Scales and eggs of leaf-curling aphids and over-wintering mites can be successfully controlled with dormant season applications of horticultural oils. To avoid plant damage and be effective, oils must be applied correctly.
Apply oils before bud break (leaf buds open) and stem elongation. Oils kill insects by blocking the insect's breathing holes, causing suffocation. To work effectively, the insects must be thoroughly coated with the oil.
Plant stomates, the gas exchange openings, may be blocked by oil applications. The resulting stomata suffocation causes brown leaves and leaf drop.
Oils should be applied when temperatures are above 40 degrees Fahrenheit. When applied below freezing, the emulsion can break down resulting in an uneven treatment and potential plant damage. Avoid making applications to drought-stressed plants. Note that summer weight oils can be used during the growing season.
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Updated Tuesday, July 22, 2014