Scale insects do considerable damage to several kinds of trees and shrubs in this area by sucking the sap. The loss of plant juices, if extensive, causes discoloration and eventually the drying of leaves or needles. This feeding may result in death of the tree or of its more heavily infested parts. The feeding damage will also weaken a plant so that it becomes more susceptible to winter injury and disease.
Most scale insects are so tiny they are hard to detect without a magnifying glass, and they move about very little, if at all, after they have begun to feed.
In the process of feeding, many scale insects excrete a sticky, sweet fluid called honey dew. The honey dew may become so abundant that cars parked beneath infested trees become coated with the substance.
Many kinds are well protected because they live under a waxy, almost impermeable shell. These kinds are called armored scales. Their eggs are deposited within the shell affording protection during this stage of development.
When the eggs hatch the young (called crawlers) can move around for a few days. It is during this stage they are spread to other plants by birds or the wind. It is also a time when they are vulnerable and easy to kill with the proper kind of insecticide. But in a short time they acquire a waxy coating which provides a large measure of protection. In other words, the sprays must be applied at specific times (soon after the eggs hatch) and the time varies with the species.
According to Dr. John A. Davidson, University of Maryland entomologist, some of the most troublesome scales in this area include San Jose scale, white peach scale, oystershell scale, euonymus scale, obscure scale, gloomy scale and hemlock scale.
San Jose scale is particularly damaging to deciduous fruit trees, pyracantha, roses, privet, dogwood, lilac, beech and birch. Davidson recommends spraying June 5 and 15, July 10 and 20, Sept. 10 and 20, with either sevin, spectracide, malathion or orthene. In all cases, follow directions on the label for mix and application.
White peach scale feeds on privet, flowering peach, lilac and various fruit trees. Spray June 10 and 20, July 10 and 20, with sevin, dursban or orthene.
Oystershell scale look like minature oysters and feed on many kinds of trees and sbrubs. Spray May 10 and 20, July 15 and 25, with sevin, trithion, malathion, volck or orthene.
Euonymus scale feed on euonymus. Spray July 10 and 20, with sevin, spectracide or cygon.
Obscure scale feed on red and white oaks. Spray July 15 and 25, Aug. 5 and 15, with sevin or volck.
Gloomy scale feed on maple trees. Spray July 1 and 10, with sevin or orthene.
Hemlock scale feed on hemlock, fir and spruce needles. Spray June 5 and 10, Aug. 15 and 25, with sevin, cygon, volck or orthene.
It is not recommended that spraying be done unless the scale insects are definitely known to be present. A close inspection of trees and shrubs with a magnifying glass should first be made. In the case of large trees, professional equipment and know-how are necessary for satisfactory control.
An important weapon against many kinds of scale insects is the dormant oil spray, and if the infestation is not severe it may be a good idea to wait and apply it late on.
This spray is one of the least harmful (to people and the environment) that can be used. It is effective against a number of over-wintering insects including aphids, mites and scale.
Dormant oils should be applied only in late winter or early spring, when temperatures are above 40 degrees and are not likely to go below freezing for 24 hours. Don not apply when the wind is blowing. Follow directions on the label for mix and application.