Dormant oil effectively smothers many overwintering pests on fruit trees. A timely application of dormant oil can delay or eliminate the onset of certain diseases and insects. The best time to apply dormant oil is just before the buds break in late winter or early spring. Spray when temperatures will stay above freezing for at least 48 hours but while the trees are still dormant. Thoroughly cover all plant surfaces. Avoid spraying if the trees are already leafing out.