvae (immature stage) do most of the damage, and are said to "strangle" trees by chewing through tissues that conduct nutrients.

Though extreme cold weather hurts the larvae, they're insulated by trees' outer bark, with larvae overwintering at the base of the trees below the snow line even better-insulated, making it highly unlikely that all the insects will be killed, according to the Minnesota Department of Natural Resources.

"We should not allow the cold to give us a false sense of security when it comes to actively managing Minnesota's ash resource," according to Minnesota DNR.

The threat from emerald ash borers illustrates the importance of diversifying trees species, limiting the damage that any one pest can do, Zeleznik said.

The Emerald Ash Borer Information Network, which draws on resources from multiple state universities and state and federal agencies, is a good general starting point to learn more about the insect and the threat it poses: www.emeraldashborer.info.

It's too early to predict how young or recently planted trees will be affected by the extreme cold weather. In general, trees on the edge of their geographic range are most likely to be hurt, Zeleznik said.

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