The Homeowners Column

The Homeowners Column

Just What is Dormant Oil?

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

During dreary winter days I envy a plant's ability to go dormant. I've considered putting a sign on my office door stating, "Dormant - do not disturb until spring". In March the plants may still be dormant, but gardeners aren't.

This time of year we see the recommendation to spray dormant oil to control insects on everything from fruit trees to lilacs. But is it just any oil sprayed when the plants are dormant? Most commercial dormant oil sprays are refined from petroleum oil. A few are made from cottonseed oil. Unlike home remedies, commercial spray oils have an emulsifier added to allow the oil to mix with water. Many of the newer commercial oils are more highly refined than past dormant oil products. The new formulations are labeled to also be used, usually at a reduced rate, during the growing season. These are generally listed as horticultural, ultrafine or summer oils. Read and follow all label directions for proper timing and rates.

Oils kill exposed insects and mites by either suffocating them (covering up breathing tubes) or by directly penetrating the outside cuticle and destroying internal cells. Spraying trees with dormant oil after bud break and leaves have emerged will still control the pests, but it may kill the young leaves or cause leaf edges to turn black if the correct oil is not used at the proper rate.

Dr. Phil Nixon and Dr. Raymond Cloyd in the U of I Extension Home, Yard and Garden Pest Newsletter outlined the benefits of making an application of dormant oil. Advantages include: (1) a wide range of activity against most species of mites and scales, including some activity on eggs; (2) minimal likelihood of insects' or mites' developing resistance; (3) generally less harmful to beneficial insects and mites than other pesticides (4) relatively safe to birds, humans, and other mammals. Disadvantages of using dormant oil are (1) potential plant damage if incorrect oil is used or used at improper rate during the growing season and (2) minimal residual activity to kill new pest infestations.

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 summer oils after egg hatch are generally required. Accurate identification of the scale is important for proper control.

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.

Summer oils are best used to control slow soft bodied insects. They do little in controlling pests such as white grubs, cabbageworms and apple maggots.

Dormant oil applications must be made when temperatures stay above freezing for 24 hours. Be sure to follow all 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.

In addition to oils other pesticides may require dormant application. Peaches should be sprayed now with lime sulfur if peach leaf curl has been a problem. Peach leaf curl appears as a thickening, curling and puckering of leaves. Fruits become swollen and deformed. This is the only spray that will control this disease. Lime sulfur may also be used on raspberries to control anthracnose. Be sure to read and follow all label directions.

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