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Will colder weather cause less insect damage to plants?

April 14, 2018 GMT

After a fairly cold winter and with having colder than average temperatures on some spring days, a question asked is if we will see fewer insects causing damage.

I wish I could say yes, at least for harmful insects since so many insects are beneficial; but the answer is most likely not.

Most insects have developed mechanisms for cold temperature survival. They lower their bodies supercooling temperature; overwinter in a phase, such as eggs, that is less affected by temperature; burrow deeply into soil or move into other protected areas.

According to Dr. Timothy Gibbs with Purdue Extension, insects suspend their usual activities and enter a dormant state to pass the cold winter months, but they do not hibernate like mammals. Instead, they produce a substance called glycerin in their blood supply that prevents freezing.

Glycerin is similar to ethylene glycol or antifreeze that we put in our cars. Glycerin in insect blood gives the insect body a supercooling ability that allows its body fluids to drop below freezing without ice crystals forming that can rupture cells and injure organs.

Individual insect species have a set supercooling temperature. Cold temperature injury is unlikely to occur to insects much above the supercooling threshold. I will use emerald ash borer (EAB) as an example.

While EAB has not yet been found in Platte County, research in other states has been done to look at cold weather effects on EAB. Research found that emerald ash borer supercooled to 25 degrees F below zero, with little mortality above that temperature.

Unless winter temperatures consistently fall to below minus 25 degrees F, cold winters will not have an effect on EAB. And the same is true for many other insect species.

Some insects migrate deep into soil to escape freezing temperatures. If they do not migrate deep enough, and soils remain frozen long enough, they will begin to die. Insects instinctively know how deep to migrate, such as the turfgrass pest annual white grub (larvae of masked chafer beetles).

Another insect that has been trapped in Platte County, but not yet been found in high enough numbers to have caused damage, is Japanese beetle. The larvae of Japanese beetle is also a grub. Research shows these specific grubs do not migrate deeper than 11 inches.

If Japanese beetle grubs are frozen for at least three weeks, they can die. In previous years, deeply frozen soils for several weeks reduced Japanese beetle populations by about two-thirds. This is one example of a soil overwintering insect that could be affected by cold temperatures.

However, both EAB and Japanese beetles are not native to Nebraska, or even the United States. Many of our pests are native and have the mechanisms needed to survive. Don’t expect to see fewer insect pests as a result of our somewhat cold winter.

As usual, practice good sanitation in yards and gardens to help reduce overwintering insects in an area. Select plants known to have few insect pest problems. Use good care practices that promote vigorous, but healthy growth that is more tolerant of insect damage.

Know what insect pests are common in the area on the types of plants you have. This will help you know what signs and symptoms to watch for so a pest is more likely to be found before it reaches damaging population levels.