Dormant Oil
TreatmentsA highly refined petroleum or plant-based oil sprayed on trees during dormancy to smother overwintering insects and mite eggs on bark surfaces.
Dormant oil, also called horticultural oil, is a highly refined oil applied as a spray to trees during the dormant season (late winter to early spring before bud break). The oil works by coating and suffocating overwintering insect eggs, scale insects, mites, and other small pests on bark surfaces. Because it kills through physical smothering rather than chemical toxicity, there is virtually no risk of pest resistance.
Dormant oil is one of the oldest and safest pest management tools available. Modern horticultural oils are so highly refined that they evaporate quickly and leave minimal residue. They are effective against a wide range of pests including scale insects, aphid eggs, mite eggs, and some caterpillar egg masses. Because the oil degrades rapidly, it has minimal impact on beneficial insects that are not present during the dormant season.
Proper timing and conditions are essential for effective dormant oil application. The spray must be applied when temperatures are above freezing and will remain so for 24 hours, before buds begin to swell and open. Applying oil to actively growing foliage can cause phytotoxicity (leaf burn). Coverage must be thorough, coating all bark surfaces, because the oil only works through direct contact. Some plant species, particularly blue-needled conifers and certain thin-barked trees, may be sensitive to oil sprays.