As you sit drinking your morning cup of coffee with the snow falling and the wind howling, the last thing you are thinking about is those poor little insects out there trying to make it through the cold adversities of winter. Instead of thinking of the winter as a kind of “black box,” careful observation during the winter can provide several clues as to which insects and mites could be potential problems for the coming season. By making written or mental notes on the severity of the winter (that is, snow levels, prolonged cold, sudden temperature changes) and knowing the basic life cycle and habits of the pest, we can catch a glimpse of how these animals survive and their potential to become problems during the growing season.
As early as the 1930s, the importance of overwintering biology on pest management was recognized. Salt states that “preparations for the control of pest species could be made more intelligently if information was available on the ability of insects to resist winter temperatures.”
Winter is a period of dormancy for most insects. They seek protected places where they are not exposed to predators or repeated freezing and thawing. Insects and mites have a variety of ways they survive the winter.
Many insects overwinter as adults, such as leaf beetles, some aphids, most leafhoppers, and many beetles. Overwintering sites include those under the loose bark of trees, fallen leaves, and other debris on the ground. Some insects, such as the elm leaf beetle and boxelder bugs, actually seek shelter indoors in homes or other protected structures.
A vast number of insects overwinter as eggs, either singly or in masses. We have probably all seen the egg mass of the eastern tent caterpillar attached to a twig and exposed. Aphid and plant bug eggs many times are situated in leaf and bud scales on woody plants, where they are protected. The notorious female gypsy moth lays her eggs near the base of trees and covers them with hairs from her body. Bagworms spend the winter as eggs in the old bags from the previous year.
An equally common life stage for overwintering is the larval and/or pupal stage. Many of the leaf-eating caterpillars (that is, mimosa webworm and cecropia moth) overwinter as pupae in a silken cocoon or other protective structure. Turf-feeding grubs (that is, southern masked chafer, Japanese beetle) overwinter in the soil, where they are generally protected from the extremes of winter temperatures.
In spite of these effective overwintering strategies, insects are still vulnerable to winter conditions. Studies on the overwintering biology of economic pests teach us much about the relationship between overwintering strategies and pest population dynamics. Here are several examples of how this information can be helpful in pest management.
The mimosa webworm is a common defoliator of honey locust trees in the South and Midwest, where it forms unsightly webbing and browning of the leaves. In the fall, the larvae descend the tree and look for overwintering sites, which may include everything from mailboxes to shutters to door frames. In more isolated areas, the larvae seek out cracks and crevices in the bark of the host tree. Once in place, the larvae spin a white silken cocoon and pupate. The silken cocoon is quite durable, waterproof, and protects them from cold, drying winds of winter. In spite of these strategies, the mimosa webworm is still vulnerable to extreme winter conditions.
What do we mean by extremes? Studies conducted in central Iowa in the early 1980s revealed that, following severe winters with prolonged cold and consistent overnight temperatures of –20° to –25°F for 2 to 3 weeks, webworm populations were greatly reduced the following season. During the winter of 1981–1982, field studies showed 100% mortality of webworm pupae in the study sites. Two weeks of prolonged cold had preceded this dramatic drop in survival. In contrast, during the 1982–1983 winter, which was mild, adult emergence in our studies was 66% with 50 to 80% defoliation of honey locust trees being common the following season.
What keeps these extremes from wiping out the population? Obviously, some of them survive, but where? Studies on their overwintering habits revealed that the larvae that overwinter behind shutters, eaves, and other protected sites on homes experienced temperatures 5° to 10°F warmer than their less fortunate “cousins” who were forced to overwinter on exposed sites on the trunk of the tree. This 5° to 10°F temperature differential makes all the difference in survival. In addition, the microhabitat of the host plant is also important. Trees that are situated in protected locations (that is, surrounded by structures in courtyards) tend to have chronic mimosa webworm infestations. In addition to protected sites, the mimosa webworm, like many temperate insects, has the ability to supercool (cool below 32°F without ice formation). However, once that supercooling point is exceeded, the freezing of body fluids and cells is lethal. During the Iowa winter of 1981–1982, overnight temperatures regularly equaled or exceeded the supercooling point of the mimosa webworm. One or two isolated nights at these temperatures may not cause dramatic mortality, but duration of exposure begins to play a major role. The longer the exposure, the greater the probability of mortality. Cold temperatures and duration of cold are a one–two punch in mimosa webworm population dynamic.
Another example is the gypsy moth. This insect is a serious defoliator of forests and woody landscape plants. The larvae feed during the months of May to early July, pupate; and then the adult moths emerge. Female moths lay their eggs in July at the base of trees or any structure that appears suitable. These eggs remain throughout the fall and winter, hatching the following spring. As with the Japanese beetle, snow cover and winter temperatures have a dramatic effect on overwintering survival of the eggs. Studies have shown that eggs die at –18o to –25oF, and ambient tem-peratures of –22o to –24°F can be lethal if they last for 14 days. Like the mimosa webworm, the eggs are able to supercool (compliments of glycerol, closely related to ethylene glycol) to –22°F.
In Maine, gypsy moth eggs were observed to survive to –26oF with snow cover and in Russia under snow cover at –60°F. The gypsy moth is a survivor. Without adequate snow cover, it is much more vulnerable to extremes in temperature. Some biologists believe the gypsy moth will not be limited by hosts but by the depth of snow cover. Observations on temperature extremes and snow depth can be helpful predictors in anticipating overwintering egg survival of the gypsy moth.
These examples illustrate how overwintering strategies and climatic factors affect pest population dynamics. Little is known about how the majority of insects overwinter. While not an exact science (just like predicting the weather), basic observations, record-keeping, and a little thought can be helpful in “predicting” what your potential populations might be when, finally, the sun is shining and the birds are singing.