Was this a harsh winter for corn rootworm?

As temperatures warm up, the questions are already rolling in from inquisitive folks wondering how insects fared the winter in Iowa. Some insects have adapted to survive freezing temperatures. A prior article delves deep into the topic of “How do insects survive the winter?” and is worth reading for more information on general strategies. The 2024-2025 winter was a particularly variable one across the state, and questions like “Was it cold enough to kill the rootworms?” have come up repeatedly.  This article will address factors that may affect overwintering mortality of corn rootworm.

Remember, northern and western corn rootworms overwinter as eggs. In general, northern corn rootworm are better adapted to our climate. Females from typical populations deposit eggs near the base of corn plants with hopes of corn being planted in the field next year. Factors like soil texture and soil moisture affect how deep females can bury the eggs. Females typically oviposit their eggs 4-6 inches below the soil surface, but in dry years can go as deep as 12 inches by utilizing drought cracks or earthworm burrows. Eggs placed deeper in the soil profile are more insulated from extreme winter environments. 

Eggs mature in the spring based on accumulating degree days (heat units). Eggs are not impacted by saturated soils or broad-spectrum insecticides used at planting. However, numerous factors beyond temperatures can influence the rate of egg development, including tillage and soil characteristics. 

Cold temperatures

Likely the biggest impact on overwintering mortality of eggs is temperature. Corn rootworm eggs have adapted to Iowa’s freezing temperatures but can eventually succumb if it gets cold enough. Three interesting takeaways from previous research:

1. Ellsbury and Lee (2004) noted northern corn rootworms could survive a brief exposure to -16.6 degrees Fahrenheit! Northern corn rootworms were more tolerant of supercooling temperatures than western corn rootworms.
2. Patel and Apple (1967) showed that six weeks of sustained cold temperatures between 14 to -9 degrees Fahrenheit prevented rootworm eggs from hatching.
3. Gustin (1981) demonstrated a lower lethal temperature limit of 18.5 degrees Fahrenheit.

This winter, air temperatures dipped below freezing numerous times during January and February. Air temperatures dropped below zero from January 19-21 throughout much of the state. The average four-inch soil temperature dropped dramatically during that period, averaging below 20°F at research stations across northern Iowa for several days (Fig. 1). Remarkably, the average (high soil temp + low soil temp divided by two) four-inch soil temperature was recorded as 15°F at Doon, and 13°F at the ISU Northwest Research and Demonstration Farm (Sutherland), and the ISU Northeast Research and Demonstration Farm (Nashua) on January 20, 2025.

The longest stretch of extremely cold temperatures was from approximately February 16-21 but sufficient snow cover during that timeframe must have limited soil temperature drops to the mid- to low-20s. There was virtually no snow cover in January compared to 2 to 4 inches in February (Fig. 2).  

Soil moisture

Female corn rootworms seek out moisture in the soil to improve the chances of larval survival the following spring. While dry soils can be abrasive to larvae, overly wet or compacted soils can drown larvae or inhibit their ability to move to corn roots.

Dry soils are more prone to temperature fluctuations than soils with sufficient moisture, but eggs laid 6-12 inches below the soil surface may be buffered from lethal air temperatures. Last September and October, Iowa went through a substantial dry spell prior to receiving significant November rainfall across much of the state, alleviating some of the dry soil and temperature fluctuation concerns headed into winter. Three interesting takeaways from previous research:

1. Ellsbury and Lee (2004) noted western corn rootworms were generally more resistant to the effects of desiccation following exposure to cold temperatures.
2. Ellsbury et al. (2001) showed northern corn rootworm infestations were lowest at poorly-drained sites.
3. Krysan (1978) verified eggs require water uptake in the spring to complete development. 

In other words, it can’t be too dry or too wet. Are rootworms like Goldilocks? As spring arrives, soil moisture will continue to be an important factor affecting the survival of corn rootworm larvae. 

Temperature fluctuations

Insects do not generate their own body heat and rely on accumulating degree days to complete development. Most insects resume development from overwintering diapause at 50 degrees Fahrenheit. It takes substantial energy to “wake up” in the spring. Insects, especially those that overwinter aboveground, can deplete energy resources if the temperatures fluctuate between 30 to 50 degrees Fahrenheit. Multiple temperatures swings were observed in Iowa this March (Fig. 3). Read more about oscillating temperature effects in a previous article, “Survival Effects of Fluctuating Temperatures on Insects” from 2020. 

Sustained and rapid freezing temperatures observed this winter could have caused some mortality of corn rootworm eggs. In addition, the erratic temperatures in March could impact survivorship. It is only once egg hatch begins in May that we’ll truly get a better idea of the 2025 corn rootworm populations in Iowa. We’ll keep you updated on predicted hatch and feeding activity throughout the growing season.

More information on corn rootworm biology and ecology can be found on the Corn Rootworm Regional Working Group’s webpage.

Additional resources:

Dean and Hodgson. 2020. Cold Hardiness of Insects and the Impact of Fluctuating Temperatures. Iowa State University Extension and Outreach Integrated Crop Management Encyclopedia. https://crops.extension.iastate.edu/encyclopedia/cold-hardiness-insects-and-impact-fluctuating-temperatures.

Ellsbury, Woodson, Malo, Clay, Clay and Carlson. 2001. Variability in corn rootworm population density and injury in relation to landscape position within fields. Proceedings of the Third European Conference on Precision Agriculture, June 18-20, 2001, Agro Montpellier, Montpellier, France (ed. by Blackmore & Grenier), pp. 653-658.

Ellsbury and Lee, Jr. 2004. Supercooling and cold-hardiness in eggs of western and northern corn rootworms. Entomologia Experimentalis et Applicata. 111:159-163.

Godfrey, Meinke, Wright, and Hein. 1995. Environmental and edaphic effects on western corn rootworm overwintering egg survival. J. Econ. Entomol. 88:1445-1454.

Krysan. 1978. Diapause, quiescence, and moisture in the egg of the western corn rootworm. Journal of Insect Physiology. 24:535-540.