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, but the record-warm winter has left many farmers wondering if 2024 will be a severe year for insects, especially now that meteorological winter is over.
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. 20 had been the coldest night during winter 2023-24 with morning low air temperatures near minus 15 degrees F in the central portion of the state and below minus 20 degrees F farther north (Fig. 1). NOAA and the US National Phenology Network provide 1.6 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 minus 16 degrees F on Jan. 20, and we had a string of subzero nights Jan. 10-21. However, four inches below the soil surface, temperatures around this time were still only barely below freezing at 27 to 28 degrees F.
Figure 1. Morning lows on Jan. 20, 2024; temperature data produced by NOAA obtained from 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 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 help forecast insect freeze mortality, which is similar to how USDA plant hardiness zone maps are used.
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 minus 25 and minus 35 degrees F with most freezing around minus 29 degrees 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 anywhere in the region excluding a few very small pockets in Northwest Minnesota (Fig. 2).
Figure 2. Predicted soybean aphid cold-exposure mortality on Jan. 20, 2024 based on average the freezing point of overwintering eggs at minus 29 degrees F (std. dev.: 3.4) and minimum air temperature. Only areas in Northwest Minnesota that reached approximately minus 25 degrees F had detectable mortality (above 5 percent).
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 minus 30 degrees F to central Minnesota. That year was a very stark comparison for soybean aphid forecasts (Fig. 3) compared to this extremely mild winter of 2024 (Fig. 2).
Those few areas with predicted mortality in 2024 show just how much difference even a few degrees difference can make for insect overwintering where temperatures just barely reaching minus 25 degrees F caused perhaps near 10 percent aphid mortality. Meanwhile, widespread 2019 temperatures colder than minus 30 degrees F caused near 90 percent mortality in the northern half of the state. Those two years document different extremes in winter temperatures, but even only a few degrees for a winter low can have significant impacts on insect overwintering ability.
Figure 3. Predicted soybean aphid cold-exposure mortality on Jan. 31, 2019 based on average the freezing point of overwintering eggs at -29º F (std. dev.: 3.4) and minimum air temperature.
Since soybean aphid populations were starting to be more abundant during 2023 after years of relatively low populations, this winter may set up the 2024 growing season to be at higher risk for soybean aphid issues. 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.
Corn Rootworm
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. Western corn rootworm forecasting though is a bit more complicated.
Their eggs overwinter in the soil where they are protected from temperature extremes by the soil's insulation, especially last winter with deep snow cover. That's why accurate soil temperatures are needed to predict overwintering for these insects.
This winter, there was potential to have some soil-dwelling insect mortality with little to no snow cover if we ever had sustained cold temperatures. A majority of overwintering corn rootworm eggs will die after at least two weeks of exposure to 18.5 degrees F or colder. However, even with an extended period of subzero air temperature during January, four-inch depth soil temperatures at Morris remained near 30 degrees F most of the winter with a brief dip near the upper 20s. There likely won't be much rootworm cold mortality this year despite the lack of snow cover.
Alfalfa Weevil
Alfalfa weevil has been a resurgent pest for alfalfa growers in recent years. It overwinters as an adult in leaf litter or stubble in alfalfa fields or nearby protected grassy areas. During the winter of 2023, 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 degrees F at the soil surface to reach 20-30 percent alfalfa weevil mortality with a small subset able to survive down to around 1 degree F. It is 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, but I would expect to see some minor to moderate alfalfa weevil mortality this year in exposed areas that may slightly reduce weevil issues in some fields. However, I would still plan to scout heavily for it in fields that have had weevil problems in previous years.
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 degrees F in the fall. Other migratory pests such as black cutworm and true armyworm cannot survive winters with persistent freezing temperatures, so this record winter still would not allow these pests to survive our winter. The problem to watch though is that populations may be larger and start the season farther north than previous years, so monitoring for moth migration will 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 degrees F insulation to morning low air temperatures below 15 degrees F. Mortality starts to be noticeable around actual exposure temperatures of minus 22 degrees F, around half freeze at minus 24.5 degrees F, and a small percentage can even survive brief exposure near minus 40 degrees F. Larger diameter parts of trees can provide more insulation; smaller trees will be closer to air temperature. This year, don't expect much if any EAB overwintering mortality (Fig. 4). Only a few areas of north-central Minnesota would even experience 10 percent EAB cold mortality this year, though EAB has not been found in many of these counties, yet.
Figure 4. Predicted emerald ash borer (EAB) cold-exposure mortality on Jan. 20, 2024 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, it's not a surprise that our record-breaking winter didn't get cold enough significantly affect some of our common overwintering field crop insects, though just getting below freezing still prevents of some of our "snowbird" insect pests from having even larger populations during the growing season. It is concerning though to see just how widespread the effects of this warm winter have been for increasing the chances of insect issues.
Warm winter temperatures don't only indicate that more insects survive the winter, but a warm spring will also lead to insects showing up earlier this year. How early remains to be seen for Minnesota, but some estimates at the time of this posting were showing measures like first leaf emergence being 20+ days earlier than normal in states such as Missouri and Iowa. Some of those locations are showing the earliest emergence on record for these spring indicators. Pest emergence will likely be much earlier than normal if current weather trends continue to hold. 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.