Horticultural/Dormant Oil Demystified
At this time of year, we often discuss spraying “dormant oil” and in summer, we talk about “summer oil.” This dated terminology applied to pest control many years ago. Dormant oils were never used during the growing season because they were unsafe for plants, and summer oils were only used as a spreader-sticker. Today, those terms simply refer to the timing of the treatment. The product that is applied during the dormant period and summer is the same; the only difference is the concentration. Oils are unique in that they have been used for more than a century and no target pest has developed resistance to them.
Horticultural mineral oil is a term used to describe a class of high quality oils formulated for agricultural use. They are produced by distilling and refining crude oils. Other terms are mineral oil, petroleum oil, spray oil, insecticidal oil, horticultural oil, superior oil, paraffinic oil. Various other petroleum products are made from crude oils, including gasoline, kerosene, fuel oil, and lubricating oil.
1. The newest horticultural oils are sometimes considered “narrow range oils” which means that the oil has been through two distillation stages to reduce the temperature range at which the oil boils. This results in an oil that has high insecticidal properties and low potential for plant injury. These oils are also sometimes referred to as “superior oil.”
2. Horticultural oils are made from crude oil that is high in paraffin. Paraffinic oil occurs in sources in the eastern U.S. and in Texas. (Oil dug from the western U.S. is high in naphthenes.) Studies have shown that paraffinic oils have greater insecticidal properties.
3. In newer oils, the unsaturated hydrocarbons have been removed. These compounds that were found in older oils were responsible for plant injury. The term for oils low in unsaturated hydrocarbons is “unsulfonated residue.” The safer oils usually have a UR value at 92% to 97%.
4. The label for oils will provide a variety of information:
a. percent oil content: the oil may be listed as “mineral,”, “petroleum,” or “paraffinic.”
b. minimum UR value: Make sure it is more than 92% so that injury to plants is reduced.
How Oils Kill Pests
Oils kill pests primarily by smothering, and work best on soft-bodied insects. Insects require oxygen to live, and oil plugs the insect’s air-exchange apparatus, causing slow suffocation. Oil works better on eggs just before they hatch because the oxygen requirement is greater. This is why we suggest spraying oil at the “delayed dormant” timing when fruit tree buds have started to swell. Oils applied during dormancy or delayed dormancy are more effective at a higher concentration (usually 1.5-2%) and the plants will not be affected. Oil applied in the summer should be mixed at a lower concentration (1%) to reduce plant injury.
Oil may also act as a repellent, delaying egg laying (of pear psylla, for example) or preventing scale crawlers from settling.
Oils are targeted at: aphids, soft scales, pear psylla, immature leafhoppers, whiteflies, mites, and eggs of most insects.
How to Prevent Plant Injury
Oils that are properly applied rarely damage plant tissue. Oils may injure plants if applied at too high a rate or on hot days. Trees most susceptible to damage are those suffering from drought stress. They have a lower tolerance for interruptions in air-exchange supply through stomates or lenticels
Oil should not be applied:
- on days where temperatures exceed 90 or are below 30 degrees F
- on dry, windy days
Symptoms of plant damage:
- dark green to purple discoloration on leaf margins
- water-soaking around stomates or lenticels
- swelling or corking of lenticels
- delay in budbreak
- leaf drop and death of buds
Vegetable oils are sometimes sold as insecticides, where cottonseed and soybean oils are the most effective. Neem oil (from seeds of the neem tree, Azadirachta indica) has some insecticidal and fungicidal properties.
Essential oils (rosemary, lavender, thyme, clove, garlic, coriander, peppermint, citronella), are said to include some fumigant and topical toxicity as well as repellent effects. They are considered minimum-risk pesticides and are exempt from Environmental Protection Agency registration. As a result, many of the products labeled for control of insect pests have not been subjected to evaluation of efficacy or plant toxicity. A recent study of several products on greenhouse plants showed that essential oils vary in their effectiveness against certain arthropod pests stated on the label, and are phytotoxic. The oils controlled spider mites and mealybug but caused significant plant injury, while green peach aphid and thrips were not affected by the oils (Cloyd, et al 2009).
Cloyd, Raymond, et al. 2009. Effect of Commercially Available Plant-Derived Essential Oil Products on Arthropod Pests. Journal of Economic Entomology 102(4):1567-1579.