Scale insects are among the worst pests of trees and shrubs. Their presence often goes unrecognized because they are quite small and move about very little, if at all, after they have begun to feed. They feed by sucking the sap, causing death of the plant or of its more heavily infested parts.Their feeding weakens the tree or shrub so that it becomes more susceptible to winter injury and disease.

There are two general groups of scales -- the armored and the soft kinds. Armored scales have a protective covering of wax under which the insect lives and feeds. Soft scales lack this protective coating.

Almost the entire life of the scale is spent in one place. The usual life span is less than a year and most overwinter in the egg stage.

A close inspection of the trunk and branches with a magnifying glass should reveal whether or not the plant is infested. Now is a very good time to do it. The scales may appear as brownish, reddish or or grayish growths or small swellings on the bark, or in the case of pine needle scale as white spots on the needles.

An infested plant can be sprayed with a dormant oil while the plant is dormant. To apply it during the growing season would burn the leaves and perhaps even kill the plant. The dormant oil will kill immature scale, scale eggs, eggs of European red mites, and adults of some species of mites that overwinter under the bark.

The spray is most effective when applied just before the buds start to break. Apply it in the morning so it will have time to dry before night when lower temperatures may occur.

Eggs of mites, aphids and scale insects which are dormant during the winter begin to stir as warmer weather approaches, and their need for oxygen increases. An application of dormant oil will prevent oxygen from being taken up and cause suffocation.

Dormant oils are easy to spply, do not leave hazardous residues, do not encourage pesticide resistance, do not seriously interfere with the build-up of beneficial insect populations including pollinators, and are not hazardous in normal use to applicators. The directions on the label should be read -- every word -- and followed closely.

Some of the most troublesome scales include oystershell scale on lilac, maple, poplar and boxwood; euonymus scale on euonymus, obscure on oak; hemlock scale on hemlocks; and San Jose scale on fruit trees, pyracantha, dogwood, rose, privet, Japanese quince and many others.

In general, dormant oils should not be sprayed on Japanese maple, sugar maple, beech, birch, hickory, walunt, butternut, douglas fir and blue spruce.