Many insects will overwinter in the egg stage on the branches of your trees and shrubs. Horticultural oils sprayed during the dormant season effectively smother these insect eggs by forming a coating of oil over them. Scale insects and certain mites that winter on plants are also suffocated when the horticultural oil blocks their spiracles, the air holes through which they breathe.
Horticultural oils may also be effective for smothering fungal spores, thus reducing the incidence of certain fungal diseases like rust or powdery mildew.
It is especially important to spray your fruit trees with horticultural oil during the dormant season. This helps kill the eggs of the codling moth and other insests which are so destructive to apples, peaches, pears, and other fruits.
For the most effective control of overwintering insect pests, it is good to make two applications of horticultural oil - once in late November after the leaves have fallen (be sure to rake up the leaves first) and again in February or March before the buds begin to swell. Be sure to coat all surfaces of the tree branches and trunk.
Read the label before you spray and be sure to follow all the label directions!