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Horticultural Oils Control Pests

Spider mites and aphids are the bane of my garden’s existence. I have managed these pests by giving the plants and bushes a nice, hard spray of water every morning (and I mean EVERY morning) in spring and summer, but that has proved impractical for me.

Enter horticultural-often called “mineral” or “narrow range”-oils, highly refined petroleum/paraffin products made specifically for controlling pests on plants. If applied correctly (always read the label), horticultural oils can be one important tool in controlling soft-bodied garden pests such as mites, aphids, white flies and mealybugs. They work primarily by suffocating the insects-and sometimes the eggs -that are on the plant at the time you spray. But unlike broad-spectrum pesticides, horticultural oils do not leave a long-term toxic residue.

“Horticultural oils…degrade rapidly through evaporation, and have very low toxicity or almost no toxicity to humans or wildlife at the rates used to control pests,” says UC Davis Integrated Pest Management (IPM) Associate Director Mary Louise Flint in Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide (University of California Press, 1998). Using horticultural oils on fruit trees during their dormant season to control overwintering insects has been a common integrated pest management practice for years. But these sprays are also effective in the ornamental garden both before and during the growing season. Here are some tips for their use:

Earlier is better: The sooner you spot an infestation, the more effective horticultural oils will be. I have found that a quick spot check on plants twice a week makes a world of difference in preventing massive infestations. Spray when populations are relatively low; you have a better chance of minimizing damage to your plants.

Know your enemy: Know the life cycle of the most-pesky pests in your garden. It can help you be more strategic about when to spray horticultural oils. If you know when and where a particular insect tends to overwinter and/or lay eggs, targeted applications of horticultural oil can minimize or prevent an outbreak. The UC IPM website is a great tool for learning about insect life cycles, the damage they do, and various ways to manage pest infestations: http://www.ipm.ucdavis.edu

Use the right oil: IPM guidelines suggest that you choose “supreme or superior-type oils with a minimum unsulfonated residue (UR) of 92 and a minimum percent paraffin (% Cp) of 60%.” Greek to you? Just jot it down, take it to the store, and consider this yet another reminder to read the label of any plant spray you are about to buy.

Also read the product label to learn how often to apply the spray; the label will mention any plants that might be harmed by its application. (By the way, don’t confuse a popular garden spray called “neem oil” with the horticultural oils discussed here. Neem is a botanical insecticide that works in a different way.)

Never spray when: the tree or plant is water stressed, it is foggy or rainy, or the temperature is expected to exceed 90 degrees. Thoroughly water in-ground plants several days before applying oils, and sooner for potted plants, which tend to dry out more quickly.

Be extra careful with annual crops: Don’t apply horticultural oil to your vegetable garden unless you carefully read the product label. Much more research has been done on the use of these oils on fruit trees and ornamentals than on the edible garden, so check the label first.

Have more questions about managing pests in the garden? UC Cooperative Extension Master Gardeners can help. Get free advice by calling the Tuolumne County Master Gardener Hotline, 209.533.5912, or by emailing mgtuolumne@ucdavis.edu. Rachel Oppedahl is a UCCE Tuolumne County Master Gardener.