What a Cold Winter Can Mean for Insects Like Soybean Aphid

February 2022

By Anthony Hanson, U of MN Extension Educator, IPM 

U of MN Extension graphic

First, I should introduce myself as a somewhat new face at the Morris WCROC. I’m Anthony Hanson, a Regional Extension Educator for Integrated Pest Management in Field Crops. I grew up farming near Brooten, MN, went to undergrad at Concordia College in Moorhead, and received a Master’s and PhD in Entomology from the U of MN in the Twin Cities, so I never strayed too far from my home turf in central MN. I cover crop issues dealing mostly with insects, but I’ll also tackle issues growers might have with other pests and the tools we use ranging from pesticides, plant genetics, scouting, or in this case, how cold weather can play a role in pest forecasting.

On Jan. 7, 2022, much of southern Minnesota saw morning low temperatures near -15°F, and portions of central and north-central MN saw lows near or below -30°F (Fig. 1). Even in my normal drive from Stearns through Pope to Stevens county that morning, there was a temperature gradient where the Morris area escaped some of the cold. Generally, this degree of cold is troublesome for farmers with alfalfa or livestock, but the cold can be a welcome event for pest management.

Cold winters help prevent many potential pest insects from establishing here or requires 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.

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 their own antifreeze compounds like glycerol. 

Where insects overwinter also affects mortality. Some insects, like corn rootworm, overwinter in in the soil, where they are protected from temperature extremes, especially when there is heavy snow cover. Others like the invasive emerald ash borer (the closest known location for most west-central area readers is probably Sauk Centre) gain some insulation by being underneath tree bark. 

The minimum winter temperature (so far, around Jan. 7 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.

Winter isn’t over yet, so we could see even colder temperatures this winter that could change how these maps look (Fig. 2). Overall, there has been very little soybean aphid mortality in the southern half of the state, while areas in Stearns county and north are predicted to have anywhere from 50 to 90% mortality so far. Here at the WCROC at Morris though, temperatures so far were only cold enough to possibly kill 5% of the soybean aphid population.

Outlook for 2022

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.  The January cold snap won't eliminate soybean aphid from Minnesota, but it could reduce early-season populations in some areas.  

However, winter 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. There may be a somewhat reduced risk of soybean aphid issues this year due to the cold, but problem fields could still easily occur across the state. Start scouting early in the year so you can stay on top of soybean aphid populations that may develop in your area. 

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

Dr. Anthony Hanson

Regional Extension Educator

Field Crops Integrated Pest Management

Email Anthony: [email protected]