Will there be fewer insect pests in the garden after chilly winter?

As we shiver through a long, cold winter, we Midwesterners tend to console ourselves with the thought that long periods of subfreezing temperatures will reduce the population of insect pests during the summer. Unfortunately, entomologists tend to dismiss this idea as wishful thinking. While winter mortality of some insect species can be significant, most have evolved mechanisms for tolerating or avoiding freezing that assure survival of more than enough individuals to maintain a respectable population. Also, most insect species can rebuild their populations quickly, even after significant winter mortality. Certain insect pests, such as leaf hoppers, are migratory and, therefore, are unaffected by local winter conditions. Others, like white grubs, burrow deeply into the soil to escape the cold.

Bean leaf beetle is a good example of the adaptability of insects. This common pest of soybean and garden beans overwinters in the adult stage in the duff of wooded areas near host plants. Entomologists at Iowa State University have developed a model that predicts the mortality of this insect based on accumulated average subfreezing temperatures (AAST). This is calculated by subtracting 32 from the daily average temperature and accumulating only negative numbers. In 2001, when the AAST was higher (more negative) than in the previous 10 years, winter mortality of bean leaf beetle was about 95 percent, compared to 41 percent in the previous, mild winter. Despite high mortality in 2001, the population was still moderately high during the following growing season due to low mortality in previous years. See: http://www.ipm.iastate.edu/ipm/icm/2001/5-7-2001/blbsurvival.html.

Insects survive cold temperature in varied and fascinating way. Some, like the springtails, can quickly empty their gut systems to avoid freezing injury. Some accumulate antifreeze compounds in their bodies like glycerol, manitol or even ethylene glycol, as temperatures begin to drop in the fall. Some insects lower the freezing point of water in their cells by producing "thermal hysterisis" proteins, so that fluids in their bodies can remain unfrozen several degrees below the normal freezing temperature of water. Still other species have the ability to move water into extracellular spaces where it freezes without causing harm. In general, since eggs and pupae are more cold tolerant that than larval or adult life stages, species that overwinter as eggs may be little affected by a cold winter.

Weather conditions can have an impact on insect mortality. It the temperature drops suddenly in the fall, insects may not have time to develop the freezing avoidance or tolerance mechanisms mentioned previously. It is difficult to predict whether sudden temperature drops in Missouri during December 2006 may have knocked some of our insect pests for a loop. For those that overwinter on or near the soil surface, snow cover can offer considerable protection. During a long cold period with little or no snow, some insects may die from desiccation and/or freezing injury. During our coldest weather in December 2006 and January 2007, soil dwelling insects were protected by a nice blanket of snow. Just like plants, insects can be adversely affected by rapidly fluctuating temperatures in spring. They may "de-acclimate" during early warm spells and suffer freezing injury if the temperature drops quickly.

The winter of 2006/2007 has been the first real winter we have experienced for some time. Does this mean that we are likely to see a significant decline in the populations of insect pests in the garden? Not too likely, unless we have some wild weather in March and April. So, don't be lax in scouting for garden pests this spr ing and don't expect to be able to sit on a mosquito-free deck next summer.

Chris Starbuck
Agricultural Engineering
573.882.4370
StarbuckC@missouri.edu