Pest forecasts heating up after a relatively cold winter

• Cold winters help prevent many potential pest insects from establishing or reaching large populations in Minnesota.
• Despite the problems the cold can cause for farmers, gardeners, and homeowners, a cold winter can be a welcome event from a pest management perspective.

Each year, Anthony Hanson, an entomologist and University of Minnesota Extension educator, provides overwintering forecasts for some of these insects, often based on UMN research, to assist farmers in decision-making. The forecasts are supported by a USDA grant and weather data from NOAA. 

Hanson says he has received many questions this winter on how much the cold will help with insect issues in 2025, “Winter lows usually need to get closer to -25 or -30°F to have really interesting forecasts. Unfortunately, the southern third of the state just didn’t get that cold from many insects’ perspectives, but this changes as you move north.”

Insect cold tolerance

For the most part, insects match the temperature of their surrounding environment, making them “cold-blooded.” Unlike warm-blooded animals, wind chill doesn’t affect most insects, but air temperature does. A major exception is honey bees, which produce their own heat and can be significantly impacted by cold and windy winters like this past year if they do not have enough food to use as a heat source.

Insects use a few strategies to survive winter in Minnesota. Many insects can survive temperatures well below freezing due to compounds that lower the freezing point of water in their bodies, similar to antifreeze in a car. Goldenrod gall fly is a great example of a native insect sometimes collected for fishing bait that uses these compounds to avoid ice formation in its body until -40 °F or colder.

While some insects adapted to survive ice forming in their bodies, others that are less tolerant of cold migrate, such as monarch butterflies. Others seek refuge in warm protected areas inside trees or our homes like multicolored Asian lady beetles. Quite a few crop and garden pests like black cutworm die out entirely each winter In Minnesota simply migrate back spring from southern states where they survived.

Crop insect forecasts

One of the simplest ways to forecast insect mortality is by using the minimum winter temperature, similar to plant hardiness zones. Soybean aphid is a major crop pest that avoids freezing. Hanson says, “If you know when they freeze, you can get an idea of how many survived the winter.” Soybean aphid eggs overwinter on buckthorn buds and freeze between -25 and -35 °F, with most freezing around -29 °F.

This year, very little mortality for soybean aphid is expected in the southern half of the state, though some areas of north-central Minnesota will see 25% mortality or more. For comparison, in 2019, a strong polar vortex brought temperatures near -30ºF to central Minnesota. That year, there was significant soybean aphid mortality throughout the state, especially compared to the extremely mild winter of 2024.

Even a few degrees of difference can significantly change insect overwintering outcomes. Hanson says, “These maps can be valuable for season-long planning, so I like to show farmers what I’d plan for on my family’s farm as an example.”

 A more detailed 2025 field crops pest forecast is available on Minnesota Crop News.

Forest insects

Where insects overwinter also can affect forecasts. Emerald ash borer is a well-known invasive pest of ash trees that overwinters a couple of inches underneath the bark of ash trees, which adds about 2 to 6 degrees of insulation to morning low air temperatures. 

Mortality starts to be noticeable when these insects experience temperatures of -22 °F, around half freeze at -24.5 °F, and a small percentage can even survive brief exposure near -40 °F.

This year, most of the northern half of the state will have 25% or more predicted EAB mortality. Unfortunately, in areas where EAB is most prevalent in the southern half of the state, the expected mortality is 10% or less.

Soil insects

Other insects overwinter in the soil, such as Colorado potato beetle, Japanese beetle, or corn rootworm and can affect a wide variety of field crops, garden plants, and turfgrass. During deep snow cover years like 2023, soil-dwelling insects were well insulated from the cold.

Due to the lack of snow this winter, upper soil temperatures reached single digits in many areas of west-central Minnesota. Corn rootworm eggs closer to the surface may see near 40-50% mortality from this, but eggs 6 inches or deeper were likely unaffected.

Turf pests like Japanese beetle do not tolerate soil temperatures below 30°F very well, though many of those beetles also burrow deep into the soil before emerging in spring.

Human pests

Nuisance insects like mosquitoes or deer flies are often at the top of people’s minds when hoping a cold winter will knock back insect populations. Many species of both those types of insects are actually well-protected from the cold because they overwinter in aquatic areas and are not exposed to severe temperatures.

Some species of mosquitoes overwinter as adults in protected areas like logs, burrows, and basements, making winter forecasting difficult. Rainfall and standing water are usually better indicators of mosquito populations.

2025 outlook

For many pest insects, this winter may have set populations back somewhat, but it wasn’t a particularly extreme winter for them either. The most impactful part of this winter was the combination of timely cold snaps when there was little to no snow cover that pushed soil temperatures down to levels we have not seen in a few years.

For farmers in the northern half of the state, they may see some partial relief from insect pests this year.

This work was supported by a federal grant from USDA National Institute of Food and Agriculture: Crop Protection and Pest Management Extension Implementation Program (grant # 2024-70006-43568).