Garden Variety: Harsh winters not always an effective pest control
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The idea of extreme cold temperatures reducing the population of Japanese beetles, bagworms and other insect pests is one that makes many Midwestern gardeners tolerate the cold a little easier. But, how much does winter really affect these insects and others? The answer depends on how and where insect species overwinter, how the cold temperatures arrive and other factors such as snow cover.
Insects may overwinter in any life stage, but most species overwinter as eggs, adults, or one of the stages between and not as multiple stages. For example, bagworms survive winter only as eggs, stink bugs survive winter only as adults, and Japanese beetles survive winter only as larvae.
Bagworm eggs and other insects that overwinter in egg form survive low temperatures in the winter because they are adapted for it. For bagworms specifically, the “bags” or woven structures created over the summer by larvae become a little protective structure for the eggs. Other insect species lay eggs in the crevices of tree bark, in bud scales of various plants, in leaf litter and various other outdoor locations which offer some protection.
Stink bugs and other insect species that survive winter as adults do so by finding sheltered locations and entering a phase known as diapause. Diapause is similar to hibernation in that metabolic activity is slowed or stopped. This allows the stink bugs and other insects that overwinter this way to survive long periods without feeding.
Insects that overwinter as larvae and pupae have much more variability in how and where they overwinter. Japanese beetles survive as larvae deep below the soil surface. Soil is very insulating and protective. Also, when snow is present, the snow cover is like a blanket providing further protection.
Emerald ash borer also overwinters as larvae, but within trees. Tree bark and wood around the larvae provide insulation and those insects are adapted to survive there.
Most insects that overwinter as eggs, larvae, or nymphs are unlikely to be affected by even the coldest Midwest winters.
Adult insects are a little easier to get rid of in winter but only under certain conditions. The highest insect pest mortality rates are most likely to occur under three types of circumstances. The first is an early sudden drop in temperature in the fall. Day length is the main trigger for diapause. If extreme temperatures occur earlier than usual and before insects are fully into diapause, they are more likely to perish than if they were already fully inactive.
The second circumstance that aids in reducing insect pest populations is when a long cold period occurs followed by a period that is warm enough and long enough to bring insects out of diapause, followed by another cold period that is cold enough to freeze the now active adult insects.
The third circumstance for high winter insect mortality is when a long colder-than-average period occurs without snow or ice cover on the ground. This one works best for insects that overwinter above the soil surface in plant debris or below the soil surface but shallow enough to be affected by the colder than average temperatures.
In very mild winters with few fluctuations or temperature extremes, insects may suffer less-than-average mortality rates and be able to reproduce earlier in the spring or later in the fall to further contribute to an increased population.
The current winter season so far has failed to produce the fluctuations or extremes needed to reduce the number of overwintering insect pests in most locations in the region, but at least it has not been so mild as to allow for a higher-than-average survival.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.