Description: Over 3,000 species of scale insects exist in backyard orchards, ornamental shrubs, greenhouses and indoor gardens throughout North America. They are such oddly shaped and immobile pests that they often resemble shell-like bumps rather than insects. Scales attach themselves to the twigs, leaves, branches, or fruits of host plants and feed on vital plant fluids. In many cases, heavy infestations build up unnoticed before plants begin to show damage. Large populations may result in poor growth, reduced vigor and chlorotic (yellowed) leaves. If left unchecked, an infested host may become so weak that it dies. Scale insects can be divided into two groups:
Armored (Hard) Scales – Secrete a hard protective covering (1/8 inch long) over themselves, which is not attached to the body. The hard scale lives and feeds under this spherical armor and does not move about the plant. They do not secrete honeydew.
Soft Scales – Secrete a waxy film (up to 1/2 inch long) that is part of the body. In most cases, they are able to move short distances (but rarely do) and produce copious amounts of honeydew. Soft scale vary in shape from flat to almost spherical.
Life Cycle: Adult females lay eggs underneath their protective covering which hatch over a period of one to three weeks. The newly hatched nymphs (called crawlers) migrate out from this covering and move about the plant until a suitable feeding site is found. Young nymphs insert their piercing mouthparts into the plant and begin to feed, gradually developing their own armor as they transform into immobile adults. They do not pupate and may have several overlapping generations per year, especially in greenhouses.
Note: Males of many species develop wings as adults and appear as tiny gnat-like insects. They are rarely seen and do not feed on plants. Females often reproduce without mating.
Control: Prune and dispose of infested branches, twigs and leaves. When scale numbers are low they may be rubbed or picked off of plants by hand. Dabbing individual pests with an alcohol-soaked cotton swab will also work when infestations are light. Commercially available beneficial insects, such as ladybugs and lacewing, are natural predators of the young larval or “crawler” stage. Natural pesticides, like insecticidal soap and d-Limonene can also be used to kill the larvae. However, these products have very little persistence in the environment, so several applications during egg-hatching will be required for effective control. Horticultural oils work by smothering insects and will control all pest stages, including adults which are protected from most other insecticides by their armor coverings.
Tip: Ants feed on the honeydew that sucking insects produce and will protect these pests from their natural enemies. An application of Tanglefoot Pest Barrier to the stalks of woody plants or to the trunks of trees will help keep ants away.
Photo Credit: UC Berkeley