How Winter Weather Affects Insect Activity
For some Utahns, November brought “Blizzard 2010”, followed by bitter cold temperatures and additional snow. What does the winter have in store for insect populations? While many might hope that pest insects would freeze to death, not all insects die during the winter. Many insects have evolved ways to avoid or tolerate the effects of cold weather. Alaskans, for example, see large populations of mosquitoes every spring following winters of below freezing temperatures.
Insects are poikilotherms (or “cold-blooded”), meaning their body temperature fluctuates with ambient temperatures. As temperatures cool, insect metabolic and enzymatic activity slows down. Thus, activity for many insects is restricted to warmer parts of the day and year. Temperature also impacts the rate of insect development. Insects regulate their temperature in a variety of ways. Some insects move to an area of preferred temperature, while others, like butterflies, bask in the sun during cool mornings to raise their body temperature. Other insects are endothermic, meaning that they must rely upon their body to produce heat. Bees, for example, generate heat with movement of their flight muscles. Take note as these mechanisms are mainly for short-term regulation of body temperature (thermoregulation).
Insects must be physiologically prepared in advance to tolerate winter temperatures. Many insects go into diapause, which is a state of arrested development in response to lower temperature, shorter day length, and changing food resources. Insects also have “cold-hardy” life-stages adapted to resist harsh climates. For example, aphids and some mosquitoes and grasshoppers overwinter as eggs (Fig. 1); wood borers overwinter as larvae within wood (Fig. 2); some moths overwinter as pupae in protected sites; and many beetles (e.g., alfalfa weevil) and true bugs overwinter as adults. Insects reduce the effect of freezing temperatures by being freeze-tolerant or freeze-intolerant. Freezing is problematic because ice crystals form and disrupt cell membranes and eventually kill an insect. Freeze-tolerant insects synthesize ice-nucleating proteins that isolate the formation of ice outside of the cells and slow the detrimental ice formation inside of cells. Most insect species, however, are freeze-intolerant and prevent the formation of ice by producing compounds including glycerol, sorbitol, and trehalose that act like antifreeze. Additionally, during diapause, insects stop feeding and purge waste to reduce any chance of ice formation on food in the gut.
To escape the harmful effects of winter, some insects migrate south to places with relatively warm winters. Monarch butterflies, for example, overwinter as adults on trees in southern California and forests in Mexico and make their way back north every spring. The susceptibility of these butterflies to harsh winter conditions was apparent in 2002 and 2004 when severe winter storms came across Mexico and millions of monarch butterflies froze to death. In central and southern Utah, where winters are warmer, corn earworm overwinters as a pupa. In the spring, some corn earworm moths will migrate back to northern Utah. Insects also combat the winter by seeking shelter in soil, debris, under bark, and in and around our homes. Some lady beetle species overwinter as adults in large populations under bark and forest debris in the mountains (Fig. 3). Snow cover and higher soil moisture maintained by frozen surface layers can act as an insulator and can significantly enhance the survival of overwintering insects. However, insect life-stages that overwinter in the soil and do not burrow deep enough may succumb to freezing if ice progresses to lower soil strata. In Utah, many adult boxelder bugs may forego outdoor overwintering and seek shelter in and around buildings and homes and become a nuisance pest.
Insect populations can decline when freezing temperatures occur out of season. Early frosts in the fall can decrease insect populations not yet prepared for cold temperatures. Insects that prematurely emerge from their cold-hardy stage or come out of diapause can also be susceptible to freezing during unpredictable spring weather. The spotted wing drosophila, an insect that can harm unripe fruit, was recently detected in Utah. Many wonder if winter will suppress this initial population and decrease the impact of this potential pest. In late November, snow was on the ground and a lone spotted wing drosophila managed to find its way into a monitoring trap. It is not known whether spotted wing drosophila will successfully overwinter in northern Utah, although survival in southern Utah is likely. Predicting an insect’s overwintering success is complex and much like forecasting the weather.
-Ricardo Ramirez, Entomologist