As you browse the shelves of your local garden supply center, you will often see bottles and jugs of pest control products labeled "oil."
These outdoor insect control products are classified as horticultural oils and are specially formulated for use on plants.
According to UGA Extension's Keith Mickler, most are petroleum-based products with an added emulsifier which allows them to be mixed with water. Horticultural oils can kill soft-bodied insects, such as aphids, mites, scales and whiteflies.
Horticultural oils are classified into three groups: Dormant oils, summer oils and superior oils. Dormant oils are the heaviest of the oils. As the name implies, they are best used during winter dormancy before plant growth in the spring. Dormant oils should not be used during the growing season unless the label indicates spring use is safe.
Summer oils are lighter and more refined than dormant oils. They are designed to be used in the spring and summer while foliage is present. While safer than dormant oils on plant leaves, summer oils still have the ability to cause leaf damage. Effects will vary by plant species.
Superior oils are the most refined of all the petroleum oils. They are excellent products for ornamental pest control at most any time of the year. Superior oil products allow greater flexibility in their use and have been tested at temperatures in the mid-90s with no damage to plants. They are safer on most plants because they evaporate faster and leave less harmful residue. Of course, the more refined the oil, the more expensive the product.
Horticultural oils work by killing insects through means of suffocation. The oil coats the body of an insect and clogs the small pores used for breathing located on the insect's outer layer (cuticle). Oils can provide better control than other insecticides because they can kill all insect life stages, including eggs.
Only insects that are present at the time of application can be controlled. Horticultural oils, unlike other garden insecticides, do not leave a long-lasting residual for prolonged control. Thorough coverage is essential. Most labels recommend a 2 percent solution for scale in the summer, according to Mark Halcomb of UT.
When mixing and spraying horticultural oils, constant agitation is required. Oil and water separate rapidly. If your sprayer sits idle for even a few minutes, aim the tip of your sprayer nozzle back into the tank and agitate the entire mixture. The emulsion in the hose and sprayer tip can separate, causing the first plants sprayed to receive a direct spot of either pure oil or pure water. The pure oil can severely burn the leaves it contacts.
Spraying oils at the right time can help you avoid unwanted plant injury. Avoid applications during periods of combined high humidity and heat. High humidity can slow evaporation of the oil, which increases its injury potential. Try to spray during the cooler parts of the day. If possible, wait to spray on a day with lower humidity.
Although horticultural oils are less toxic to humans than other insecticides, it is still important to use them with caution. Always read and follow all label directions.
Clark MacAllister is the Dawson County extension agent. For more information, call (706)265-2442.