Scale insects — the pest disguised as a scab
Scale insects are one of the most deceptive pests, because they often don't look like insects at all. Once they reach adulthood they develop a hard, waxy shell and attach themselves to the plant in one spot — resembling small brown bumps or scabs rather than living pests. They feed by sucking sap, which gradually weakens the plant and can slow its growth.
What to look out for
- Stationary bumps — Small tan or brown raised bumps along stems, joints (where the leaf meets the stem), and the midrib — the central vein — on the underside of leaves.
- Sticky residue (honeydew) — Like mealybugs, scale insects excrete sticky honeydew while feeding. If the leaves feel tacky or look shiny but you don't see fuzzy white pests, check for hard bumps instead.
- Yellow patches on leaves — Leaves may develop yellow spots or patches near where the scale insects are attached.
- Sooty mold — If honeydew accumulates, a black soot-like fungus may grow on the leaves. The mold itself does not harm the plant, but it is a sign of an active infestation.
- The scrape test — Use your fingernail or a dull tool to gently nudge one of the bumps. If it pops off easily and looks moist underneath, it is likely scale.
Examples of scale insects in real homes
Why they’re difficult to eliminate
Scale insects are protected by a hard waxy shell that acts like armour. Many sprays cannot penetrate this barrier — so the insects must either be removed manually, or treated with products that dissolve the protective coating or are absorbed by the plant itself.
How to treat them
- Isolate immediately — Move the affected plant to a separate room to stop the infestation spreading to other plants.
- Rinse the plant — Take it to the sink or shower and use a steady stream of water to wash away as many pests as possible.
- Prune heavily infested areas — Remove heavily infested leaves or stems with clean scissors and dispose of them immediately.
- Use an insecticide — Apply an insecticide thoroughly across the entire plant, including the stems and the undersides of leaves. Repeat weekly for about 5 weeks to break the pest life cycle.
- Target with alcohol — Dip a cotton swab in rubbing alcohol (isopropyl alcohol) and press it onto each scale. The alcohol dissolves the protective wax and kills the insect, letting you wipe it away.
- Use a systemic treatment — Add a systemic insecticide to the soil so the plant absorbs it. When scale insects feed on the plant's sap, they ingest the insecticide and die.