Controlling insect pests with horticultural oils
By Dawn Pettinelli - UConn Home & Garden Education Center
Feature Article - posted Thu., Apr. 7, 2011
In use for more than 100 years, horticultural oils are one of the safest and most effective insecticides gardeners have. In the past, heavier viscosity, petroleum-based “dormant oils” were used prior to bud break, primarily on tree fruits. Horticultural oils have been improved upon and many are now made from plant oils or even fish oils. They can be used either when plants are still dormant or during the growing season, depending on the formulation. Generally horticultural oils are applied to woody plants, but new uses for them on bedding and greenhouse plants are being discovered all the time.
Most insecticides function as a contact or stomach poison, but horticultural oils kill by suffocating the insect. Because of this mode of action, insects cannot become immune to oil sprays and they are also safer for the applicator and many animal species. The oils are, however, toxic to fish. Researchers are also finding that oil sprays might act as a feeding deterrent to certain species of insects and discourage the egg laying of others. Most recently, they are being employed as an ingredient in sprays used to control fungal diseases like powdery mildew.
Horticultural oils offer excellent control of pear psylla, scales, mites and aphids on apple and pear trees. The pear psylla is a tiny insect usually overwintering on tree bark and in litter. As they feed on the sap of the pear foliage, they secrete a sticky, clear exudate known as honeydew. Often the honeydew becomes colonized by a nonpathogenic fungus called black sooty mold. Eventually, this will weaken the tree, as the sooty mold will cover the leaf tissues, blocking the sunlight and reducing photosynthesis. Aphids produce similar symptoms to the pear psylla, although they are often noticed sooner because the new growth tips begin to become distorted. The wooly adelgids that attack hemlock can also be controlled by horticultural oils.
Several species of spider mites can also cause problems with fruit trees, as well as some ornamental plants. Leaves appear stipled with a bronzy, grayish cast. Spider mites are very tiny insects and difficult to see without a hand lens. Fine webbing will be noticed between branches and leaves, which is quite unlike the meticulously designed webs of beneficial spiders. A heavy infestation could cause premature leaf drop and small, poorly colored fruit. Spider mites are most likely to occur during dry, hot weather.
Scale insects are also sucking insects that attach themselves to the tree stems, branches and sometimes, leaves. You will usually see small, raised bumps which may be whitish, grey, brown or reddish in color, depending on the type of scale. Ornamentals, most noticeably euonymus, can also be afflicted with scale insects.
Horticultural oils can be used on a wide variety of plants. Sometimes, however, they can cause damage to leaves or needles, so it is wise to spray an inconspicuous part of the plant first and note any problems. Do not apply oils or any type of pesticide if temperatures are above 85 or below 40 degrees F. Generally, an application of horticultural oil is made to pear trees when new green leaf tissue is starting to show and repeated in seven to 10 days. Apple trees need only to be sprayed once, when the new buds show one-half inch of green tissue.
Oil sprays can also be used quite effectively to control scale and sometimes, mealy bugs, on houseplants. Again, test first to see if damage occurs to the plant. For heavy infestations, be sure to cover the plant completely. A second or even third application may be needed at later dates.
As with all pesticides, read the directions carefully. Often two different dilution ratios are given – one for dormant plants and one for those actively growing. If you have questions of using horticultural oils to control pests, see the fact sheet on the UConn Home & Garden Education Center website, www.ladybug.uconn.edu, call (toll-free) 877-486-6271, or contact your local Cooperative Extension Center.