What does this Warm Winter Mean for Insects?

Except for a few extremely cold days, Iowa has experienced a mild winter. In fact, February 2024 was the warmest February in Iowa’s weather history, and this winter is one of the warmest ever for the state. In addition to warmer temperatures, the total snowfall for our state has been low (Figure 1) and below average compared to a normal year (Figure 2). Much of the snowfall this winter was during a two-week period in January. As a result, many people are asking how a dry and mild winter might impact overwintering insects.

We can make some generalizations about how winter conditions affect overwintering insects, but it is complicated because of the different overwintering strategies used by insects. While mild winters in general favor overwintering insects, specific conditions during these winters could also result in either no effect or will hinder survival of them.

Some winter survivorship factors are highlighted here:

1. Most insects adapt to cold winters by slowing preparing in the fall and staying dormant until the spring. Large temperature swings can be detrimental to insects; the body can be injured or death can occur. We would expect some insect mortality due to cold intolerance when temperatures oscillate below freezing to above 50 degrees.
2. Insects that overwinter aboveground (e.g., bean leaf beetle adults and soybean aphid eggs) may be more likely to survive with fewer cold days. But a lack of snow cover can expose insects to below-freezing temperatures, and could increase mortality compared to years with insulating snow.
3. Insects that overwinter belowground (e.g., Japanese beetle grubs) will not likely be affected by a mild winter because soil temperatures are more constant. However, there could be more survivors than normal if the frost layer is shallow.
4. All insects develop based on temperature. A warm winter day may cause insects to become active (e.g., woolly bear caterpillars) when they normally would be dormant. Activity uses up stored fats they depend on to survive until the spring. Without access to food, these active insects could starve to death before food becomes available.
5. Perhaps most complicated of all is the actual overwintering mechanisms used by insects. Some insects enter diapause for the winter, which is a sort of hibernation for insects. Diapause duration depends on the insect species, but sometimes diapause is broken by environmental cues, which could cause some insects to break diapause too early. Other insects do not enter diapause but rather become freeze-tolerant or freeze-avoidant. An encyclopedia article details cold hardiness of insects.

Also, there are other factors to understand before we can predict how successful insects will be in the spring and summer. The same survival factors outlined above also apply to beneficial insects, like predators and parasitoids, and insect-killing pathogens. So ultimately it might not matter too much if more pests survive in a mild winter, because more beneficial insects will likely survive and help regulate spring populations. The uncertainty of insect survival in the winter can make predicting pest populations very difficult. For many insects, spring conditions are much more important than winter conditions for a few reasons. First, if diapause is broken in the spring and cold temperatures occur, insects could suffer mortality. Second, soil-dwelling insects are more likely to be affected by soil moisture than soil temperature. For example, an important source of corn rootworm larval mortality is saturated soils in May and June.

Lastly, there have been questions about how to calculate degree days with winter days that exceed the lower developmental temperatures (i.e., 50 degrees for most insects). In other words, should we include those warm days in estimating insect development? This is a difficult question to answer, given we do not have a lot of experience with predicting insect development with an especially warm winter. Our educated guess is to calculate accumulating degree days from 1 January for most insects. The Pest Maps and Forecasting tool provides degree day calculations for several important insect pests. As we observe actual insect development this spring and summer, we will see if temperature models are accurate or need to be slightly modified.