Landscape - Dormant Oil Use

Dormant oils should be applied in late winter or early spring for control of cool season insect and mite pests. The following is an article on using dormant oil.

DORMANT OIL. Late February and March is a normal time to apply dormant oils to trees and shrubs before bud break. In this mild winter, we may see great benefit and for past problem areas it is good IPM. The rate is usually 3-4 gallons of oil to 100 gallons of dilute spray. Soak the bark of twigs, branches and the trunk to run off; you will be more effective and economical if the vegetation is pruned first. Consider pruning out the eggs of tent caterpillar or moth pupae before you spray. They are easier to spot before vegetation appears. The dormant oil will help kill overwintering eggs, mites, some scales and other overwintering stages it might contact. For example, adult and nymphal populations of white pine sheath mite might be impacted. Contact is the key as there is no residual with oils.

Late February and March always is a tough month to plan oil coverage. You need wait until the weather warms and it is not likely to freeze overnight. We usually say it should be 45 degrees F or more. Spray on a day when plant material is dry and the wind is not strong, so the oil has a chance to dry slowly, allowing for penetration of the chemical into the insects/insect eggs you are targeting. Do not spray if leaves are just opening, as they often are most vulnerable to chemical damage at this early stage. Do not include a fungicide or sulfur compound with your spray. Avoid brambles and most maples and evergreens with bluish color such as blue spruce. Check the label for plants such as black walnut, beech, holly, evergreen magnolias and redbud since they often exhibit some sensitivity to dormant oils.

Reprinted from an article by Dr. Dewey Caron, Extension Entomologist, University of Delaware in the Ornamentals Hotline from UD Cooperative Extension.