Horticultural oil sprays are applied to leafless deciduous trees to control overwintering insects or prevent disease. The term "dormant oil spray" is used synonymously with horticultural oil spray. These sprays are applied from late November to late February, or until bud break in the early spring. Horticultural oils are petroleum-based products or sprays made from selected vegetable oils and neem seed oil.
Horticultural oil sprays made from cottonseed, soybean or neem oils are considered the least toxic disease and insect remedy. Petroleum-based horticultural oil is not safe for edible fruit, according to University of Washington Botanic Garden horticulturalists. Petroleum, vegetable and neem seed horticultural sprays are used in the early winter to disrupt the life cycle of insects that overwinter in trees. Neem seed oil compounds inhibit insect feeding, repel pests, disrupt insect growth and kill fungi.
Horticultural oil controls insects by smothering them or interfering with their metabolism. Controlling insect populations also controls disease where insects spread bacteria or viruses. Watermelon mosaic disease, squash mosaic and powdery mildew are controlled with horticultural spray oils. Petroleum-based spray contains naphthene and paraffin compounds. Paraffin is more toxic to insects and less toxic to plants, while naphthene is less likely to injure plants but is less insecticidal. Neem-derived insecticides contain azadirachtin, which controls whiteflies, aphids and other soft-bodied insects that spread disease on deciduous trees.
Actively growing insects or mites are more susceptible to horticultural oil sprays than dormant ones. Insect dormancy ends in the late winter or early spring. Most horticultural oil spray product labels recommend applying the product when the outside temperature is between 4.44 and 32.2 degrees Celsius. Optimum humidity is 45 to 65 per cent. It is essential that the oil evaporate quickly and that the plant not dry out. Irrigate the area the day before spraying.
Home gardeners most often apply horticultural oil with a hose-end sprayer. Backpack sprayers are also used, but you need to take care to clean the sprayer thoroughly to prevent residue oil from mixing with other sprayed substances. Spray dormant oils in the morning when temperatures are cool and plants retain water. Avoid spray drift onto sensitive plants and trees. Dormant oil--sensitive plants include Japanese and red maple, junipers, cedars, spruce, Douglas fir and black walnut.
Effect on Beneficial Insects
Beneficial insects such as green lacewings and lady beetles scatter before the spray reaches them. The horticultural oil residue does not harm them when they return to the tree. Beneficial predatory mites that deter harmful insect pests cannot move fast enough to avoid spray contact and are killed by horticultural oils. Use companion planting near deciduous trees to provide a habitat for predatory mites.
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