Q. What are the current recommendations for using horticultural oil to protect mature trees from insect attack?
A. Spraying trees or shrubs with a horticultural, or dormant, oil is recommended as a way to help control repeated attacks of scale, mites and aphids. Some of the common pests that can be controlled with the oil include euonymus scale, lecanium scale, cottony maple scale and obscure scale, honeylocust mite, European red mite and spruce spider mite. The oil is applied when the tree or shrub is dormant, before it buds out (usually in March), when the temperature is at least 40 degrees, with no danger of rain or frost within the following 24 hours. Never apply chemicals on a windy day. Thorough coverage of all bark and branches is necessary to suffocate any overwintering insects or eggs. Read label directions thoroughly since certain plants have a phototoxicity to the oil and should not be sprayed, including amur maple, arborvitae, beech, blue spruce, Japanese maple, redbud and sugar maple.