Almost every phone call home turns into an extensive update of the goings-on at my family’s farm. The latest conversation with my dad about the approaching planting season, for example, landed on current soil conditions and the fact that the last cold snap froze the ground almost 48 inches deep.
Freezing temperatures aren’t necessarily ideal for fieldwork, but they can be advantageous from a pest control standpoint. In a recent article from University of Minnesota Extension, Anthony Hanson explains how cold weather drives migratory pests south and prevents overwintering insects from living to see the next growing season. Cold weather does, in fact, have its merits. But was it cold enough for significant insect mortality this winter?
A recent resurgence
Hanson, an integrated pest management specialist, notes that alfalfa weevils overwinter as adults in leaf litter and alfalfa stubble. He says this species has been particularly resurgent in the Gopher State in recent years, especially in regions where insects have been provided refuge from extreme cold under a blanket of snow. Although snow cover is helpful to insulate plant roots and protect them from winter injury and winterkill, it can be a tradeoff for pest control.
It takes soil surface temperatures of about 13ºF or lower to kill 20% to 30% of an alfalfa weevil population; however, a small subset of pests is expected to survive soil surface temperatures as low as 1ºF. Hanson explains that, as cold-blooded animals, insects match their body temperatures to their surroundings.
“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,” he states.
For this reason, the temperature at which ice does form in an insect’s body is considered in addition to air temperatures to predict pest populations. Hanson writes that exposure to cold may not kill insects immediately, but rather have a delayed effect weeks later, making it more complicated to forecast pest mortality.
It can also be difficult to estimate soil surface temperatures considering variations in vegetation. Therefore, Hanson uses 2-inch soil temperature data as a conservative estimate to create annual pest population predictions, which he bases on the coldest night of the year.
So far during the 2024 to 2025 meteorological winter, the coldest night in Minnesota was recorded on January 21 when air temperatures fell below -25ºF and 2-inch soil temperatures were about 5ºF. Hanson notes this followed a string of several subzero nights, and according to the National Oceanic and Atmospheric Administration (NOAA), this was the same time frame most states throughout the Midwest and Fescue Belt experienced their coldest winter night, too.
“I would still plan to scout heavily for [alfalfa weevil] in fields that have had weevil problems in previous years, though this is one of the few years where there was both enough cold and lack of snow cover to suggest we may see a reduction in alfalfa weevil populations in some areas,” he says.
Caution in Kansas
From Minnesota to Kansas, extension specialists in the Sunflower State are advising farmers to start scouting for alfalfa weevil now as accumulated growing degree days are well ahead of average. Anthony Zukoff, an entomology associate with Kansas State University Extension, suggests using growing degree days to estimate insect development can help guide scouting efforts.
“Alfalfa weevil eggs begin hatching after 300 degree days have accumulated,” Zukoff writes in a recent extension article. With that said, since there is no telling whether eggs were laid during the previous fall or in early spring, he recommends scouting for alfalfa weevil after 180 growing degree days starting from January 1.
After 300 growing degree days, alfalfa weevils enter the first and second instar, or growth stage, in which their feeding damage can look like pinholes on plant leaves. As the larvae continue to develop into the third and fourth instars between 450 to 750 growing degree days, so does their ability to defoliate alfalfa.
When temperatures rise, so will the rate of plant growth and alfalfa weevil development; however, Zukoff says dramatic temperature drops can slow alfalfa growth, putting plants at a disadvantage in keeping up with insect damage.
Other pests
Unlike alfalfa weevils, pests like potato leafhoppers and true armyworms typically migrate south to the Gulf States to avoid persistent cold. These insects flee the Midwestern scene when low temperatures reach roughly 20ºF in the fall and return with favorable conditions in the spring.
Hanson notes sufficiently chilly temperatures likely eliminated the risk of these migratory pests uncharacteristically overwintering in the Upper Midwest. Moreover, the widespread presence of cold weather may have encouraged a more extensive migration, which may delay the return of potato leafhopper and true armyworm.
“Some of these pests may be pushed farther south than last year, depending on cold exposure in the Southern states, but they will eventually return to Minnesota, though hopefully in lower initial numbers,” he reports.
Overall, Hanson says the combination of timely cold snaps during periods with little to no snow cover were important factors of pest control this winter. Even with lower-than-average soil and air temperatures, though, he only expects to see moderate mortality of major field pests.
“Years with moderate mortality like this can be one way to have slight reductions in populations that can mean the difference between a farmer having economically damaging levels of a pest versus tolerable populations that aren’t worth treating with pesticides,” Hanson states
