Cold weather’s effects on insects and plant disease organisms
Several gardeners I work with have recently expressed hope that the extreme cold temperatures and abundance of snow and ice that we have experienced the past few weeks in central Ohio will kill or reduce populations of insects that gardeners battle each growing season.
Unfortunately, cold weather has little effect on most species of insects, as they have complex biological systems and behaviors that allow them to survive even the coldest winter conditions. Many insects have lived on the planet for millennia in varying weather conditions and through many different cycles of climate change.
Although we might sometimes wish for a world free of insects while swatting mosquitoes on the patio on warm August evenings, the fact is that most insects are not harmful to plants or animals, including humans.
There are millions of different species of insects on Earth and very few are harmful. In fact, most insects are beneficial, performing important ecosystem functions such as pollination of food and ornamental crops, keeping populations of harmful pests in check, and contributing to the food web for other insects, arthropods, birds and some animals.
Insect survival tactics
While certain insects such as leafhoppers, armyworms and black cutworms do not survive cold winters, many insects survive by going through a diapause, which is a form of physiological dormancy exhibited by insects and other arthropods such as spiders. It is similar, but not entirely equivalent to hibernation exhibited in some mammals.
Diapause allows insects and their relatives to survive periods of cold winter weather and a lack of available food sources that would otherwise lead to death of the insect.
Some insects survive the winter as eggs that were laid by adults during the previous season or in larval forms. Protective bags built by bagworms (Thyridopteryx ephemeraeformis) easily seen hanging on junipers and arborvitae during the winter contain eggs that will hatch when warmer temperatures arrive in spring.
Certain insects are freeze-tolerant, thanks to compounds in their bodies called cryoprotectants, which are similar to the antifreeze that prevents your car engine from freezing in winter. Yet other insects such as stink bugs and lady beetles will find ways to enter structures
and overwinter in wall voids, attic spaces and basements.
Snow acts as insulation
Many insects spend the winter burrowed underground in the soil or in mulches and under fallen leaves. Other insects overwinter under tree bark or in crevices in the branches of trees and shrubs, or in hollow plant stems of herbaceous perennial plants such as milkweed and coneflower. Other insects will
diapause under rocks.
A layer of snow actually serves as insulation for insects overwintering in these locations, keeping the temperature significantly warmer than the air above the layer of snow.
Plant disease organisms also survive cold
Unfortunately, most fungi that cause plant diseases are unfazed by cold winters. Many plant pathogenic fungi spend the winter in a dormant state, usually on old plant material or in the soil.
Many fungi create special survival structures in cells to survive extended cold and dry periods. Some plant pathogenic fungi, such as those that cause many of the rust diseases, never survive the winter in Ohio, and instead must be reintroduced by airborne spores every summer.
A severe winter in the Southern U.S., where they do overwinter, may decrease their levels in Ohio the following season.
Spring temperatures more critical
More than winter weather, early onset of warm temperatures in spring is a greater determinant of the potential pressure of certain insect populations during the growing season. In general, the earlier in spring that warm temperatures occur, the greater the potential for larger populations of certain insects throughout the growing season.
So next August, when you are swatting mosquitoes on your forearm and picking tomato hornworms off of your tomato plants, just keep saying to yourself, “Most insects are beneficial ... most insects are beneficial!”
Mike Hogan is an associate professor at Ohio State University and an educator at the OSU Extension.hogan.1@ osu.edu