Scale Insects on Shrubs, Fruit Trees & Ornamentals

title: "Scale Insects on Shrubs, Fruit Trees, and Ornamentals"

Scale insects are easy to misidentify because they don't look like insects at all. The protective covering that gives them their name disguises the insect underneath. Gardeners often assume they are looking at disease, a bark abnormality, or plant growths before they realize they are seeing an insect infestation that may have been building for years.

Per UC IPM, scale insects are among the most common and damaging plant pests in ornamental and fruit gardens. They feed by inserting piercing-sucking mouthparts into plant tissue and extracting plant sap. Heavy infestations cause yellowing, dieback, and can kill young or stressed plants over time.

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Scale insect biology

Scale insects are classified into two major groups based on whether they have a detachable shell:

Armored scales produce a hard, waxy shell (the "scale") that is separate from the insect's body. Per UC IPM, this shell is made of wax secretions plus the cast skins from previous molts and is firmly attached to the plant surface. The shell protects the insect from many pesticides.

Soft scales produce a waxy covering that is fused to the body. They are larger than armored scales, do not have a detachable shell, and produce honeydew — a sticky sugary excretion that attracts ants and supports sooty mold growth.

Per Missouri Botanical Garden, most scale species overwinter as eggs under the protective covering of the mother. In spring, eggs hatch into "crawlers" — tiny mobile first-instar nymphs that are the only stage that moves and the primary stage susceptible to contact pesticides.