CHAMPAIGN, Ill., 3/9/23:  Above normal air temperatures this winter kept Illinois average soil temperatures higher than usual. These mild conditions are favorable for insect pests that overwinter in Illinois, but many other factors will affect insect populations for the upcoming growing season, according to scientists at the Prairie Research Institute (PRI), a part of the University of Illinois.

Temperatures
Statewide temperatures from December through February averaged 34 degrees, which is 4 degrees higher than the long-term average, said Jennie Atkins, PRI Water and Atmospheric Resources Monitoring (WARM) program manager. Southern Illinois was particularly warm with temperatures in the 70s during the last week of February, 20 to 25 degrees higher than normal.

This warmer winter weather led to higher soil temperatures across the state. Soil temperatures at 4 inches under bare soil averaged 38 degrees, or 3 degrees above the long-term average and 2 degrees warmer than last winter. All Illinois regions saw warmer soils with average temperatures ranging from 35 degrees in the north to 42 degrees in the south.

Despite the warmer temperatures overall, the state did see periods of freezing temperatures at most monitored locations. Most notably during the extreme cold of Dec. 22–24, soil temperatures fell as low as 18 degrees. The lowest temperatures were in the central region where there was little snow to insulate the soils. 

In the first week of March, soil temperatures were high and reached the 60s in the southern and parts of the central regions on March 6 and the 40s and 50s in the north. Lows ranged from the mid-30s to mid-40s.

Insect pests
This mild winter has sparked questions about the potential for higher than average pests this coming growing season. It’s a complicated answer as not all pests overwinter here in Illinois, but the conditions have been favorable for the ones that do, said Kelly Estes, PRI state agricultural pest survey coordinator. Insects can overwinter in any life stage, including egg, larva, pupa, or adult, and in different locations.

Some insects overwinter in the soil. For example, rootworms overwinter as eggs and Japanese beetles as grubs. Others seek shelter in leaf debris, grass, or even under tree bark.

Warmer air and soil temperatures are certainly favorable to insect survival. However, it’s important to note that these conditions are also beneficial for natural enemies, such as parasitoids, predators, and pathogens. It’s also important to remember that spring conditions will also affect insect survival. Late freezes and excessive moisture are just two examples of factors that can impact insect populations.

“While there is no easy answer to predict what we might experience this coming growing season, we can say for certainty that insects will be around,” Estes said.

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 Media contacts: Jennie Atkins, Ph.D., (217) 333-4966, jatkins@illinois.edu; Kelly Estes, (217) 333-1005, kcook8@illinois.edu

The Illinois State Water Survey’s WARM program collects hourly and daily weather and soil information at 19 stations across the state. Daily and monthly summaries can be found at the WARM website (warm.isws.illinois.edu/warm/) and in the Illinois Water and Climate Summary (https://warm.isws.illinois.edu/warm/climate.asp).