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.

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.

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