Horticultural oils are highly refined so that compounds toxic to plants are removed. Considered effective and safe, they can be used to control insects as well as diseases. The types of oils available for pest control include dormant oils and summer oils.
Dormant oils are used during the winter season when plants are dormant to control over-wintering stages of insects such as aphids, spider mites and scales. An oil applied during the dormant period will suffocate the over-wintering eggs of aphids and spider mites, or suffocate the adult in the case of scales.
Light horticultural oils, sometimes called “summer oils”, are a lighter more viscous version of the dormant kind and can be applied to actively growing plants during the growing season. In terms of insect control, summer oils can be used to control aphids, mites, thrips, scales, mealybugs and their eggs.
Oil phytotoxicity (damage to a plant) can occur if the product is not used properly. Plant damage can occur when: 1) too much is used; 2) plants are under water stress; 3) temperatures are over 90 F; and 4) when dormancy is mistaken (i.e. spraying too early in the fall). Wait until December to February to apply dormant oils. Temperatures must be above 45 F.
For detailed tips about how to use these products see the file Using Horticultural Oils