AMES, Iowa – Midwest winter weather has been warmer than normal, which means farmers may be wondering how pests will respond to the higher temperatures. 

It just depends on the insect says Erin Hodgson, Iowa State University professor of entomology: “Some insects are just more cold hardy than others.”

For example, in Iowa, she said most species that have adapted to the winters have developed strategies to overcome cold temperatures.

“However, surviving the winter is generally more difficult with oscillating temperatures, or when days go above 50 and below freezing several times,” she said.

“The odds of surviving increase if they are underground because the temperatures are more consistent (e.g., Japanese beetle, corn rootworm),” she added. “However, insects that overwinter above ground can experience more extreme temperature fluctuations (e.g., bean leaf beetle).”

Douglas Richmond, Purdue University professor of entomology, said insect mortality can be relatively high over the winter, so mild winters tend to increase survival of some insects.

“However, since a lot of factors influence the likelihood of a pest outbreak, mild winters don’t necessarily translate into greater pest problems the following season,” he said. “Mild winters may, however, increase insect activity periods by extending the length of the season.”

Hodgson said spring temperatures can drive the success of potential pests for the rest of the growing season: “A slow warm up, timely planting of crops, making sure food is available, and adequate moisture, all can promote success.”

Justin Glisan, state climatologist of Iowa in Des Moines, said the potential for a historically strong El Niño typically means unseasonably warm winter temperatures.

“Of course, there will be cold-air outbreaks, but overall, mild winters generally mean pests may overwinter better than in cold wintertime conditions,” he said.

Hodgson said whether dry soils matter depends on the insect: “For insects that spend part of their life cycle underground, snow cover and crop residue can reduce severe cold temperatures. Soils that are too dry or saturated can cause mortality.”

Glisan said dry soils react quicker to changes in temperatures, and will freeze faster and deeper.

“Moisture in soil profiles provide some insulation to temperature swings, and hence, they won’t freeze as fast,” he said. “Snow cover acts as an insulator as well; snow reflects incoming sunlight, the fraction of which depends on the age of the snowpack.