It is still early enough to spray dormant oils for control of insect and mite pests on trees and shrubs that have not broken bud, especially in the central and northern portions of Illinois. Spraying either evergreen or deciduous plants with a dormant oil after bud break may kill newly expanding leaves or cause leaf edges to turn black.
Dormant oils, derived from paraffinic crude oil, are the heaviest of the petroleum-oil sprays and have a low unsulfonated residue (UR). UR is a measure of phytotoxic compounds remaining after distillation and refining. A high UR (greater than 92%) indicates a highly refined product with less probability of phytotoxicity. Dormant oils have a UR value below 92%, meaning a higher risk of phytotoxicity.
Dormant oils kill exposed insects and mites by either suffocating them (covering up the spiracles or breathing pores) or by directly penetrating the cuticle and destroying internal cells.
Dormant oils are effective in controlling certain scales that overwinter as nymphs or adults such as cottony maple, euonymus, lecanium, and obscure scale. However, dormant oils provide minimal control of oystershell and pine needle scale because both these scales overwinter as eggs. In addition, eggs are generally stacked on top of each other, and the dormant oil may not contact the bottom layer. As a result, applications of insecticides after egg hatch are generally required.
Honeylocust mite, European red mite, and spruce spider mite are controlled with dormant oil sprays, because they overwinter as exposed eggs on plants. Dormant oil sprays do not kill two-spotted spider mite, as they overwinter on the ground in leaf debris.
Dormant oil applications must be made when temperatures stay above freezing for 24 hours. Be sure to follow label directions because oil sprays may damage certain plants, including amur maple, Japanese maple, redbud, and sugar maple. In addition, the foliage (needles) of Colorado blue spruce can be discolored (change from blue to green) by dormant oil applications.
It is generally thought that the use of dormant oils would be less prone to resistance. However, this may not be true. For example, a Christmas tree plantation of Scots pines was sprayed with dormant oils for over 10 years to control pine needle scale (although dormant oils are not that effective for this scale). Eventually, the scales became more difficult to control. It was discovered that the scale covers were thicker than normal, making it harder for the dormant oil to penetrate. Remember--insects don't read the entomological literature.