December 2023 through February 2024 has been dubbed “The winter that wasn’t,” due to historically warm temperatures across the country and a lack of snowfall in the northern states. Not only is the unprecedented weather pattern causing concerns about soil moisture and alfalfa winterkill, but it also raises questions about pest populations this spring and summer.
In a recent article from University of Minnesota Extension, Anthony Hanson explains how freezing temperatures regulate the population of insects that overwinter in the Midwest. The extension educator adds that cold weather also drives migratory insects south; however, these natural cycles of pest control may have been interrupted this past year.
Even if it gets below 32ºF, Hanson says insects contain compounds that lower the freezing point of water in their bodies, similar to antifreeze in a car. Knowing this, researchers use minimum winter air temperatures of an area to estimate insect freeze mortality rates for different species.
No snow, no cold
Some insects, like alfalfa weevil, overwinter above the soil surface in plant stubble and leaf litter. The snowpack that is so important for winter survival of alfalfa also protects alfalfa weevils, but without adequate snow cover in recent months, the pests may have been subject to harsher conditions. With that said, Hanson says soil surface temperatures must be around 13ºF before alfalfa weevil mortality rates reach 20% to 30%.
“It is difficult to get accurate widespread estimates of temperatures at the soil surface due to variation in vegetative cover and other insulating effects of soil, but I would expect to see some minor to moderate alfalfa weevil mortality this year,” he asserts. To be sure, producers must scout hayfields heavily this spring, especially where there is a history of alfalfa weevil infestation.
Other forage pests, like potato leafhopper, migrate to the Gulf Coast every year once air temperatures hit 20ºF in Minnesota, Hanson says. Despite the record warm trend, he affirms migratory insects would not have survived the Midwestern winter. Even so, present conditions could pull these pests back up north ahead of schedule.
“The problem to watch is that populations may be larger and start the season farther north than previous years, so monitoring migration will be important this year,” Hanson asserts.
Some more speculation
Iowa experienced an exceptionally warm winter as well, with February 2024 being the warmest February on record for the Hawkeye State. Despite this, Erin Hodgson and Ashley Dean with Iowa State University Extension highlight the following survivorship factors of overwintering insects that indicate the mild winter could, in fact, have neutral to positive effects on pest control.
• Large temperature swings from below 32ºF to above 50ºF may disrupt insect dormancy and heighten mortality rates due to cold intolerance.
• Greater insect activity with above average temperatures could deplete insects’ energy stores before food is available, causing pests to starve to death.
• Mild winters usually have little to no effect on insects that overwinter belowground since soil temperatures are more constant than air temperatures; however, a shallow frost layer could enhance these insects’ survival.
Hodgson and Dean note these survivorship factors apply to beneficial insects, too. Therefore, if a mild winter does amplify pest pressure, there will likely be more beneficial insects to regulate it. Overall, the professor and field crop entomology specialist say spring conditions and soil moisture will likely have a larger influence on insect survival than winter weather.
