Surviving Kentucky winters is a challenge for cold-blooded arthropods. A few species, such as the monarch butterfly, cede cold weather to the hardy and fly south for the winter. Those that remain use a state of arrested development called diapause to survive this inhospitable time of year. Clearly, they cope with their challenges more successfully than the media family whose term they share.
Diapause occurs at different stages of the life cycle, depending on the insect species. Preparations include increasing fat stores, producing antifreeze compounds, and moving to some sort of shelter. Their antifreeze allows “super-cooling” so body liquids can drop below freezing without the formation of lethal ice crystals.
The insect species that inhabit Kentucky vary in their susceptibility to winter conditions. Sudden early freezes, days when high temperatures do not climb above freezing, and icy rain without the protection of a blanket of snow can help to “reset” insect populations or distributions that have benefited from precious mild winters.
Understanding wintering strategies and recognizing wintering forms may allow the use of cultural practices to expose pests to the elements and predators or to protect beneficial species.
Arthropod Overwintering Strategies
While the above tactics are generally effective, the potential for survival varies depending on the insect’s stage of development and degree of exposure to the elements. Here is a look at some overwintering strategies.
Eggs are a particularly hardy stage in the life of insects. Hard “shells” or protective coatings produced by females can protect against both predators and temperature extremes (Figure 1).
Silk bags containing hundreds of eggs (Figure 2) would seem like an ideal wintering retreat but 24 hours at -1oF or below can reduce survival by 50%.
Eastern tent caterpillars are winter-hardy insects that are among the first to be active in spring. A varnish-like coating of spumaline, produced by the female moth, keeps eastern tent caterpillar eggs from desiccation and provides some protection from tiny wasps that parasitize the eggs. Egg removal may be a potential management strategy on small trees. On the other hand, preservation of praying mantid egg cases (Figure 4) can mean some more natural control during the spring and summer.
Some insects spend winter belowground, protected by several inches of soil, where they can escape unusually cold winters. This tactic usually is only “uncovered” when turning ground during soil preparation.
May beetles, Japanese beetles, and green June beetles are among the beetles that spend the winter in the soil as white grubs. Even the harshest of winters are unlikely to have a great effect on them. Many of these species are “annual grubs;” most of their feeding was done in late summer so they will feed little if any in spring. A few species spend more than one year as larvae and will continue to feed.
Full grown tobacco and tomato hornworms burrow into the soil and pupate (Figure 6). They pass the winter in the soil near the site where their host plants were grown.
Colorado potato beetles and woolly bear caterpillars are examples of insects that spend winter in relatively exposed places, such as under leaf litter or near the soil surface. They are more likely to be killed during cold snaps, especially if there is no snow cover or other protection.
Large numbers of lady beetles (Figure 8) and face flies congregate in sheltered voids for the winter months and remain active if their lair remains warm. However, they do not reproduce during that time and disperse in spring to resume their lives as predators.
By Lee Townsend, Extension Entomologist