How do insects survive the winter?

October 5, 2016 2:05 PM
Blog Post

Right now, I have five different insects walking around my office! Of course, they are accidental invaders along with many other nuisance pests active right now. It’s also a sign of the quality of the building I work in [sigh]. I’ve been asked several times, “how cold does it have to get to kill insects?” Perhaps it is important to understand why cold temperatures kill insects. Insects are unlike mammals and birds because they must generate their own heat (called ectotherms). Insects die when they are exposed to temperatures below the melting point of their body fluids. If they want to survive our cold Iowa winters, they must avoid freezing or tolerate freezing. Over time, insects have developed several strategies to survive cold temperatures and none of them involve wearing fleece.

boxelder bug
Boxelder bugs. Photo by Joseph Berger,

Some insects just move into human structures in the fall and keep warm until spring. A common example are the multicolored Asian lady beetles aggregating on and in houses every year. Even if they are protected inside, they will likely die before spring if they don’t get food and water. Some insects also migrate to warmer climates to avoid freezing. A classic example is monarch butterflies moving from Canada to Mexico every year. Sounds pretty good about now!

multicolored Asian lady beetles
Multicolored Asian lady beetles mass on structures every fall. Photo by Robert Koch, University of Minnesota.

Most of our persistent insects in Iowa have to overwinter outside, and two strategies have evolved to survive extreme conditions: freeze avoidance and freeze tolerance. Freeze-avoidant insects keep body fluids liquid and freeze-tolerant insects can handle the formation of internal ice. Wait a minute, what? I know…either strategy seems fantastical.

The main strategy for insects living in the northern hemisphere, where we have cold temperature for long period of time, is freeze avoidance. Freeze avoidance can be achieved a few ways. Sometimes insects enter a “dry” hibernation by getting rid of all the food and water in their body. That way, ice can’t form inside the body and kill them. Water needs food or dust particles in order to crystallize; water can cool down to -42°C without freezing if particles are absent. Other insects have a super waxy coating on the exoskeleton that protects against ice formation on the body. Amazingly, some freeze-avoidant insects also produce cryoprotectants, such as glycerol and sugar, to reduce the lethal freezing temperature of the body. So yes, cryoprotectants act like the antifreeze in your car. I can’t make this stuff up!

Most insects living in the southern hemisphere, where the climate is more variable, employ freeze tolerance. These insects can stand ice formation in the body. Some will actually initiate freezing their body at relatively high temperatures in order to prepare for a longer hibernation. An example of a freeze-tolerant insect is the woolly bear. [Side note: several winter festivals celebrate the woolly bears kinda like Groundhog's Day.]

woolly bear caterpillar
Woolly bears overwinter as cold-hardy caterpillars. Photo by IronChris, Wiki.

No matter the overwintering strategy, all insects will eventually die if it gets cold enough. However, the lower lethal temperature is different for each species. Insects can overwinter in any life stage - some are belowground and some aboveground. It gets complicated quickly, and so I will save that for another time. Find out more about how insects survive the winter from this Wiki page.


Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...