The Backyard Jungle

The history of scale insects and keeping your garden parasite free.

| November/December 1984

scale insects - brown shells on bown bark

Oystershell scales are oblong, armored scale insects.


Part IX: Scale Insects

by Ron Westby

In predator-prey relationships, one axiom stands before all others: What can't be seen (that is, recognized) generally won't be eaten! And insects seem to have all but cornered the market on effective disguises. Moths blend in with tree bark . . . katydids resemble leaves ... and inchworms mimic twigs (when alarmed).

However, the slyest members of the whatyou-see-may-not-be-what-you-get set are definitely the scale insects. These crusted con artists look more like bird droppings or disease organisms than bugs . . . and are often ignored, even by gardeners. That's too bad, because scales are related to aphids and can become just as abundant . . . often literally encrusting the stems of trees and shrubs.

Young scales, called crawlers, are fairly mobile; but they soon grow sessile under the protection of their tough outer shell. These insects do their damage by sipping the sap from a host plant (usually orchard trees and ornamentals), a process that can cause a gradual decline in vigor of the plant or tree, stunt its growth, and even kill it. And the foliage-killers are so adept at hiding that they often exist unnoticed till winter causes the leaves to drop from their deciduous-tree hosts.


Of the 4,000 or so species of scale insects, few ever exceed 3 cm (1.2") in length, and most range in size from a barely discernible 1 mm to a tiny 1.5 cm. The most prevalent types, in the U.S., are the soft scales (family Coccidae) and the armored scales (family Diaspididae). Despite the names, both of these develop hard shells, but the soft scales grow theirs as an extension of their exoskeleton, whereas the protective "bubbles" of the armored group are not attached to the bugs' bodies.

In general, all scales have similar (and fairly simple) life cycles. They go through an incomplete metamorphosis (egg to nymph to adult). In the nymph stage, scale insects are equipped with legs, but when they reach adulthood, these appendages drop off, and the creatures become sedentary. A female lives out her life beneath a protective shell, sucking plant juice and laying eggs (about 1,000 a year!) till there's little left of her. An adult male-with a set of feeble wings able to propel him only a few feet at a time, and with no digestive tract-exists only to copulate, after which, in most cases, he will die.

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