Dormant oil sprays are used to control insect pests on plants and trees in the very early spring before leaves break out of buds. Some types of oil can also be used as a foliar spray for controlling pests like aphids. Oils kill insects through suffocation, from toxicity due to naturally occurring substances, and by interfering with how the pests feed.
Commercial Horticultural Sprays
Most of the oil sprays available commercially are a purified mineral oil base. Chemicals that can harm plants have been removed, and a chemical to allow the oil to emulsify better with water has been added. These sprays will kill any leaves they land on, so be sure the buds are still tight, and cover any nearby vegetation.
Homemade Oil Spray
You can make your own dormant oil spray by mixing 2 quarts of light motor oil with 1 lb. of fish oil or 1/2 cup of dish detergent. Mix this one part of oil to 20 parts of water immediately before spraying, making sure to shake well as you spray. This mixture will not combine well with water, so care must be taken to ensure even application of the oil.
Some types of vegetable oil can be used in horticultural sprays. The most effective types are cottonseed oil and soybean oil; other types have no insecticidal value. The addition of natural insecticides such as garlic or pyrethrin can greatly boost the performance of a dormant oil spray.
From Leprosy to Pests
The neem tree, famous for its use as a leprosy treatment, is the source of neem oil used in many horticultural applications. Neem oil has azadirachtin, a proven insecticide, and other compounds that are both insecticides and fungicides. It can be purchased at many garden supply outlets, and mixed one part of oil to 100 parts of water to spray for many types of insects. The addition of a little dish soap will help the oil mix with water. Neem oil spray can also be used as a foliar spray.