Was this winter cold enough to reduce insect populations in 2025?

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

Cold winters help prevent many potential pest insects from establishing in Minnesota or require species that cannot survive our winters to migrate up from southern states each year. Extreme cold can also knock back species that are established here. The cold can be a welcome event for farmers from a pest management perspective and has left many farmers wondering if 2025 will see a reduction in pest numbers, especially now that meteorological winter is over. The forecasts for the 2025 growing season are mixed depending on specific location within the state.

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. 21 had been the coldest night during winter 2024-25 with morning low air temperatures near -25 °F in the central portion of the state and below -30 °F in a few areas farther north (Fig. 1). The US National Phenology Network and the federal National Oceanic and Atmospheric Administration (NOAA) provide 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 interpolated data is generally within a couple degrees of individual weather stations. Here at the U of M's West Central Research and Outreach Center in Morris, the morning low was -24 °F on Jan. 21, and we had a string of subzero nights prior to to the 21st.

Figure 1. Coldest temperature experienced between Nov. 2024 through Feb. 2025. Jan. 21 was the coldest night for most of the Minnesota. Temperature data produced by NOAA obtained through the U.S. National Phenology Network.

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

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. 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. (Fig. 2). 

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. 

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 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. (Fig. 2). 

Since soybean aphid populations were starting to be more abundant during 2024 after years of relatively low populations, the 2025 growing season may be at higher risk for soybean aphid issues, especially in southern parts of the state. Soybean aphids may be slightly slower to build up in northern areas that saw 25 to 50% mortality, but remaining aphids and those migrating in from farther south still can pose a risk to soybean crops that will warrant scouting in 2025.

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 reaching their minimum of 5.3 °F on Jan. 21, I would expect to see significant mortality in that area, especially since the soil surface would be even colder at that time. Even in other nearby areas in west-central Minnesota, soil temperatures can vary significantly from that Morris reading depending on the amount of snow or vegetative cover, so there are still likely refuge areas where alfalfa weevil had some survival. I would still plan to scout heavily for it in fields that have had weevil problems in previous years, though this is one of the few years where there was both enough cold and lack of snow cover to suggest we may see a reduction in alfalfa weevil populations in some areas.

Figure 3. Hourly soil temperatures from Dec. 4, 2024 to March 4, 2025 at 2 and 4 inches at West Central Research and Outreach Center weather station at Morris, MN. Despite prolonged cold air temperatures in February, the coldest soil temperatures were actually recorded in January when less snow was present.

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 hatch 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 March 7, 2025 for 4 and 8-inch depths were 10.4 and 16.8 °F, respectively. For shallow eggs at 4-inch depths, this should result in about 50% acute cold mortality for those eggs. Deeper eggs likely had more insulation and less mortality.

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, soil temperatures at Morris were under this threshold for 1.5 weeks at 4-inch depths. For western corn rootworm, duration-based cold mortality predictions would be 50% (similar to acute mortality estimates), while the more cold-hardy northern corn rootworm would have less mortality from those 1.5 weeks.

Overall, this winter may somewhat reduce corn rootworm populations, but problem fields with high populations in 2024 may still be at risk in 2025, especially since a subset of the population is likely overwintering in deeper and warmer soils.

Migratory crop pests

Potato leafhopper affects crops such as soybeans, edible beans, alfalfa, and potatoes, though it typically has to migrate up from states bordering the Gulf of Mexico each year. It only overwinters in areas with at least 260-270 frost-free days and is usually no longer found in Minnesota once temperatures reach 20 °F in the fall. Other migratory pests such as black cutworm and true armyworm cannot survive winters with persistent freezing temperatures, so our colder winter this year would not allow these pests to survive Minnesota winters. Some of these pests may be pushed farther south than last year depending on cold exposure in southern states, but they will eventually return to Minnesota, though hopefully in lower initial numbers. Monitoring for moth migration will still be important this year, such as the UMN Black Cutworm Reporting Network.

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. 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. Mortality starts to be noticeable around actual exposure temperatures 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. 4). Most areas with 50% predicted mortality in northern counties do not have confirmed cases of EAB, though some areas such as Hibbing and Cass counties with EAB finds do fall into this range. Unfortunately in areas where EAB is most prevalent in the southern half of the state, especially the southeast, expected mortality is 10% or less.

Figure 4. 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.

Growing season outlook

In short, this was a complicated winter for pest forecasting. We experienced prolonged cold snaps this winter that may have made it seem like we had an extremely cold winter for humans. 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. There's no one pest insect in these forecasts where it can be said the risk for 2025 is greatly reduced across the state as it did not get that cold. Years with moderate mortality like this though can be one way to have slight reductions in populations that can mean the difference between a farmer having a economically damaging levels of a pest versus tolerable populations that aren't worth treating with pesticides.

While we're moving out of winter, it's too early to say when the growing season will start. How early remains to be seen for Minnesota as some estimates at the time of this posting were showing measures like first leaf emergence fluctuating between being 20 days earlier and 20 days late in southern states. University of Minnesota Extension also tracks temperature-based forecasts for insect pests of field and specialty crops, so more updates will be coming this spring.

Read more about insect overwintering at Tough Buggers: Insect strategies to survive winter in Minnesota or contact 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).