When the weather turns cold, we don’t think much about insects. What exactly happens to them during winter? The term overwintering is used to describe how insects navigate the challenges of the winter months and successfully come out on the other side. Insects typically have two different approaches to overwintering. Some utilize the “get out of Dodge” tactic and relocate to a more hospitable environment. The other group, those that don’t migrate, must figure out a way to handle the freezing temperatures where they live.
Insect ‘Snowbirds’ Take a Vacation
We often hear the slang ‘snowbird’ used to refer to people who escape the harsh weather of winter by temporarily moving to warmer locales. Well, you could say there are also insect ‘snowbirds’ that employ this same method. Monarch butterflies are the classic example. They cannot survive in freezing conditions and will fly thousands of miles to their ‘summer home.’ Some fly as far as 3,000 miles to reach Central Mexico, where they overwinter.
While most insects typically don’t travel this far, there are others that look for ways to stay cozy in the cold. Some common insects, like the brown marmorated stink bug, may attempt to relocate into your house! It is worth remembering, though, that insulated structures are comparatively recent developments over evolutionary time. So even those insects that sneak inside still have some level of cold tolerance.
Finding a Way to Fight the Cold
All the insects that stick around during cold weather have had to adapt to survive. Otherwise, internal formation of ice crystals would ultimately kill them. As such, insects can alter their physiology in a number of different ways to stop these crystals from forming. Some produce cryoprotectants. These are substances that protect biological tissues from freezing, similar to the way antifreeze works in cars. Another strategy is the creation of a wax-like coating that covers the insect’s exoskeleton and acts as a barrier to the weather. Some insects may even expel all food and liquids so that ice crystals cannot form. Using methods like these, insects spend their winters tolerating the cold as larvae, pupae, or even adults.
However, none of these strategies are foolproof. Temperatures can only get so cold before an insect dies. With that in mind, most insects will look for extra protection. Seeking shelter in places like bark crevices, tree cavities, leaf litter, or soil can help further buffer against extremes.
Why Overwintering is Important in Tree Care
Knowing why and how insects overwinter can help guide tree care decisions. Therefore, it’s important to have an arborist that fully understands local tree pests, their history and how they commonly overwinter.
Cold winters are more likely to result in higher mortality for those insects that overwinter in exposed locations (e.g. on bark) versus those that are well-protected (e.g. inside of a tree). High overwintering mortality may mean a less severe outbreak in the following growing season. Similarly, a mild winter can result in lower rates of overwintering mortality. Either outcome will influence the way impacted tree pests should be managed on a property.
These same considerations apply to natural predators in a landscape. Both climate and site conditions matter. Habitats with a greater diversity of plant material of all types, shapes and sizes (e.g., overstory trees, understory trees, shrubs, ground cover, mulch and organic matter) are better able to support overwintering beneficial insects. When releasing biocontrol agents as an additional method to control damaging pests, it’s important to ensure those biocontrol insects are able to handle the temperature extremes in the release area and have access to resources such as food and shelter.
When it comes to winter, it is true that arborists can’t change the weather! However, an understanding of insect overwintering dynamics helps us better manage pest populations – and best protect trees and shrubs.