I have oystershell scales on my lilac. How can they be controlled?
The oystershell scale is a common insect pest in Iowa. It can be found on a wide range of trees and shrubs. Plant hosts include ash, lilac, willow, maple, apple, pear, plum, cotoneaster, linden, and viburnum.
The oystershell scale is a small (1/20 to 1/8 inch long), elongated, oyster-shaped insect. The insect is concealed beneath a waxy, shell-like covering that varies from light to dark brown. Osytershell scale overwinters as eggs underneath the scale covering of the mother. In Iowa, eggs typically hatch from mid-May to early June. The tiny white to pale yellow “crawlers” move about on the plant looking for suitable feeding sites. The “crawlers” begin to develop a protective, waxy cover almost immediately after they begin to feed.
Oystershell scale attaches itself to the branches and trunks of trees and shrubs and sucks sap from plant cells. Heavy scale infestations may cause yellowing of foliage, stunting, and dieback of twigs and branches. Weakened trees and shrubs are vulnerable to additional insect and disease problems.
Contact insecticides do not penetrate the protective coverings of most scale insects. The best way to control oystershell scale is to apply an insecticide when the crawlers are present. Effective insecticides include insecticidal soap, horticultural oil, Sevin, permethrin, and others. The insecticide must be applied before the tiny insects develop their protective, waxy coverings.