If there is any one thing that dampens our enthusiasm for outside gardening, it's insect control. It seems unfair to put forth the effort and expense to produce a beautiful landscape just to have it devoured by a Pandora's box of bugs.
There is one precaution you can take now, though, that will spare you many hours and quarts of chemicals later in the season. A single spray of dormant oil now, before new growth begins, will curb later populations of such persistent pests as aphids, mites, tent caterpillars and scale insects.
Wise gardeners got on the dormant oil bandwagon as early as 1763, although oils were not commonly used until the latter 19th century. Up until then, unmodified petroleum, turpentine and kerosene were applied directly to the plants, and although they did a fabulous job on the pests, the plants were also frequently victims. Things improved in 1874 with the development of a kerosene-and-soap emulsion in water, but the field really expanded in the early 1900s through the use of refined petroleum oils.
Today's dormant oils are highly refined petroleum derivatives. As the name suggests, they should only be used during the dormant period of a plant's life cycle. If you've ever sprayed dormant oil on a tree or shrub after it's leafed out, you know the result is a plant that looks like it's been blowtorched! There are some highly refined summer oils, but these should only be used on tolerant plants for positively identified pests, and all label directions and precautions should be carefully followed.
Dormant oils kill pests and their eggs by surrounding them with a film of oil which suffocates them. This is just about the only way to control most insects or mites in the egg stage. Oils are usually applied in a mixture with water and an emulsifier to help keep the oil evenly dispersed. Because the oils are non-toxic, they do not contaminate spray equipment the way herbicides do. The fact that the oils are not poisons make them a lot more attractive to many homeowners who do their own spraying.
There are a few rules to follow when using dormant oils. Only one spray a year should be applied, and it must be done before bud break in the spring. Apply oils on a sunny day when the temperature is between 40 and 65 degrees, and there is no chance of a freeze within 24 hours. Oil will not dry and perform properly in cloudy or cold weather. Since good coverage is critical to achieve good control, avoid spraying on a day that is breezy or windy, when the oil could be blown off target.
There are several different brands and refinements available on the market, so read labels carefully to be sure you get what you need. In general, dormant oils can be used on deciduous trees and shrubs, fruit trees and hardy evergreen plants, but not on needleleaf ornamentals. You also should not use dormant oils on such deciduous trees as sugar and Japanese maples, hickory, walnut, butternut, redbud, beech or birch. The only way to be sure if an oil is safe for use on your plant is to read the label.
Although a dormant oil is an efficient way to prevent many pest problems from developing later, it is not a panacea. Some insects and mites may survive the treatment, and with their incredible reproductive capabilities become a problem by summer time. Some insects, such as oystershell, euonymus and white wax scales, are simply resistant to the oil. All things considered though, it is well worth your time and effort to put out a dormant oil spray within the next six weeks.
Sargent is a Virginia Tech Cooperative Extension agent in Newport News. Anyone desiring further gardening information can call the service at 599-8899.