What does a record-warm winter mean for 2024 insect forecasts? (2024)

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 -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 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 -29º F (std. dev.: 3.4) and minimum air temperature. Only areas in Northwest Minnesota that reached approximately -25ºF had detectable mortality (above 5%).

For comparison in 2019, there was significant soybean aphid mortality throughout the state. This was the year we had a strong polar vortex thatbrought temperatures near -30ºF to central Minnesota.That year was a very stark comparison for soybean aphid forecasts (Fig. 3) comparedto 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 -25ºF caused perhaps near 10% aphid mortality. Meanwhile, 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, 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 moreabundant during 2023 after years of relatively low populations, this winter mayset 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 aphidpopulations. Even with increased aphid risk, this does not mean there will be a benefitto preventative insecticide use. Insecticidal seed treatments generally aren’t effective for soybean aphid as the seed treatment loses efficacy after about40 days after planting, and aphids typically move from buckthorn to soybeansafter this time. Instead, be proactive about scouting this year. This may also be a good year to consider aphid-resistant varietiesif available in your area.

Corn rootworm

Where insects overwinter also affects mortality, so insectslike soybean aphid that are primarily exposed to air temperatures during winterare a bit more straightforward to forecast. Western corn rootworm forecastingthough is a bit more complicated. Theireggs overwinter in the soil where they are protected from temperature extremesby 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 insectmortality with little to no snow cover if we ever had sustained coldtemperatures. A majority of overwintering corn rootworm eggs will die after atleast two weeks of exposure to 18.5 °F or colder. However, even with an extendedperiod of subzero air temperature during January, four-inch depth soiltemperatures at Morris remained near 30 °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 growersin recent years. It overwinters as an adult in leaf litter or stubble inalfalfa fields or nearby protected grassy areas. During the winter of 2023, weevils were well-protected and insulated by snow cover from temperatures that wouldcause it to freeze. As a tradeoff, that same insulation also protects alfalfa from winterkillas 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 reach20-30% alfalfa weevil mortality with a small subset able to survive down toaround 1 °F. It is difficult to get accurate widespread estimates of temperaturejust at the soil surface due to variation in vegetative cover and other insulatingeffects of the soil, but I would expect to see some minor to moderate alfalfa weevil mortalitythis year in exposed areas that may slightly reduce weevil issues in somefields. However, I would still plan to scout heavily for it in fields that have had weevilproblems 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 Mexicoeach year. It only overwinters in areas with at least 260-270 frost-free daysand 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 survivewinters with persistent freezing temperatures, so this record winter stillwould not allow these pests to survive our winter. The problem to watch thoughis that populations may be larger and start the season farther north than previous years, so monitoringfor 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 °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, don't expect much if any EAB overwintering mortality (Fig. 4). Only a few areas of north-central Minnesota would even experience 10% 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 tosurvive winter in Minnesota or contact Anthony Hanson at hans4022@umn.edu.