Dormant Oil Spray

Now is the time to apply dormant oil spray. One of the best pest control tactics is the application of a dormant oil spray to yard plants during the early spring.

For a long time, farmers and gardeners have realized the importance of this spray in controlling overwintering mites, aphids and scale on fruit trees. Homeowners should be aware that they too can purchase and apply a horticultural oil to many of the trees and shrubs surrounding their home.

Most oils available as insecticides today are termed superior-type horticultural oils, and are considered appropriate for use both in the dormant season and in the growing (or verdant) season.

Horticultural oils are paraffinic, degrade rapidly through evaporation, and at rates used to control pests, have very low toxicity or almost no toxicity to humans or wildlife. Properly used, they also have low toxicity to plants. This distinguishes them from the more familiar aromatic and napthenic oils used as motor fuels and solvents, which can be toxic to wildlife and plants.

Horticultural oils kill mites and insects primarily by suffocating them, although there may be additional toxic reactions in some species. Oils are most effective against eggs, immature forms and soft-bodied adults. In the dormant season, oils are useful against scales, mites, aphids, psyllids and plant bugs. Remember, oils affect pests present at the time of application but do not kill pests arriving after application.

If applied properly and at the recommended rates, a dormant oil spray will not be phytotoxic to most species of deciduous tree or shrub commonly found in Missouri. This is true for most conifers, except for the blue spruces in which the oil removes the bluish frosted material (glaucus bloom) from the needles. Always remember to first read the label regarding any plant species that may be sensitive to oil.

There may also be some additional precautions listed on the label when applying oils during the summer. We still have time to apply a dormant oil spray, but do not delay or it will soon be too late.

Bruce A. Barrett, Extension Entomologist