Cleaning up garden and flower beds in the fall is an effective way to control various insect pests. Many insects survive the winter buried in the soil (5-25 cm) or on its surface. Debris left on the soil surface will in fact help the insects to survive.

In winter, the temperature of the soil is higher than the air temperature. Various components in the soil buffer it against severe freezing temperatures. For this reason. insects are able to survive in the soil, protected from the winter cold. Debris left on the soil surface further protects the soil, thus making it more hospitable for overwintering insects and increasing their chances of survival. If you remove the debris, the soil will freeze to a greater depth and more insects will likely die during the winter.

When and How to Remove the Debris

The best time to remove the debris is in early October, after the insects have buried themselves in for the winter. You can remove the debris earlier or later, still with effective results in terms of controlling insects. When removing the debris, it is wise to till the garden and flower beds. Tilling will bring the insects up the soil surface, where they are more susceptible to killing temperatures.

Drenching the soil in the fall with an insecticide to control insect pests is not recommended. In order for an insecticide to be effective, the insect must be active. Insect larvae, pupae and adults overwintering in the soil are not active, and therefore pesticide control at this time of year is ineffective.

Garden Cleanup - Insects that overwinter on the soil surface under garden debris or buried in the soil include the beet leaf miner, cabbage maggot, Colorado potato beetle, flea beetle, imported cabbage worm, onion maggot and spinach carrion beetle. Incorporating organic matter in the fall is recommended, but it should be well tilled into the soil; if left on the soil surface, the organic matter will help protect insects over the winter.

Flower Beds and Shrubbery - In flower beds and shrubbery, a number of insects overwinter successfully under debris or in the soil near the base of ornamentals. Such insects include the currant fruit fly, imported currant worm, pear slugs, rose curculio and spring cankerworm. Removal of the leaf litter and a shallow tillage under ornamentals will help control many of these insects.