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We sure have experienced very cold air temperatures and even
colder windchills this winter. 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 with 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.
Some insects just move into human structures in the fall and
keep warm until spring. Think about boxelder bugs and multicolored Asian ladybeetles
aggregating on 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 ladybeetles mass on structures every fall.
Photos by Robert Koch.
But 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 -42C 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. [Sidenote:
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.
