Is it cold enough yet? Insect overwintering forecasts for 2026

Anthony Hanson, Regional Extension Educator - Field Crops Integrated Pest Management

Cold winters help prevent many potential pest insects from being an issue in Minnesota. Many questions have been coming in from farmers about how how the January cold-snap might affect insects lately, and even though meteorological winter is not over yet, the recent January cold snap should give us a good idea of how much effect winter had on overwintering insects.

Each year, I try to get a rough snapshot of how winter may have helped us out with reducing pest insect populations by using temperatures on the coldest night of the year. So far for most of the state, Jan. 23 was the coldest night during winter 2025-26 with morning low air temperatures near -20 °F in the southern and central portions of the state and below -30 °F in roughly the northeastern quarter of the state (Fig. 1). Most areas in and near Minnesota reached their winter low (so far) the day prior or after Jan 23. The National Oceanic and Atmospheric Administration (NOAA) provides approximately one square-mile resolution daily temperature data used in Fig. 1 that helps give a region-wide picture of daily high and low temperatures. This dataset is generally fairly accurate being within a couple degrees of individual weather stations. Here at the U of M's West Central Research and Outreach Center in Morris, the low air temperature was -19.4 °F on Jan. 23.

Figure 1. Approximate coldest temperature experienced between Nov. 2025 through Feb. 1, 2026 based on Jan. 22-24 nightly lows. Temperature data provided by NOAA using the Unrestricted Mesoscale Analysis  dataset. Click the map for higher resolution imagery.

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. Even so, many insects can survive temperatures well-below freezing due to antifreeze compounds like glycerol that lower the freezing point of water in their bodies similar to antifreeze in a car. For many insects, like soybean aphid, we can forecast mortality based on when ice does form in their body. The minimum winter air temperature each year can be a simple way to forecast acute insect freeze mortality, which is similar to how USDA plant hardiness zone maps are used. Insects can also die from exposure to cold temperatures that do not immediately kill them, but instead, cold mortality often occurs after weeks of exposure. This can be more complicated to forecast compared to acute mortality.

Soybean aphid

This year, very little mortality for soybean aphid is expected where much of the state's soybeans are grown in the southern half of the state and along the Red River Valley. (Fig. 2). Much of north-central and northeast MN will see between 50-95% mortality. Soybean aphid overwintering predictions are a little easier to make with air temperature alone because they overwinter as eggs on buckthorn buds where there is little protection from cold exposure unless small plants are under insulating snow cover. Eggs will freeze between -25 and -35 °F with most freezing around -29 °F. Some egg mortality can also occur above freezing due to dehydration and late-fall cold snaps. 

Soybean aphids may be slightly slower to build up in northern areas of Minnesota that saw over 50% mortality, though remaining aphids and those migrating in from farther south still can pose a risk to soybean crops that will warrant scouting in 2026. For other states, northern Wisconsin also had cold enough temperatures for significant mortality. However, other soybean growing areas of the US largely did not even approach these temperatures.

Figure 2. Predicted soybean aphid acute cold-exposure mortality from coldest winter temperature experienced based on average the freezing point of overwintering eggs at -29º F (std. dev.: 3.4) and minimum air temperature. Click the map for higher resolution imagery.

For comparison in 2019, there was significant soybean aphid mortality throughout the state. This was the year we had a strong polar vortex that brought temperatures near -30ºF to central Minnesota, especially compared to the extremely mild winter of 2024. Even a few degrees difference can significantly change insect overwintering outcomes where temperatures just barely reaching -25ºF caused perhaps near 10%  aphid mortality. Widespread 2019 temperatures colder than -30ºF caused near 90% mortality in the northern half of the state. 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. This winter's predictions aren't as welcoming for farmers as they were in 2019, but they are an improvement over 2024. 

Overwintering is only one piece of the puzzle for affecting pest populations, so it’s possible other factors like beneficial insects and pathogens might help suppress aphid populations. Even with increased aphid risk, this does not mean there will be a benefit to preventative insecticide use. Insecticidal seed treatments generally aren’t effective for soybean aphid as the seed treatment loses efficacy after about 40 days after planting, and aphids typically move from buckthorn to soybeans after this time. Instead, be proactive about scouting this year.  This may also be a good year to consider aphid-resistant varieties if available in your area.

Alfalfa weevil

Where insects overwinter also affects mortality, so insects like soybean aphid that are primarily exposed to air temperatures during winter are a bit more straightforward to forecast. Others are not so straightforward to provide statewide forecasts, but you can get an idea of risk from local soil temperature data. Alfalfa weevil has been a resurgent pest for alfalfa growers in recent years. It overwinters as an adult in leaf litter, stubble in alfalfa fields or nearby protected grassy areas. During deep snow cover years like 2023, these weevils were well-protected and insulated by snow cover from temperatures that would cause it to freeze. As a tradeoff, that same insulation also protects alfalfa from winterkill as long as the plants don't go through too many freeze-thaw cycles in spring.

It takes exposure around 13 °F at the soil surface to reach 20-30% alfalfa weevil mortality with a small subset able to survive down to around 1 °F. However, there aren't readily available alfalfa weevil mortality models that capture other parts of the temperature range. It is also difficult to get accurate widespread estimates of temperature just at the soil surface due to variation in vegetative cover and other insulating effects of the soil, though weather stations with 2-inch soil temperature data can be a conservative estimate. With 2-inch soil temperatures at Morris in west-central MN reaching their minimum so far of 15.4 °F on Jan. 31, I wouldn't expect much mortality this year. Areas farther north and with less snow cover might see slight reductions in alfalfa weevil pressure in 2026, though I would not be pulling back on alfalfa scouting plans.

Corn rootworm

Corn rootworm forecasting is a bit more complicated than alfalfa weevil.  Their eggs overwinter in the soil where they are protected from temperature extremes by the soil's insulation, especially in years with deep snow cover.  Eggs are typically found at 4 to 6 inch depth, but during dry years where cracks in the soil are present, eggs can be found up to a foot deep. That is why accurate soil temperatures are needed to predict overwintering for these insects. 

There are two species of corn rootworm in Minnesota, the northern and western corn rootworm that vary in their cold tolerance. About 40% of western corn rootworm eggs survive after exposure to 10.4 °F, but egg survival drops to near 0% at 0.5 °F. Under dry conditions, about 55-50% of northern corn rootworm eggs can still hatch after brief exposure to 10.4 and 0.5 °F, though under wet conditions, hatch rates for those same temperatures drop to about 20%. Exposure temperatures of -6.7 °F across soil moisture conditions reduce northern corn rootworm hatch rates to 5%. At Morris, the coldest soil temperatures recorded up to Feb. 1, 2026 for 4 and 8-inch depths were 19.4 and 23.0 °F, respectively. At current winter lows, not much acute mortality should be expected for corn rootworm.

Duration of cold exposure can also be used to predict corn rootworm mortality, though these types of models are complicated by winters that vary significantly in soil temperature over time. A general rule of thumb across rootworm species is that around half of overwintering corn rootworm eggs will fail to hatch after at least two weeks of exposure to 18.5 °F or colder. Across the entire winter, 4-inch soil temperatures at Morris have note quite reached this temperature yet, much less for two weeks.

Emerald ash borer

While it's not a field crop pest, I often get questions if winter will cut back on emerald ash borer (EAB) populations when preparing field crop forecasts. EAB overwinters a couple inches underneath the bark of ash trees, which adds 2 – 6 °F insulation to morning low air temperatures below 15 °F. EAB cold mortality starts to be noticeable around actual exposure temperatures (i.e., not accounting for insulation from trees) of -22 °F, around half freeze at -24.5 °F, and a small percentage can even survive brief exposure near -40 °F. Larger diameter parts of trees can provide more insulation; smaller trees will be closer to air temperature. 

This year, most of the northern half of the state will have 25% or more predicted EAB mortality (Fig. 3) after adjusting for tree insulation. EAB has become widespread across much of the state now, so this recent cold-snap could help slow down EAB in north-central and northeastern MN. Unfortunately in areas where EAB is most prevalent in the southern half of the state, especially the southeast, expected mortality around 10%. This includes areas of western MN and especially Fargo, ND where EAB was just recently confirmed.

Figure 3. Predicted emerald ash borer (EAB) acute cold-exposure mortality from coldest winter temperature experienced based on average freezing point -24.5 °F (std. dev.: 6.6), minimum air temperature, and added average insulation between green and black ash species at 4.6 ft height. Click the map for higher resolution imagery.

Growing season outlook

The story for this winter so far is that while January seemed cold to us humans, it wasn't quite cold enough to set back most of our regular insect pests in most of the state except in northeast MN. A good rule of thumb in Minnesota is if air temperatures reach -30°F in your area, you'll likely see some reduced insect populations, though this is just a starting point. Some migratory pests like potato leafhopper, black cutworm, or true armyworm generally can't survive even warmer Minnesota winters, but will migrate from the south again in the spring .Winter isn't over yet though, so if we see colder weather than we had in January, they will be another pest forecast update.

Read more about insect overwintering at Tough Buggers: Insect strategies to survive winter in Minnesota or contact Dr. Anthony Hanson at hans4022@umn.edu.

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).