A. Oil sprays applied after trees have dropped their leaves in fall and before the buds open in spring are used to kill pests that overwinter on the trees. "Oils generally are most effective against small, immobile or slow-moving, soft-bodied pests, such as aphids, scales, leafhopper nymphs, whiteflies, and mites," says Lee Reich, Ph.D., the author of Grow Fruit Naturally (Taunton, 2012).
Horticultural oils, also called superior or stylet oils, may be petroleum-or plant-based, and are diluted at higher concentrations of oil for use during dormancy or at lower oil concentrations for spraying when plants are actively growing. Because oil sprays can injure plants, Reich says, it's important to follow product label instructions and to spray when conditions are favorable. "Avoid damage by spraying when the air is dry and temperatures are between 40 and 90 degrees Fahrenheit," he explains. Do not spray plants that are moisture-stressed, and do not apply oil sprays within 2 weeks of spraying sulfur. Observe these precautions whether you're applying oil sprays in winter or summer. In addition to controlling certain insect and mite pests, oil sprays may be used to limit some disease problems, including powdery mildew of gooseberries and pseudomonas blight of pears, Reich adds.
Ask Organic Gardening is edited by Deb Martin.