Many people have asked if the cold temperatures we have been experiencing in the US will help to kill off some of our insect pests. While some pests may be negatively impacted, the vast majority will be unscathed by even sub-zero temperatures. This is because as a group insects have evolved many different overwintering strategies to protect themselves from the cold.
Strategy #1: Migration
Some insects simply book it out of town when the weather starts to change. Much like people from New England or the Midwest who have second homes in the South, these insects will travel long distances in order to stay warm.
The most famous example of this is the monarch butterfly, which is continually on the move throughout its native range. Each year they travel up from Mexico as the US and Canada are warming, using successive generations to go through the Midwestern US and into Central Canada. Eggs are laid as they go, hatching into new caterpillars that become the next group of butterflies on the move. Eventually they reverse course, moving southward out of Canada, and the last generation of the year will end up in overwintering sites in Mexico.
Aside from monarchs, other Lepidopterans such as painted lady butterflies and the Madagascan sunset moth will also migrate. Outside of the moths and butterflies, short-horned grasshoppers can migrate long distances as can dragonflies like the green darner or the globe skimmer.
Strategy #2: Freeze Avoidance
Besides migrating away from the problem, insects may also take winter head on. They can overwinter in any life stage (egg, larvae, nymph, pupa, or adult) but freezing could be lethal to any of these stages. Most insects will undergo behavioral or biochemical changes that ward off freezing. One way to accomplish this is for larvae or nymphs to choose a dry hibernation site to eliminate the chance of ice crystallization from an outside source. If a larvae pupates in a site like this, they will be protected as well, the same is true for eggs that adults lay. Being situated in this drier habitat allows the insect to become supercooled, or to have water in them that is still liquid below the normal freezing point for water. Adult insects that overwinter may end up using our homes to find these nice, dry places to wait out the cold. This is how we end up having issues with multicolored Asian lady beetles and brown marmorated stink bugs.
Alternatively, some insects may produce what we call “cryoprotectants” which are natural antifreezes that keep them ice free. The most common antifreeze in insects is called glycerol. Some can produce ethylene glycol, which is the same stuff you put in your car to prevent it from freezing up!
Strategy #3: Freeze Tolerance
A less common strategy for insects is to freeze and thaw during the winter, in what is known as freeze tolerance. Freeze tolerant insects actually encourage their body fluids to freeze and avoid supercooling, inducing their fluids to freeze at relatively high temperatures. These insects produce internal compounds that cause ice crystals to form. They control where these compounds are produced and therefore where ice is formed in their body, shunting it into non-lethal areas. This type of overwintering strategy is more common in the southern hemisphere but in our part of the world, the famous wooly bear caterpillar is a freeze tolerant insect.
No matter the method, winter rarely has a large impact on insects or their relatives. While overwintering strategies can be as simple as leaving the cold behind or as complex as making their own antifreeze, all of them ensure there will always be bugs around!