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CHAMPAIGN -- Illinois’ milder winter soil temperatures may have an impact on pest populations, according to researchers at the University of Illinois’ Prairie Research Institute.

Overall, soil temperatures were milder than normal, according to Jennie Atkins with the Illinois State Water Survey’s Water and Atmospheric Resources Monitoring (WARM) Program. At depths of 4 inches under bare soil, temperatures averaged 35.2 degrees this winter, or 1.4 degrees above the long-term average. Soils averaged 1.8 degrees cooler than last winter.

Temperatures rose and fell throughout the season. Late December and early January had a 12-day period when soil temperatures averaged below freezing, falling to a seasonal low of 11.7 degrees in Belleville on Jan. 2. The highest of the season was 65.2 degrees, reported at Dixon Springs on Feb. 20.

Soils warmed up in February and reached a state average of 47.6 degrees on Feb.28, 12.8 degrees warmer than normal. Daily highs were in the 40s and 50s with lows falling into the 30s.

Temperatures under sod followed a similar pattern with a winter statewide average of 36.6 degrees, 1.6 degrees warmer than normal, but 2.4 degrees lower than in winter 2016-17.

The mild temperature trends over the winter favor insect survivability, according to Kelly Estes, state agricultural pest survey coordinator with the Illinois Natural History Survey. Ultimately, though, there are several factors that play a part in determining spring insect pressure: the previous year’s population, winter temperatures, spring weather, and more.

The extremely cold period in January may have impacted some insect survival rates. However, the cold was not enough to completely wipe out pest populations.

Insects can survive cold temperatures through various genetic adaptions. What will impact insects overwintering in the soil is repeated temperature swings through freeze/thaw cycles combined with exposure to moisture. Despite these cold temperatures, their duration was relatively short and didn’t impact the average winter temperatures.

Although a mild winter is favorable for insect survival, it is only going to affect insects that spend their winter here in Illinois. Several important crop pests will migrate into the Midwest this spring and summer from the southern states. Despite some bitterly cold temperatures in January, there is still a possibility for an active insect season.

Sources: Jennie Atkins, PhD, 217-333-4966,; Kelly Estes, 217-333-1005,


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