Where did the insects go?

When I published the recent blog post on reptile overwintering strategies I received a question from someone curious about what insects do to survive the winter. Those of us in colder climates may notice a sharp decrease in insect activity over the winter. Where do they go? They have a variety of strategies for surviving the cold.

Insects are ectothermic. They do not create their own body heat.

Some insects cope by migrating. When birds migrate, individuals carry out the round-trip journey on a yearly basis. Insects don’t live nearly as long as birds, which complicates things during a long journey. The lifespan of a single insect is not long enough to survive the trip to an overwintering location and back again. Insects have to carry out these migrations over the course of several generations.

The monarch butterfly is the most famous example of a migrating insect. Monarchs have been known to travel over a thousand miles to reach overwintering havens in Mexico. After overwintering, monarchs start their way north again. They’ll lay their eggs on the way, and their offspring continue the journey north after their parents have died. The butterflies go through several more generations before they reach summer homes in Canada and northern United States. 

Photo by Raina Kumra.

Monarchs cluster in the thousands at overwintering locations.

Of the insects that do not migrate, some are freeze-tolerant (they can survive even if their internal fluids freeze) and the others are freeze-avoidant (they would die if their body fluids freeze). Most insects are freeze-avoidant. Some freeze-avoidant insects are able to change their internal chemistry so their body fluids freeze at a lower temperature. Other insects rid themselves of as much water as possible before winter so ice won’t form inside them.

The southern hemisphere has a lot more variation in the weather than the northern hemisphere. Weather can change very quickly, so insects have had to adapt to be able to survive a sudden freezing. These are the freeze-tolerant insects. Freeze-tolerant insects are able to control where ice forms in their bodies so they don’t get harmed.

Woolly bear caterpillars hatch in the fall and are freeze-tolerant. This helps them survive in the cold areas where they live, including the arctic! 

Insects in a climate with several months of prolonged cold may enter diapause. Diapause is a hibernation-like state of suspended activity. Much like hibernating mammals, insects will build up extra energy stores before entering diapause. They then search out a protected shelter. Some dig under the ground while others hide under tree bark or nestled within plant tissue. Insects remain dormant until the weather improves enough for them to become active again. Some insects go through diapause as adults. In other species the adults breed before winter and die in the cold, leaving their immature eggs and larva to go through diapause. 

Ladybugs in search of somewhere to overwinter can sometimes swarm into houses. If left alone, they will depart again in the spring.

Ants are a rare case of insects that are able to remain active during the winter months. Underground ant colonies can extend below the frost line and keep the ants warm enough to survive. Ants living there feed on stored food during the winter and don’t emerge from their nests until spring.

- Kate Dzikiewicz, Paul Griswold Howes Fellow