Wallaces Farmer

Extreme cold can impact some insects and plant pathogens, but others stay hunkered down and survive.

Rod Swoboda

February 27, 2019

3 Min Read
farmstead in the winter
A BLANKET: Rootworm eggs that are buried more deeply in the soil, especially with snow cover, are less likely to be killed by extremely cold temperatures.

You wonder what effect this winter’s extremely cold temperatures might have on insect pests and crop disease pathogens. It would be nice if the frigid winter would help reduce overwintering pest populations and lessen the potential for damage this coming growing season, but that’s not necessarily a given.

Entomologists and plant pathologists say, based on past experience, the most serious crop pests Iowa farmers deal with are unlikely to be impacted much by these conditions.

Many of the insects and disease pathogens in Iowa and the Upper Midwest are adapted to survive cold winters. Extreme cold can impact some insects and plant pathogens, however. Different species of insects overwinter in different life stages. For example, corn rootworm eggs overwinter in the soil, but migratory insects such as black cutworm and fall armyworm don’t overwinter here because they can’t survive. They migrate into Iowa each spring from Southern states or Mexico.

Some survive, some can’t
“It is not easy for insects to survive Iowa winters,” says Erin Hodgson, Iowa State University Extension entomologist. “Some literally can’t; they freeze to death [corn earworm, black cutworm] or migrate to warmer climates [potato leafhopper].”

Insects are ectotherms. They are unlike mammals and birds because they must generate their own heat, Hodgson says. “Insects die when they are exposed to temperatures below the melting point of their body fluids, termed the lower lethal temperature. Over time, insects have developed several strategies to survive cold temperatures, and none of them involve wearing fleece.

“Many insects have adapted to cold temperatures by entering diapause, which is like hibernation. Diapause is a developmental arrest to survive adverse seasons where individuals slow down metabolic activity.”

Insect survival strategies
To prepare for diapause, insects store energy reserves and move to protected sites, such as in the soil, under leaf litter or inside plants. Some species aggregate together, such as multicolored Asian lady beetle, Hodgson says. “Aggregation in the winter happens for many reasons but usually masses of the insects form as a result of chemical communication within the species. Diapause is specific to a certain life stage for insects. For example, European corn borer overwinters as fifth instars; bean leaf beetles overwinter as adults; and soybean aphid overwinters as eggs.”

Insects can use two survival strategies for cold temperatures: freeze avoidance or freeze tolerance. “Freeze-avoidant insects keep their bodily fluids liquid, and freeze-tolerant insects can handle the formation of internal ice,” she says. “Insects in Iowa generally avoid freezing because the winter lasts for a long time. Some freeze-avoidant species adjust or adapt for winter by creating cryoprotectants, or antifreeze proteins, in their body, usually 20% to 30% of fresh body weight.”

Fluctuating temperatures
Generally, warmer winters mean more insects are likely to survive because they aren’t exposed to lower lethal temperatures, Hodgson  says. “A winter with widely fluctuating temperatures can be difficult for insects. Insects need time to gradually prepare for winter, and repeated cold and warm cycles can eventually burn up all their stored energy reserves. The polar vortex of 2019 produced extremely harsh air temperatures and wind chill factors.”

This winter certainly killed those insects susceptible to cold temperatures, Hodgson says. But the insects that are overwintering in the soil are insulated from air temperatures. Snow and crop residue can add layers of protection, too.

“No matter the overwintering strategy, all insects will eventually die if it gets cold enough,” she says. “I anticipate a drop in field crop pest activity, but we will know more as spring approaches.”

 

 

 

About the Author(s)

Rod Swoboda

Rod Swoboda is a former editor of Wallaces Farmer and is now retired.

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