This fall, Iowa’s temperatures have fluctuated between freezing and 80 degrees. I started thinking about how insects are adjusting to the temperature swings as they make preparations for winter. My biggest decision the last two weeks was to wear flip flops or fleece. But insects go through a surprisingly complicated process to switch gears from active in the summer to apparently “sleeping" in the winter.
Most insects in Iowa need to go through a period of inactivity or diapause to survive the harsh elements (aka, cold weather and a lack of food). Diapause can be described as having little or no development, greatly reduced activity and metabolism, and increased resistance to environmental stresses. Any life stage can go through diapause, but is usually fixed for a particular insect species. Examples include:
Egg: grasshoppers, corn rootworm, stalk borer
Larvae: European corn borer, Japanese beetle, colaspis beetles
Pupae: seedcorn maggot, fall webworm, sod webworm
Adults: bean leaf beetle, green stink bug, lady beetles, billbugs
Diapause begins with external stimuli that signal upcoming changes in the environment BEFORE they happen. Usually that means colder temperatures in temperate zones. But in other parts of the world, hot and dry weather may trigger diapause. Hormones inside the insect start to change and are not easily reversed. So once the body starts to enter diapause, that behavior is permanent even if the weather improves.
The most important external stimulus is decreasing photoperiod or day length. Shorter days cue insects to start the process of finding suitable shelter. A common trigger for insects is experiencing less than 13 hours of light a day for 1-3 weeks. Another important stimulus in Iowa is cold weather. Insects are motivated to move before it freezes. Usually photoperiod and temperatures go hand-in-hand in nature. And in the case of field crops, their food source is often eliminated and stimulates diapause behavior.
Insect diapause is huge obstacle to overcome! You should see all types of insects starting the overwintering process right now.
Bean leaf beetle adults overwinter in leaf litter.
Photo by Purdue Cooperative Extension Service.