For decades, fruit growers have routinely applied a refined petroleum oil spray to their trees in the early spring. This is done to kill the eggs and adults of certain spider mites and other insects that spend the winter on or under the bark. It is also effective against scale insects. Petroleum oil sprays have many advantages. They are different from other insecticides in that they act mechanically rather than as a true "poison". Oils act by covering the insect or its eggs and smothering them. They are not persistent in the environment long and have extremely low toxicity to non-target organisms.

These sprays are called "dormant" oils because they must be applied before the plant begins pushing new growth. If applied after the plants have broken dormancy, oils can "burn" the new growth. This type of injury is called phytotoxicity.

Proper timing is of great importance with dormant oil sprays. To be effective, the temperature of the air and the surface of the bark has to be warm enough to allow the oil to spread uniformly. It must be able to flow into the nooks and crannies where eggs and insects may be hiding. However, too much warm weather will push tender new growth and increase the risk of phytotoxicity.

The general rule for oil applications is to spray after 2 or 3 consecutive days of temperatures over 40 degrees. A single day of adequate temperatures is not enough since it usually takes several days for the wood of the tree to warm properly. Avoid applying oils when rain is expected within 24 hours.

Dormant oils can generally be used on fruit trees, winter creeper (Euonymus), crabapple and magnolia. Check the label for other species.

Several tree species are sensitive to dormant oils. Generally, avoid using oils on beech, birch, blue spruce, boxwood, butternut, Douglas fir, hickory, mountain ash, walnut, Japanese maple, Norway maple and sugar maples. Blue spruces sprayed with oils tend to lose their blue color for a year or more.

Oil sprays should not be used within two weeks of any fungicide application containing sulfur. Avoid mixing oils with the insecticides Sevin or dimethoate. Always check the label of the product for other restrictions.

In recent years, new refining methods have produced oils that may be used during both the dormant and growing seasons. These "horticultural oils" are used at a different mixing rate depending on the season. The label will indicate the dilution to use for each season. Normally, the summer rate is half as strong (i.e. more dilute) as the dormant season rate.