Scale insects – so called because that’s what they look like. The young insects are small, soft-bodied and mobile. When the eggs hatch the young, six-legged scale insect “crawlers” disperse throughout the new tissue seeking suitable sites in which to feed. They attach themselves to the plant with long, threadlike mouthparts (stylets) which are six to eight times longer than the insect itself. Once attached, they lose their legs and antennae, and begin to secrete a waxy layer over themselves.
The immobile, “shell stage” of scales are adult females; males are small, fly-like and infrequently seen. Scale insects can be basically divided into two groups: soft scales and armored scales. Soft Scales generally secrete an attached, thin, waxy layer over themselves. The soft covering they secrete cannot be separated from the scale’s body. Soft scales typically move between branches and leaves during their lifecycle. Armored Scales shed skins and wax that is unattached to their body to form their hard, shell-like cover. These covers can be separated from the scale’s body. Hard scales typically do not move to leaves during their lifecycle. Most species of armored scales overwinter as eggs beneath the female cover. In spring, these eggs hatch into tiny mobile crawlers which migrate to new feeding sites.
Scale insects feed on plant sap slowly reducing plant vigor. They can occur on leaves, twigs, branches or trunks. Heavily infested plants grow poorly and may suffer dieback of twigs and branches. Their small size and general lack of mobility make them difficult to notice by the casual observer. Large colonies remove substantial quantities of plant fluids and cause wilting, but seldom kill their host.
As Scale insects feed on plant sap they secrete a sugar-rich sticky liquid called honeydew. Armored scales do not produce honeydew. This honeydew serves as food for ants and wasps and supports the growth of black sooty molds. It often falls on leaves or needles, branches, fruits or anything else immediately underneath the infested area of the plant. The sooty mold fungi grow on the honeydew.Sometimes plants not actually infested by insects may be affected if a tree above them is being attacked by a honeydew producing insect and the honeydew drops onto them.
Scale insects can be controlled by using dormant oil treatments generally applied in very early spring, before bud break. Summer oils can also be very effective but some plants are sensitive to these treatments. Check labels to make sure your plant is not harmed by the treatment you are considering. Many other insecticides, including insecticidal soaps, can be used only against the mobile crawler stage of scales since adult scales are protected from insecticides by a waxy covering. Natural enemies to scale include tiny parasitic wasps and predators such as ladybugs. It is very common for ladybugs to move onto a plant with a growing scale infestation; before deciding upon a treatment, look for adult and immature ladybugs on your plants.