The end of the growing season signals the end of the battle with insect pests in vegetable gardens. But the end of a battle is not the end of the war and new battles will be fought next season.
Where do all those insects retreat to for winter? Some are snowbirds and head south for the winter. Others overwinter in the garden, while some spend the winter in nearby cracks and crevices of structures and on weeds.
The snowbirds include armyworm, corn earworm (also known as tomato fruitworm) and striped and spotted cucumber beetles. Since these insects do not overwinter in the garden, sanitation is not considered a control method for these insects.
However, many insects do overwinter in the garden and cleaning up and destroying plant debris can reduce their numbers. Reducing the population of insect pests limits the amount of damage they cause and provides more control options.
Insects that overwinter on plant debris in the garden include cabbageworm, cabbage loopers, and squash bugs. The cabbage caterpillars overwinter as pupae inside cocoons attached to plant debris, usually the host plant. Squash bugs spend the winter as adults hiding in plant debris.
Insects that overwinter in the soil are the adults of Colorado potato beetles, the eggs of grasshoppers, and the pupae of squash vine borers and onion maggots. Fall tillage of soil reduces these insects by exposing the insects to colder temperatures. Removing plant debris removes an insulating layer that also protects insects from extreme temperatures.
Bean leaf beetles spend the winter as adults in nearby sheltered areas, preferring to spend the winter in the plant litter of windbreaks and woodlands.
Some insects spend the winter on weeds near the garden. Fall sanitation not only includes cleaning up or tilling under vegetable debris in the garden, but control of nearby weeds as well.
When cleaning up plant debris, can it be added to the compost pile? The general recommendation is to not add insect infested plants, diseased plant debris, or weed seeds to home compost piles.
Most plant diseases and weed seeds, as well as some insects, are destroyed during composting when temperatures in the pile center reach 140° to 150°F. However, in many home compost piles, it is difficult to mix materials thoroughly enough to bring all waste to the center where it will be exposed to these temperatures.
Finally, it is often asked if insecticides applied to bare soil in fall will kill overwintering insects. The answer is not very often, if at all. Overwintering insects are often in the pupae or egg stage where they are protected from insecticides. Applying insecticides to soil to try and control overwintering insects is not a responsible or effective use of a pesticide.