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Cold kills some insects, but they'll be back

Cold kills some insects, but they'll be back

From the Deep freeze: Read the latest reports on dangerous winter weather in the Quad-Cities series
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Cold temperatures must be putting a dent in our bug population, right? So that next summer we can expect fewer Japanese beetles on our roses and a slowdown in the destruction of ash trees by emerald ash borers?

Probably not, said Donald Lewis, entomologist with Iowa State University Extension & Outreach in Ames.

"Expecting insect pests to freeze to death is mostly wishful thinking," he wrote in an email. "Some insects die every winter. We have fewer insects in the spring than in the fall because they die of exposure, dehydration, disease or cold.

"This natural die off does not eliminate the species. A few always survive to repopulate the next breeding cycle."

Two in particular worth noting:

Japanese beetles: These garden pests are responsible for eating hundreds of kinds of flowers (especially roses), fruits (especially raspberries) and trees (especially lindens). This doesn't kill the plants; it just makes them look bad.

The grub stage of these beetles is in the ground now, 4 to 6 inches deep in the soil. "They will survive and be unaffected by the brutal air temperatures above the snow," Lewis wrote. "Four years ago the Japanese beetle population declined over the winter because of the combination of a dry fall and prolonged deep ground freeze," which we don't have here in the Midwest yet, Lewis wrote.

And although Japanese beetle numbers were down in many places the season afterward, the population has completely recovered now, he said.

Emerald ash borer: Research cited by Lee Frelich, director for the Center for Forest Ecology at the University of Minnesota, suggested the EAB larvae that live under the bark of trees do indeed die in cold temperatures.

He says about 5 percent die at zero; 34 percent at minus 10, 79 percent at minus 20 and 98 percent at minus 30.

During a polar vortex experienced in January of 2014 in the Twin Cities, "we think there was major mortality (of EAB) and that gave us a few years longer to have our ash trees around," he said.

But by 2016-17, populations had rebounded to pre-2014 levels, so the decrease is only temporary, he said.

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