The use of oils as part of a control program is becoming more favored instead of using synthetic pesticides. Oils can be distilled from petroleum such as horticultural oils, Volck oils, summer oils, dormant oils or mineral oils or oils can be extracted from plants and animals such as neem oil or fish oils. Oils are generally effective against aphids, scale crawlers, mealybugs, spider mites, whiteflies and small caterpillars.
When oils are applied, a thin layer covers the insect or mite. The oil clogs the spiracles or pores through which they breathe causing death by suffocation. Oils can also disrupt membrane function or structure or disrupt feeding. Multiple applications may be needed for control.
Sometimes oils can injure a plant causing leaf scorching, defoliation, reduced flowering and stunted growth however there are some items on the label to be aware of such as unsulfonated residues, viscosity, and distillation. Usually the higher the unsulfonated residue (UR), the less likely for plant injury. Also the lower the viscosity, the less likelihood for plant injury. In addition, the distillation range is a measure of the purity of the oil fraction so distillation ranges of 80°F or less are considered appropriate.
It is best to apply oils when conditions are lower than 85°F and 90% humidity is recommended, since the longer the wet oil remains on the foliage, the greater the chance of phytotoxicity. Also it is not advised to treat stressed plants with oils and some plants are sensitive to oil such as azalea, hibiscus, impatiens, photinia, and spruce so they should not be treated.