By Angie Peltier and Anthony Hanson, UMN Extension educators, and Jochum Wiersma, UMN Extension small grains agronomist
February 11, 2026’s Strategic Farming: Let’s Talk Crops session discussed numerous forecasting tools to aid producers in crop, pest and disease management. This webinar series runs through March and registrations are still being accepted: https://extension.umn.edu/courses-and-events/strategic-farming.
To watch this episode visit: http://z.umn.edu/StrategicFarmingRecordings.
As ‘cold blooded’ organisms, most insects are not impacted by wind chill, but rather temperature. For those pests that overwinter above-ground like soybean aphid (overwinters on buckthorn bud scales), air temperature impacts winter survival. Soybean aphid eggs tend to freeze at -29 °F and so in years such as 2019, when a polar vortex brought arctic air into most of Minnesota, winter aphid mortality was estimated to be between 50 and 95% statewide. The 2025-26 winter has so far not resulted in temperatures likely to be lethal for soybean aphids in the primary soybean-producing regions of Minnesota. For other insects such as corn rootworms and bean leaf beetle, that spend their winters in soil and leaf litter respectively, soil temperature and snow cover also impact winter survival.
Alfalfa weevils overwinter at the soil surface in plant material and other leaf litter as adults. While those areas of the state covered by the Minnesota Ag Weather Network or the North Dakota version of the network (NDAWN) collect soil temperature data, none collect temperature data from the top four inches of soil or the area just above the soil or in leaf litter where pests like alfalfa weevil overwinter. Researchers that have access to 2 inch soil temperatures can use this data to estimate when weevils are likely to be exposed to temperatures below 13 °F, a temperature that causes 20-30% mortality and at or below 1 °F, a temperature that causes 95% winter mortality.
Last winter, on January 21, 2025, temperatures dipped down to 6 °F for one night in Stearns County, Minnesota. As there was little to no snow cover, this single temperature dip reduced alfalfa weevil pressure for some in the 2025 growing season. With the snow cover experienced by most of the state during the coldest portions (so far) of the 2025-26 winter that occurred toward the end of January, the alfalfa weevil population is unlikely to have experienced significant temperature-related mortality.
The cold temperatures and the duration that western corn rootworm eggs are exposed to these temperatures, impacts survival. Soil temperatures below 10 °F can reduce survival by 60%. Soil temperatures dropping to 0 °F can reduce survival to 95%. Northern corn rootworm survival is also impacted by soil moisture, with higher rates of egg hatch following a winter with dry soils that dip below 10 °F (45-50%) than after a winter with wet soils at the same temperature (20%). When soil temperatures drop to ~-7 °F, egg hatch for both northern and western corn rootworms drops to ~5%; one can also anticipate a ~50% decline in egg hatch when soil temperatures drop below 18 °F for ~2 weeks.
To assist producers in managing migratory pests, researchers deploy something called a pheromone trap. Pheromones are sex hormones secreted by female insects looking to attract a mate. Commercial companies sell pheromone lures that can be used to bait traps that then attract and capture males of the species when they arrive in Minnesota. Degree day calculations use historical weather trends and knowledge about how temperature drives specific pests’ growth and development.
For example, when eight or more black cutworm moths are trapped over a 2-night period, degree days are used to project a potential cutting date. Cutting dates can help us to better pinpoint when damage to a corn crop may occur, and therefore when scouting should commence. This sort of information is of particular importance to those that are growing conventional or non-Bt corn hybrids.
Similarly, with tar spot of corn, the ipmPIPE website has a historical and growing season map that is updated with positive confirmations of tar spot sightings each year to assist folks in their scouting efforts.
Begun in 1989, NDAWN is now the largest mesonet (a network of collectively owned/operated automated weather stations) in the United States. Folks that farm in Minnesota’s Red River Valley have long benefited from the NDAWN stations that were sited near sugarbeet piling stations operated by the Crystal Sugar Cooperative.
Multiple degree day models are available on the NDAWN website to help crop producers better time management practices that are closely related to corn, wheat, sugarbeet, canola and sunflower developmental milestones. Also included are maps/tables that show the number of insect degree days that have accumulated after a given calendar date when the base temperature used in the calculation is 40, 45, 50 and 55 °F, corresponding to the base temperature used for degree day calculations for (for example) cabbage maggot, variegated cutworm, alfalfa weevil and corn earworm, respectively. Also included in the NDAWN suite of tools is degree day information on the sugarbeet root maggot to assist with insecticide application timing and crop susceptibility to and emergence of wheat midge.
Also available are disease risk maps available for Cercospora leaf spot in sugarbeet and white mold in canola. In small grains, NDAWN hosts forecasting tools developed for tan spot, Stagonospora blotch and leaf rust. A forecasting tool for Fusarium head scab or “scab”, a fungal disease associated with steep yield and quality losses due to mycotoxin contamination, is not housed on the NDAWN website, but rather on the US Wheat & Barley Scab Initiative’s Fusarium Risk Tool webpage.
In some Minnesota regions, oats production is increasing and with increased production there is new interest in forecasting crown rust risk. The fungus that causes crown rust requires an alternate host to complete its lifecycle. Buckthorn, the alternate host of crown rust, is unfortunately well established in the understory of wooded areas throughout Minnesota. Although there is not a forecasting model specifically tied to oats, crown rust requires environmental conditions that are remarkably similar to leaf rust in wheat. Wiersma recommends consulting the leaf rust model to better understand the risk of crown rust in oats.
These tools provide crop producers with much more information than their forebears could even dream of. But despite the ready access of these tools, there is still no substitution for scouting fields to ground truth growth stages predicted by NDAWN or determine whether there are foliar disease symptoms present in the lower portion of a small grains crop’s canopy. For several foliar diseases, disease presence in the lower canopy produces the spores that infect plant parts important to yield; in spring wheat, it is important to protect the flag leaf as this one leaf alone accounts for approximately 45% of yield.
North Dakota Ag Weather Network (Includes weather station and degree day tools for Minnesota)
https://www.ipmpipe.org/
Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
Cold-weather impacts overwintering insect pests
Those pests that cause problems for Minnesota’s agronomic crops that are also able to survive our harsh winter weather conditions use numerous survival strategies. Ice crystal formation is what causes cold-weather-related mortality and many of the survival strategies are related to avoiding the formation of ice crystals. Similar to motor vehicles, which require antifreeze for their engines to work properly in cold weather, insects also require their own antifreeze-type compounds. By excreting water before the temperature drops below freezing, the concentration of sugars and other chemicals increases. This concentration of solutes produces a natural antifreeze that can lower the temperature at which pests freeze and die, with some pests able to lower their freezing point to -30 °F or lower. Other insects (ex. Goldenrod gall midge) are able to survive temperatures as low as -60 °F despite the fact that ice crystals do form in their bodies. Other insects, such as emerald ash borer, are afforded natural protection by where they choose to feed, under the bark, which provides some insulation from open air temperatures, while others still (ex. bean leaf beetle) survive in leaf litter where they are typically insulated by a blanketing snow cover.As ‘cold blooded’ organisms, most insects are not impacted by wind chill, but rather temperature. For those pests that overwinter above-ground like soybean aphid (overwinters on buckthorn bud scales), air temperature impacts winter survival. Soybean aphid eggs tend to freeze at -29 °F and so in years such as 2019, when a polar vortex brought arctic air into most of Minnesota, winter aphid mortality was estimated to be between 50 and 95% statewide. The 2025-26 winter has so far not resulted in temperatures likely to be lethal for soybean aphids in the primary soybean-producing regions of Minnesota. For other insects such as corn rootworms and bean leaf beetle, that spend their winters in soil and leaf litter respectively, soil temperature and snow cover also impact winter survival.
Alfalfa weevils overwinter at the soil surface in plant material and other leaf litter as adults. While those areas of the state covered by the Minnesota Ag Weather Network or the North Dakota version of the network (NDAWN) collect soil temperature data, none collect temperature data from the top four inches of soil or the area just above the soil or in leaf litter where pests like alfalfa weevil overwinter. Researchers that have access to 2 inch soil temperatures can use this data to estimate when weevils are likely to be exposed to temperatures below 13 °F, a temperature that causes 20-30% mortality and at or below 1 °F, a temperature that causes 95% winter mortality.
Last winter, on January 21, 2025, temperatures dipped down to 6 °F for one night in Stearns County, Minnesota. As there was little to no snow cover, this single temperature dip reduced alfalfa weevil pressure for some in the 2025 growing season. With the snow cover experienced by most of the state during the coldest portions (so far) of the 2025-26 winter that occurred toward the end of January, the alfalfa weevil population is unlikely to have experienced significant temperature-related mortality.
The cold temperatures and the duration that western corn rootworm eggs are exposed to these temperatures, impacts survival. Soil temperatures below 10 °F can reduce survival by 60%. Soil temperatures dropping to 0 °F can reduce survival to 95%. Northern corn rootworm survival is also impacted by soil moisture, with higher rates of egg hatch following a winter with dry soils that dip below 10 °F (45-50%) than after a winter with wet soils at the same temperature (20%). When soil temperatures drop to ~-7 °F, egg hatch for both northern and western corn rootworms drops to ~5%; one can also anticipate a ~50% decline in egg hatch when soil temperatures drop below 18 °F for ~2 weeks.
How forecasting can help manage crops and migratory pests
Those pests that are unable to survive the cold Minnesota winter like black cutworm, potato leaf hopper, true and fall armyworm and corn earworm, need to travel northward on large-scale weather systems each growing season. Because they are dependent upon moving long distances on the jet stream, migratory insects don’t cause problems during every growing season in every part of the state.To assist producers in managing migratory pests, researchers deploy something called a pheromone trap. Pheromones are sex hormones secreted by female insects looking to attract a mate. Commercial companies sell pheromone lures that can be used to bait traps that then attract and capture males of the species when they arrive in Minnesota. Degree day calculations use historical weather trends and knowledge about how temperature drives specific pests’ growth and development.
For example, when eight or more black cutworm moths are trapped over a 2-night period, degree days are used to project a potential cutting date. Cutting dates can help us to better pinpoint when damage to a corn crop may occur, and therefore when scouting should commence. This sort of information is of particular importance to those that are growing conventional or non-Bt corn hybrids.
Forecasting and monitoring disease risk in crops
One of the tools used to monitor pest and disease presence is the ipmPIPE website. University-based Extension specialists add disease sightings to a map on the website to alert folks to the presence of southern rust of corn. Southern rust is a periodic disease caused by a pathogen that cannot survive the Minnesota winter; it can significantly limit a crop’s yield potential if rust spores arrive to Minnesota early enough in the growing season.Similarly, with tar spot of corn, the ipmPIPE website has a historical and growing season map that is updated with positive confirmations of tar spot sightings each year to assist folks in their scouting efforts.
Minnesota Ag Weather Network – Disease and pest forecasting
The Minnesota Ag Weather Network is expanding to include one weather station every 20 miles throughout the primary crop producing regions of the state. These stations monitor air temperatures at multiple heights (to detect conditions that favor temperature inversions), soil temperatures (bare and under turf), relative humidity, wind speed and direction, solar radiation, atmospheric pressure, precipitation, dew point and wind chill. Data (both historical and current conditions) from these stations can be observed through the NDAWN (North Dakota Ag Weather Network) website.Begun in 1989, NDAWN is now the largest mesonet (a network of collectively owned/operated automated weather stations) in the United States. Folks that farm in Minnesota’s Red River Valley have long benefited from the NDAWN stations that were sited near sugarbeet piling stations operated by the Crystal Sugar Cooperative.
Multiple degree day models are available on the NDAWN website to help crop producers better time management practices that are closely related to corn, wheat, sugarbeet, canola and sunflower developmental milestones. Also included are maps/tables that show the number of insect degree days that have accumulated after a given calendar date when the base temperature used in the calculation is 40, 45, 50 and 55 °F, corresponding to the base temperature used for degree day calculations for (for example) cabbage maggot, variegated cutworm, alfalfa weevil and corn earworm, respectively. Also included in the NDAWN suite of tools is degree day information on the sugarbeet root maggot to assist with insecticide application timing and crop susceptibility to and emergence of wheat midge.
Also available are disease risk maps available for Cercospora leaf spot in sugarbeet and white mold in canola. In small grains, NDAWN hosts forecasting tools developed for tan spot, Stagonospora blotch and leaf rust. A forecasting tool for Fusarium head scab or “scab”, a fungal disease associated with steep yield and quality losses due to mycotoxin contamination, is not housed on the NDAWN website, but rather on the US Wheat & Barley Scab Initiative’s Fusarium Risk Tool webpage.
In some Minnesota regions, oats production is increasing and with increased production there is new interest in forecasting crown rust risk. The fungus that causes crown rust requires an alternate host to complete its lifecycle. Buckthorn, the alternate host of crown rust, is unfortunately well established in the understory of wooded areas throughout Minnesota. Although there is not a forecasting model specifically tied to oats, crown rust requires environmental conditions that are remarkably similar to leaf rust in wheat. Wiersma recommends consulting the leaf rust model to better understand the risk of crown rust in oats.
These tools provide crop producers with much more information than their forebears could even dream of. But despite the ready access of these tools, there is still no substitution for scouting fields to ground truth growth stages predicted by NDAWN or determine whether there are foliar disease symptoms present in the lower portion of a small grains crop’s canopy. For several foliar diseases, disease presence in the lower canopy produces the spores that infect plant parts important to yield; in spring wheat, it is important to protect the flag leaf as this one leaf alone accounts for approximately 45% of yield.
Audience questions
Hanson and Wiersma fielded many audience questions, including: Is it the lethal temperature or the duration of time at the lethal temperature that results in pest mortality?, Do temperature fluctuations complicate forecasts?, Does buckthorn grow in every US state?, How is climate change impacting where insect pests are able to survive the winter?, With the polar vortex hitting the southern US this winter, will it have any impact on Minnesota crop pests that overwinter there?, Given that crown rust and soybean aphid both survive the winter on buckthorn, should we concentrate efforts on buckthorn eradication?Additional Resources
Hanson, A. 2026. Is it cold enough yet? Insect overwintering forecasts for 2026. MN Crop News.North Dakota Ag Weather Network (Includes weather station and degree day tools for Minnesota)
https://www.ipmpipe.org/
Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
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