A Few Degrees Make a Difference

Just a few degrees can make a difference in winter survival rates for some pests. Dr. Anthony Hanson, University of Minnesota Extension Integrated Pest Management, reported that researchers found just 5 degrees makes a big difference in soybean aphid mortality rate.

When the winter low temperature hit -25°F in 2024 in Minnesota, researchers estimated a 10% mortality rate. In the exceptionally cold winter of 2019, temperatures dipped to below -30°F in the same area and the estimated mortality rate for soybean aphid skyrocketed to near 90%.

"Those two years document different extremes in winter temperatures where even only a few degrees for a winter low can have significant impacts on insect overwintering ability," says Hanson.

Soybean aphids are rarely a problem in home gardens, but we can apply the winter temperature research to common landscape pests and surmise that yes, extreme cold will make a difference in pests’ ability to successfully overwinter.

Overwintering Depends on More Than Temperature

Low temperature readings play a part in determining which pests successfully overwinter, but they don’t tell the whole story. Survival rate is influenced by a multitude of factors including duration of a cold period—a short stretch of low temperatures is not nearly as taxing on pests as a long cold period.

Timing of the extreme cold events is a factor too. If a cold snap strikes early in the winter season, some pests might not have moved into dormancy or their overwintering site, making them more vulnerable to cold conditions than they are later in the winter season.

Another factor influencing pest overwintering is the availability of habitat. Some pests overwinter in leaf litter and organic debris. If these materials aren’t available, pests are more susceptible to the elements.

Snow Mediates Cold Temps

Snow cover plays a big role too. Snow acts as an insulator; it moderates the temperature of everything underneath it, including the critters in their winter homes. Dr. John Tooker, professor of entomology at Penn State, highlights slugs as an example of a pest that benefits from snow cover.

"Higher amounts of snow insulate slugs against colder temperatures," Tooker says. The more snow on the ground, the more likely pests, including slugs, overwinter with ease.

Pest Adaptations

Insects and pests of all kinds have been dealing with cold weather challenges for eons. Some pests avoid the cold altogether by migrating to warmer climates where cold snaps are far less likely. Others have the ability to manufacture a chemical that works like antifreeze in their body to prevent freezing. And still other pests survive low temperatures by freezing. They use special proteins to regulate the way their body freezes, minimizing damage to cells.

Keep in mind that while the pests are adapting to cold temperatures, beneficial insects and predators are adapting too. If more pests survive winter than usual, it's likely that more beneficial insects and predators survive too. And the web of life continues.

Predictions Are Difficult

With a multitude of factors influencing pests’ ability to overwinter, it's tough to provide a definitive answer to how many pesky critters will be present in the landscape from year to year. One thing we can count on for sure: you’re bound to encounter a few garden pests no matter how low the mercury drops.

"For most species, the influence of cold winter temperatures and lots of snow is likely to be minor," Tooker says. "Many pest species are well adapted to living in our climate."