Scale insects are common pests of many trees and shrubs. They are often overlooked since they are immobile for most of their lives and they will generally blend in with the host plant. Heavily infested plants are often covered with small disklike or waxy coverings. The scale is underneath each covering. They feed on juices of the plant.
Trees heavily infested with scale may look water stressed. Leaves turn yellow, twigs may drop and limbs may die. And there may be cracks and a gummy substance on the bark. Keeping your trees and shrubs well watered and fertilized will help prevent scale problems. Scales can be rubbed off the tree by using your hand or a brush. On non-evergreen trees horticultural oil can be used during the dormant season by following the label directions. An insecticidal soap can also be used to reduce the number of scale insects.
Because of their protective covering, scales can be fairly resistant to many conventional pesticides. Insecticide treatments work best during the crawler stage of their life cycle. This is generally in the early spring and summer. To determine when they are in the crawler stage you can monitor them, by wrapping two-sided sticky tape around branches throughout the tree or shrub. The emerging crawlers will stick to the tape. The crawlers will look like tiny, pale yellow specks on the tape. When you determine they are in the crawler stage, an insecticide may be used. Continued monitoring and multiple sprays will be needed to control some types of scale insects. Check with your county Extension office, Master Gardener hotline or local nursery for more scale information.