Scale insects are unusual plant pests named for the protective, scale-like covering that conceals their bodies as they feed on a host plant. Their appearance is so atypical that people often mistake them for fungal growths, small blisters, or natural plant abnormalities rather than actual insects. Understanding the visual characteristics of these pests is the first step in identifying an infestation.
The Immobile Adult Form
The adult female scale insect is the form most frequently encountered and misidentified on plant surfaces. These insects are sessile, permanently affixing themselves to the plant and losing their ability to move after the first molt. Adult scales are relatively small, with their protective coverings typically measuring from 1/16 inch up to 1/4 inch across, depending on the species.
The appearance of this outer layer varies dramatically in both shape and texture. Shapes include circular, oval, elongated, or one that resembles a miniature oyster shell. The texture may be hard and crusty, waxy and smooth, or even cottony and mealy.
Because the adult female remains underneath this secreted covering, she does not exhibit the visible legs, distinct head, or segmented body typical of most insects. Instead, the insect looks like an unmoving, inconspicuous bump or spot on the stems, bark, or leaves. These protective shells often blend in with the plant material, appearing in colors like white, brown, grey, or black.
Distinguishing Soft Scale from Armored Scale
Soft scales and armored scales are differentiated by the structure of their protective shell. Soft scales are generally larger, with mature individuals often reaching 1/8 to 1/4 inch in length. Their bodies typically have a convex, dome-shaped, or even hemispherical profile, sometimes resembling a small tortoise shell.
The covering of a soft scale is a waxy secretion that is fused to the insect’s body, making it impossible to separate easily from the insect itself. These scales feed deeper within the plant, tapping into the phloem tissue, which causes them to excrete a sugary, sticky substance called honeydew.
Armored scales, by contrast, are smaller, usually measuring between 1/16 and 1/8 inch long. They are flatter, often presenting a shield-like or oyster-shell shape on the plant surface. The covering is a distinct, non-living shield composed of wax and shed skin layers, which is not attached to the insect’s soft body. This hard shell can be scraped off, revealing the soft, typically yellow or orange insect underneath. Armored scales feed on individual plant cells and do not secrete honeydew.
The Appearance of the Mobile Crawler Stage
The first instar nymphs, commonly referred to as crawlers, are the only mobile stage. These crawlers are minute, often smaller than the head of a pin, making them difficult to observe without a magnifying lens. They are generally oval and flat in shape, resembling a very tiny, conventional insect.
Crawlers possess functional legs and antennae, which they use to move across the plant surface in search of a permanent feeding site. Their color is usually light, ranging from yellow or orange to light brown, which helps distinguish them from the often darker, mature adult shells.
Once they settle, the crawlers insert their mouthparts and begin metamorphosis into the immobile adult form. They lose their legs and antennae during the first molt as they secrete the protective waxy covering characteristic of their species. The appearance of these minute, active crawlers confirms that a new generation of scales is establishing itself on the host plant.