Jump in emerald ash borers, crop pests likely after record-warm winter

Minnesota’s warmest winter on record is likely to have consequences for the summer of 2024.

Our lack of extreme cold last winter means that many insects that are considered pests survived the winter in greater than usual numbers. That may have big implications for ash trees and Minnesota crops this summer.

Anthony Hanson is the regional extension educator for field crops integrated pest management at the University of Minnesota. He keeps track of winter temperatures and insect mortality in Minnesota’s typically frigid winters.

Anthony posted this article that suggests a very low insect mortality occurred this winter. He opens with comments on the nature of extreme cold in Minnesota and the likely impact of the lack of extreme cold last winter.

Cold winters help prevent many potential pest insects from establishing in Minnesota or require species that cannot survive our winters to migrate up from southern states each year. Extreme cold can also knock back species that are established here. The cold can be a welcome event for farmers from a pest management perspective, but the record-warm winter has left many farmers wondering if 2024 will be a severe year for insects, especially now that meteorological winter is over.

The lack of extreme cold last winter means insect survival rates are high.

The coldest temperature at Minneapolis-St. Paul International Airport last winter was minus 8 degrees on Jan. 14-15. International Falls bottomed out at minus 23 degrees on Jan. 20.

Anthony notes that the lack of extreme cold last winter means insect survival rates are likely much higher than in several winters. And it’s even warmer where those little critters can burrow a few inches into soils or tree bark.

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. So far for most of the state, Jan. 20 had been the coldest night during winter 2023-24 with morning low air temperatures near -15 °F in the central portion of the state and below -20 °F farther north (Fig. 1). NOAA and the US National Phenology Network provide 1.6 square-mile resolution daily temperature data used in Fig. 1 that helps give a region-wide picture of daily high and low temperatures. This interpolated data is generally within a couple degrees of individual weather stations. Here at the U of M's West Central Research and Outreach Center in Morris, the morning low was -16 °F on Jan. 20, and we had a string of subzero nights Jan. 10-21. However, four inches below the soil surface, temperatures around this time were still only barely below freezing at 27 to 28 °F. 

Anthony notes that insects like the soybean aphid escaped this winter with very little mortality in Minnesota.

Compare that map to Jan. 31, 2019, when the bitter polar vortex dropped temperatures to minus 30 or colder across much of Minnesota. Soybean aphid mortality was more than 90 to 95 percent across more than half of Minnesota.

What about emerald ash borer?

Our record-warm winter is not good news for stopping the recent attack of the emerald ash borer in Minnesota. The lack of extremely cold temperatures this winter suggests that most of will Minnesota see less than 5 to 10 percent ash borer mortality.

While it's not a field crop pest, I often get questions if winter will cut back on emerald ash borer (EAB) populations. 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 actual exposure temperatures of -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. This year, don't expect much if any EAB overwintering mortality (Fig. 4). Only a few areas of north-central Minnesota would even experience 10% EAB cold mortality this year, though EAB has not been found in many of these counties, yet.

Climate change connection

Data from the National Oceanic and Atmospheric Administration and the nonprofit climate news and science group Climate Central show winters have warmed between 5 and 6 degrees on average in Minnesota since 1970.

Anthony notes that many insects need only a few degrees warmer temperatures in winter to survive in greater numbers. That’s why these pests are sensitive to even relatively small changes in climate.

From the climate change perspective, some of the maps help give context for just what a few degrees difference in a winter minimum temperature can make. That's been especially helpful for some farmer audiences I work with when discussing IPCC predictions.

Our record-warm winter means it’s likely that farmers and ash trees may have a harder than usual time dealing with the potential for increased insect numbers this year.

Stay tuned.