Insecticidal oils or horticultural oils are used against soft-bodied insects: aphids, thrips, mites, young whiteflies and scale insects, larval forms of caterpillars and beetles, and insect eggs. Scheduled applications may help prevent fungal diseases such as powdery mildew, black spot on roses, and some forms of Alternaria leaf spot.
Insecticidal oils block pests' oxygen intake, thus killing them, and are fairly non-selective. They will kill soft-bodied beneficials, but have no residual toxicity.
Insecticidal oils may be used on dormant plants before buds open, or in spring or summer on plants in leaf.
Refined mineral oil sprays or horticultural oils are available commercially, and are specially formulated for insect control. Typically, these are petroleum-derived products, and as such only the ultra-purified versions (known also as ultra-refined or superior oils) can be used when plants are in leaf - older dormant oils contained plant- and animal-toxic impurities.
As with insecticidal soaps, insecticidal oils can damage plants.
Spot-treat small areas and observe for 24 hours prior to treating entire plants, and test on a few plants prior to treating many.
A mixture of ordinary vegetable oil and liquid soap, mixed 15:1, may be used as an alternative to commercial preparations. Dilute such mixtures further, about 20:1, and spray to control various pests. Because these oils are less miscible in water, and form larger particles, they do not spray as well or as evenly.
Mixed with sodium bicarbonate - one good tablespoon of baking soda, one tablespoon of insectical oil per 4 litres or 1 gallon of water - can be used as an effective preventative for fungal disease. It is, however, not a true fungicide - it does work against existing infections.
Some plants, especially non-woody species, do not take well to being sprayed with these solutions. Ferns and walnut trees should not be treated with insecticidal oils. Seedlings, recent transplants and fresh cuttings should not be treated, nor should drought-stressed plants. Ideally, plants should be watered or irrigated prior to application.
Spray on a calm, cool, cloudy day; the oil should dry before sun reaches treated plants, as the oil can act as a magnifying lens and cause burn damage. Temperatures should be above freezing and below 30C/86F; avoid foggy or very high-humidity days. Treat all parts of the plant. Use insecticidal oils at recommended concentrations and rates. Agitate the oil-and-water mixture frequently to keep the oil evenly in suspension. Do not use within a month of using sulfur-based products.
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