Scale Insect Identification: Armored vs Soft Scale

Scale insects look like small bumps rather than bugs: UC IPM describes them as circular, elongate or oval bumps on bark, leaves or fruit, with no obvious head or legs and coverings 1/25 to 1/4 inch long. They feed by sucking plant sap. To confirm scale, check three things:

• Do the bumps line up along veins and stems? Scales settle on stems and the undersides of leaves (RHS).
• Is anything sticky or black? Soft scales excrete honeydew, which grows a black sooty mould (UF/IFAS; RHS). Ants climbing the plant often mean honeydew.
• Do tiny yellow or orange specks move? Those are crawlers, the only stage with working legs (UC IPM).

Knowing the type matters because treatments differ (see below).

• Cover: an armored scale's cover lifts off the insect underneath; a soft scale's cannot be removed (UC IPM).
• Honeydew: armored scales produce none; soft scales produce plenty (UC IPM).
• Shape: armored scales are tiny and flat; soft scales are rounded and convex, up to 1/4 inch (UC IPM).
• Examples: San José scale and California red scale are armored; black scale and brown soft scale are soft (UC IPM). UF/IFAS gives sizes for Florida species: purple scale 2 to 3 mm and Florida red scale 2.0 to 2.2 mm across (armored); soft brown scale 2.5 to 4 mm; cottony cushion scale 10 to 15 mm including its white egg sac.

UC IPM lists whitefly nymphs, psyllids and mealybugs as insects that resemble scales. Mealybugs are covered in white wax and keep their legs, so they can walk; see aphids for another sap-feeder that leaves honeydew. UC IPM also warns about so-called diamond scale on palms, which is not an insect at all but the fruiting bodies of a blackish fungus. A lady beetle larva or pupa stuck to a leaf can also look like a bump; see ladybug identification before you spray.

Old, dead scale covers can stay on a plant for a long time, so check before you treat:

• Squish test: press a few scales with a fingernail. Live scales exude fluid; dead ones do not (UC IPM).
• Exit holes: a tiny round hole in the cover means a parasitic wasp has emerged from the scale it killed (UC IPM).
• Colour: parasitized scales can look puffy or darker than healthy ones (UC IPM).
• Crawler tape: wrap transparent tape tightly around a few infested twigs and check weekly. Crawlers stick to it and show as yellow or orange specks, which tells you when they are hatching (UC IPM).

• Decide whether to act. RHS says healthy, well-tended plants tolerate light populations. UF/IFAS advises chemical treatment only when plant injury begins and parasites, predators or pathogens are absent.
• Prune heavily infested twigs and branches if they are limited to a few parts of a small plant (UC IPM).
• Control ants. Ants protect honeydew-producing scales from their natural enemies; UC IPM suggests sticky barriers on trunks and slow-acting ant baits.
• Target crawlers. Crawlers are more susceptible than eggs, settled nymphs or adults (UF/IFAS). Apply horticultural oil soon after most crawlers have emerged, or during winter on deciduous plants (UC IPM).
• Systemic insecticides such as imidacloprid control most soft scales but not most armored scales or cottony cushion scale, and UC IPM says not to apply them during or shortly before flowering because of the risk to pollinators.
• Biological control: RHS lists the nematode Steinernema feltiae as an option for soft scale, and parasitic wasps and lady beetles attack many scales (UF/IFAS).

Adults sit under a protective cover and do not move, so sprays that hit them often do little. The insects that can be reached easily are the crawlers, and they appear only at certain times, which is why UF/IFAS and UC IPM both time treatments to crawler emergence. The other common mistake is using the wrong product for the type: a systemic that works on soft scale may not touch armored scale (UC IPM). Identify the type, confirm the scales are alive, and time the treatment.

Field identification usually stops at armored or soft scale. Naming the species is specialist work: the USDA-linked IDtools scale insect tool, built for species of quarantine significance, uses keys for slide-mounted adult females, with fact sheets and an image gallery (last updated April 2014). For a garden or house plant, a clear photo plus the plant name is usually enough for an extension office to tell you what you need to know. Photograph the bumps and any moving crawlers, and include the leaf or stem for scale.

Research and drafting for this page used AI assistance. Every figure, date and quotation was then checked line by line against the sources listed at the bottom of the page, each of which was opened while writing. Nothing here is a diagnosis. If a bite, sting or contact with an insect is followed by trouble breathing, swelling of the face or throat, spreading redness, fever or any symptom that worries you, get medical care; in the United States Poison Help (1-800-222-1222) is free and open 24 hours a day.