Insect forecasting for 2023: was winter cold enough to help with Minnesota pests?

Dr. Anthony Hanson

Extension Educator & Assistant Professor - 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, like potato leaf hopper or black cutworm, to migrate up from southern states. Extreme cold can also knock back species that are established here. Generally, cold weather is troublesome for farmers and livestock, but the cold can be a welcome event for pest management.

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. 2019 was a prominent example where much of the state was below -30°F which caused significant mortality for pests like soybean aphid. So far, Feb. 3 has been the coldest day during winter 2022-23 for much of the state with morning low air temperatures below -20°F in the northern half of the state and near -15°F in the south (Fig. 1).

Figure 1. Morning lows on Feb. 3, 2023; temperature data from U.S. National Phenology Network

There were a number of cold snaps this year, but even this most recent one didn't quite hit -30°F except for a few areas in north-central MN. Combined with the extended cold, the heavy snow cover this year adds another complicating layer to forecasting. It makes more work for us humans, but actually gives insects under the snow some insulation. At the U of M's West-Central Research and Outreach Center in Morris, we saw a low of -16°F on Feb. 3, but four inches below the soil surface, temperatures were still only barely below freezing at 30°F and have only ranged between 32 to 30°F there since Dec. 1.

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 matter to 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.

Soybean aphid

The minimum winter air temperature each year (so far, around Feb. 3 this year) can help forecast insect freeze mortality (Fig. 2), which is similar to how USDA plant hardiness zone maps are used. Soybean aphid predictions are a little easier to make with air temperature because they overwinter as eggs on buckthorn buds where there is little protection from cold exposure unless they are under insulating snow cover. Eggs will freeze between -25 and -35 °F. Some egg mortality can also occur above freezing due to dehydration and late-fall cold snaps.

Figure 2. Predicted soybean aphid cold-exposure mortality on Feb. 3, 2023 based on average the freezing point of overwintering eggs at -29º F (std. dev.: 3.4) and minimum air temperature.

Winter isn’t quite over yet, so we could see even colder temperatures that could change how these maps look. Overall, there has been very little soybean aphid mortality outside the northern third of the state. Sustained low temperatures just above lethal temperatures can cause more mortality than what is projected on the map (e.g. multiple nights below -20 degrees F). However, this type of extended stress is much more difficult to use to predict pest risk than using the coldest night of the year as a ballpark.  

Overwintering mortality is only one piece of the puzzle when forecasting if an insect pest will be a problem. In the case of soybean aphid, winged aphids are very mobile and can colonize new areas by field-to-field flight or long distance travel on air currents, which can vary year-to-year. Many insect populations can grow quickly, so even a small percent of the surviving population can still cause problems for soybean growers under the right conditions. While the risk may be a bit more elevated this year due to relatively warm winter temperatures (for the aphids at least), it's not a guarantee we'll see more problem fields with aphids this year. Populations have been relatively low with only scattered problem fields across the state in the last few years, so at least we aren't in the highest risk scenario of coming into the growing season with high populations from last year with little cold mortality. Start scouting early in the year so you can stay on top of soybean aphid populations that may develop 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 a bit more complicated.  Their eggs overwinter in the soil where they are protected from temperature extremes by the soil's insulation, especially this year when there is heavy snow cover adding extra insulation. A majority of overwintering corn rootworm eggs will die after at least two weeks of exposure to 18.5°F or colder, though with soil temperatures remaining near 30°F, there likely won't be much rootworm mortality due to the cold this year.

Alfalfa weevil

Alfalfa weevil has been a resurging pest for alfalfa growers in recent years. It overwinters as an adult in leaf litter or stubble either in alfalfa fields or nearby protected grassy areas, so it is currently well-protected and insulated by snow cover from temperatures that would cause it to freeze. 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. Considering how much insulating snow cover we had throughout this winter, and how warm the soil still is I would not expect much help from winter with alfalfa weevil this year. 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 this spring.

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. Emerald ash borer (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 -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. Unfortunately in southern areas of the state where EAB has been found, don't expect much if any EAB overwintering mortality (Fig. 3). The Duluth area is the only other part of the state with known EAB finds where mortality might range in the 10-25% range.

Figure. 3 Predicted EAB cold-exposure mortality on Jan 31, 2019 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.

In short, it didn't get cold enough this winter to push back some of our common insect pests, and the snow cover gave a bit more protection than in recent years unless you were in an area with continually exposed ground.

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

References

1. Armbrust et al. 1969. Lethal limits of low temperature for the alfalfa weevil in Illinois. Journal of Economic Entomology 62: 463-467. 
2. Christianson, L.D.E. & R. C. Venette. 2018. Modest effects of host on the cold hardiness of emerald ash borer. Forests 9: 346.   
3. McCornack et al. 2005. Physiological constraints on the overwintering potential of the soybean aphid (Homoptera: Aphididae). Environmental Entomology 34: 235-240.  
4. Woodson, W.D., & R.D. Gustin 1993. Low temperature effects on hatch of western corn rootworm eggs (Coloptera: Chrysomelidae). Journal of the Kansas Entomological Society 66: 104-107