Apply Dormant Oil to Reduce Insect Pests

Damage from many leaf feeding insects can be reduced by applying a dormant oil spray to the bark of trees and shrubs during the winter dormant season. The caterpillars you refer to are the larval stage of butterflies and moths. They live over as pupae in cocoons on the bark of trees and some shrubs. Late in the season, the adult moths and butterflies also deposit eggs in the bark crevices, which overwinter and hatch when weather warms in the spring.

Neither the pupae in the cocoons nor the eggs are susceptible to most pesticides. However, they are living breathing organisms and require oxygen to survive. Dormant oil spray plugs up air intake areas and prevents oxygen from reaching the eggs and pupae inside. Without oxygen, they die and do not develop into larvae and adults. Dormant oil is also very effective in killing the eggs of mites and scale insect eggs and adults which grow on the bark.

Dormant oil spray is considered to be an organic pesticide since its action is not chemical in nature. The old dormant oil sprays could only be used in the dormant season because they would damage green leaf tissue. Pesticide oil sprays are now available for year round use which do not damage green foliage. This smothering technique works well for other insects during the growing season. These oils have become one of my favorite organic insect controls.

Dormant oil is not 100% effective in preventing insect damage. Spray has to be thorough enough to cover all bark areas of branches 2 inches or more in diameter, although most overwintering insects are on the trunks and larger lower branches. Also, insects can come from areas adjoining yours which are not treated.

Lime is the most effective way to reduce the natural acidity of our soils. Vegetables and lawns in particular respond to lime application. I like to apply 3 to 5 pounds of lime per 100 square feet at least every other year to vegetables. Apply as soon as the soil becomes dry enough to till or spade. Mix it into the soil along with compost, bark dust or other organic materials.