Is there something that can be sprayed on plants while they are dormant to rid them of insects before they do damage?
You can rid your defenseless trees and shrubs of inconspicuous killers by using a prophylactic spray. The spray that should be used is referred to and sold under many brand names as dormant oil, horticultural oil or scale emulsion.
These are highly refined oils (not motor oils!) that spread uniformly on the bark of trees and shrubs to which it is applied and coat non-mobile, dormant insects on the tree smothering them to death. Heavier oils may have to be applied with a tank (pump-up) sprayer, which can apply the fully diluted product, rather than with a hose-end sprayer, which may become clogged. Applicators should frequently shake sprayers to agitate the water and chemicals mixed since plant damage can occur if a concentrated oil spray, caused by solution separation, is applied. Mix dormant oil at the recommended rate on the product label.
It is best to spray before buds begin to swell. If buds of trees and shrubs have begun to swell slightly, go ahead and spray. Although some of the buds may be damaged, the benefits of spraying dormant oil far outweigh the possible repercussions. Applying a dormant oil spray this late will also serve to cover pruning cuts and can serve as a second attack on stubborn pests which were not killed by an earlier oil application. The closer the application is made to budbreak, the greater the kill. Do not spray trees that are in full bloom.
Spraying of dormant oil should occur on a clear day when the temperatures are expected to remain over 50 degrees F. for at least twenty-four hours. The ideal temperature for application is between 40 and 70 degrees F. in order to get the oil to spread out over the tree and cover all crooks and crevices. Try to avoid applying dormant oil when severe freezing trends are expected in the 3-4 days following application.
CAUTION: The use of a dormant oil mixture will not only kill, but also annihilate, annual flowers such as pansies, bluebonnets or snapdragons growing under or near plants to be treated. To avoid this, completely cover such tender vegetation BEFORE spraying nearby trees and vines with dormant oil.