With winter approaching, plants are acclimating and insects are altering their behavior and physiology in response to shorter day lengths and cooler temperatures. Plants are accumulating sugars and proteins as they become dormant for the cold winter months ahead. Overwintering plants survive temperatures below 32°F by tolerating or avoiding freezing. Some plants, such as zoysiagrass, survive by tolerating ice formation in spaces between cells. Other plants, including fruit trees grown in Missouri, survive low winter temperatures by avoiding freezing. Water inside freeze-avoiding plants remains in an unfrozen state due to supercooling.
Insects utilize one or more methods for winter survival including migration, freeze avoidance or freeze tolerance. Some insects avoid cold weather by migrating to warmer climates. Probably the best example of migrating insects are Monarch butterflies that begin flying south in late August to overwinter in Mexico. Green darners, a common species of dragonflies, also migrate to Texas and Mexico to avoid freezing temperatures. Another way insects avoid the cold is by seeking shelter. For example, adult Asian lady beetles overwinter in cracks and crevices in roof shingles, windows, and siding on structures. All life stages of European honey bees overwinter inside hives. Midge larvae reside in galls for winter protection. Leaf litter is another common shelter for overwintering insects such as bean leaf beetles, stink bugs, and grasshoppers. Other insects can be found within bud scales (mites) or under loose bark of trees (codling moth larvae), in leaf litter, underneath rocks (pillbugs), or in the soil (Japanese beetles) during winter (Figure 1).
Many insects spend the winter in a state of dormancy called diapause. In the fall, shorter day lengths and exposure to cool temperatures trigger hormone production, which lowers insect metabolism and their energy requirement. When conditions become favorable near spring, insects resume their activity. Insect eggs, larvae, pupae, or adults can undergo diapause, depending on the species.
Freeze-avoiding insects lower the temperature at which their body fluids freeze. Thus, they can withstand temperatures below 23°F. Some insects avoid freezing by emptying their gut to prevent internal ice formation. Certain insects can also produce antifreeze proteins in their hemolymph (body fluid) that bind to ice crystals during their formation to prevent crystals from growing. Other types of antifreeze compounds synthesized by insects are polyols and sugars, which lower the freezing point of their hemolymph. Glycerol is a common type of polyol and sorbitol is a sugar that lowers the temperature at which an insect freezes. Codling moth and emerald ash borer larvae and forest tent caterpillar eggs survive winter by freeze avoidance.
Freeze-tolerant insects are able to withstand the formation of ice internally. In the fall, ice nucleation-active agents in the insect's hemolymph or in other parts of the body promote non-lethal ice formation extracellularly (i.e., outside cells) at temperatures usually between 23 and 14°F. Even though ice forms within the insect, it is non-lethal. Woolly bear caterpillars are an example of a freeze-tolerant insect.
For the upcoming winter, the National Oceanic and Atmospheric Administration (NOAA) is predicting a weak La Nina event. Thus, there is much uncertainty in the forecast for Missouri weather. At best, we can look for insects seeking shelter and follow their lead to avoid freezing.
REVISED: February 21, 2017