It is still early enough to control scale and some other insects with dormant oil spray, particularly in central and northern Illinois. In this approach, a highly refined petroleum oil is applied to a dormant tree or shrub. If sprayed once, buds begin to break; leaves on both deciduous and evergreen ornamentals will likely turn black or have black edges. Dormant oil is applied to deciduous plants when the temperature will stay above freezing for the next 24 hours. Evergreens are more susceptible to damage, so application is safest when temperatures stay above 40°F for the next 24 hours. Mixing at the lowest recommended rate is also a good idea when spraying evergreens.

The oil covers and suffocates or penetrates and destroys cells of exposed insects and mites. It kills scales that overwinter as nymphs or adults but is not effective against scale eggs. Of the common Illinois scales, only oystershell scale and pine needle scale spend the winter in the egg stage. Honeylocust mite, European red mite, and other mites that overwinter as exposed eggs on the plant will also be killed. Two-spotted spider mite typically spends the winter on the ground and is not controlled with dormant oil.

Petroleum oil is phytotoxic to some ornamentals. Japanese red maple, amur maple, black walnut, sugar maple, and redbud can be damaged by the spray. Petroleum oil spray can remove the blue bloom on Colorado blue spruce and Koster spruce. Be sure to check the label for warnings of phytotoxicity before applying a dormant oil.