How To: Use Horticultural Oils For Controlling Pest Problems

Whether they're called dormant oils, summer oils or horticultural oils, there are many pest problems that you can control using these products. They are effective on both soft and armored scales including San Jose, lecanium, euonymus, cottony and oystershell, as well as on European red mites, aphids, webworms, leafhoppers, tent caterpillars, mealybugs and adelgids. The oils kill insects by smothering all stages of growth and the film of oil on tree bark can suppress some insect egg-laying. Because the newer oils are lighter and purer, you can use some of them even on annual and perennial plants and some greenhouse crops. There are a few items to keep in mind while using horticultural oils.

  1. Horticultural oils are not preventive. The pest must be on the host now or, in the case of dormant applications for overwintering pests, must have been present during the past growing season.

  2. Purchase the product with the most pests listed on the label. At the time of purchase, check the label for susceptible plant species to avoid causing phytotoxicity. Arborvitae, beech, some maples and junipers have shown sensitivity to horticultural oils. Blue spruce will lose its blue color if sprayed with horticultural oils.

  3. Horticultural oils are either miscible oils or concentrate emulsions and you should never use them full strength. Follow directions for mixing with water. Some synthetic pesticides can be mixed with horticultural oils. The oil helps the spreading properties of your application. Do not mix with sulfur-containing pesticides — it will damage plants. Some sulfur pesticides persist for extended periods, possibly requiring a 30-day wait before and after horticultural oils can be applied. Check the label to verify how long you need to wait between applications.

  4. The mixture must be constantly agitated and applied to dry plant tissue. Wet tissue prolongs the drying time and increases risk of phytotoxicity. For this same reason, avoid spraying when relative humidity is greater than 90 percent.

  5. Apply when temperatures are between 40 and 95∞F within 24 hours of spraying. If it is too hot, plants may be experiencing temporary water stress and be damaged; if it is too cold, the oil can break down and become ineffective.

  6. Phytotoxicity appears first as light yellow tissue turning to a water-soaked appearance, then to a dark purple before leaves are dropped. You may want to experiment on a small population before treating a larger area to be sure you are not treating sensitive species.

  7. Spraying horticultural oils when plants are dormant does avoid harming some beneficial insects, except those that overwinter on the plants. During the growing season, some beneficial insects may be present. Despite the low toxicity of horticultural oils to humans and animals, you should be careful not to overuse them.

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