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I have recently received several questions about the extreme cold temperatures and insect populations. While it would seem impossible for insects to overwinter under these conditions, Ken Johnson, horticulture educator posted an explanation on his blog page.

In his article, Ken said, “I hate to be the bearer of bad news, or perhaps good news (depending on your thoughts on insects) but, for the most part, most insects will survive just fine. Insects use a variety of strategies to survive through the winter”.

The first strategy some insects will use is to avoid the cold altogether. Some insects that we find in Illinois won't survive our freezing winter temperatures. So, instead of trying to make it through the winter here, they'll survive elsewhere. An example of this would be monarch butterflies, which migrate to Mexico for the winter and return in the spring. Other insects using this strategy include armyworms and potato leafhoppers, with populations moving north from southern states in the spring.

For those insects that stick around, low temperatures aren't necessarily the problem; the formation of ice crystals in their bodies is what makes survival difficult. If ice crystals rapidly form in their bodies, their cells will burst, resulting in damage and likely death. Some insects, like woolly bear caterpillars, will avoid this by using chemicals to control the way in which and where they freeze and therefore minimize damage to their cells.

Other insects produce chemicals in their bodies to avoid freezing altogether. As temperatures begin to cool, these insects will start creating anti-freeze chemicals. These chemicals allow the insects bodies' to supercool (reach temperatures below freezing, 32ᵒF). Therefore, the insects won't freeze until they reach their supercooling point. Some insects that take this approach include Japanese beetles, emerald ash borer, and codling moth.

Just because temperatures don't hit the supercooling point of an insect it doesn't mean some won't be killed. For example, a study by the U.S. Forest Service showed that 5% of emerald ash borer larva die at 0ᵒF, 34% at -10ᵒF, 79% at -20ᵒF and 98% at -30ᵒF.

While air temperatures in many places in Illinois got colder than the above supercooling points, most insects will overwinter in protected areas where temperatures did not get nearly as cold. For example, white grubs in the soil are insulated from cold temperatures by the soil. Emerald ash borer overwinters under tree bark, also benefiting from the insulation it provides. Additionally, snow is also an excellent insulator, and all of our snow offered another layer of protection.

While the brutally cold temperatures may have seemed like they lasted forever, in reality, they only lasted a relatively short time and likely didn't drop temperatures in these protected areas low enough to 'wipe out' these pest insects. So, while the cold temperatures we've had have likely killed some insects, come spring it should be business as usual.

For more information on University of Illinois Extension programming in Coles, Cumberland, Douglas, Moultrie and Shelby counties, visit our website at or call us at (217)345-7034.

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