Scale Insects
Scale insects are sap-suckers that live under a protective shell or waxy coating, looking like small bumps glued to bark, stems, and leaf veins.
Scale Insects at a glance
| Also known as | Hard scale, soft scale |
|---|---|
| Affected parts | Stems, branches, leaf undersides/veins |
| Most at risk | Many broadleaf and tropical trees, junipers; indoor trees especially |
| Main cause | Sap-sucking scale insects under protective shells on bark and leaves |
| Severity | Moderate; slow but persistent, weakens the tree |
| Reversible? | Yes, but needs repeated treatment (shells protect them) |
What to look for
- Leaves: Small raised bumps along veins and on undersides; yellowing and premature leaf drop; sticky honeydew and black sooty mold on foliage below.
- Whole tree: Slow loss of vigor; ants on the trunk; sooty mold blackening the foliage.
Overview
Scale insects are sap-suckers that live under a protective shell or waxy coating, looking like small bumps glued to bark, stems, and leaf veins. Because the shell protects them, they are easy to overlook and stubborn to treat. They weaken the tree slowly and, like aphids, excrete honeydew that leads to sooty mold and ants.
Causes
Crawlers spreading from infested plants; warm, sheltered conditions (indoors, greenhouses); and undisturbed trees where colonies build up unnoticed.
Commonly confused with
Mealybugs are a mobile, cottony soft-scale relative (white, fluffy, in leaf axils). Aphids/whitefly also give honeydew but move. Lenticels or natural bark bumps can look like scale — but scale can be scraped off and crushed.
Which species are affected
Wide range: many broadleaf and tropical species (ficus, citrus-relatives, privet, camellia, olive), junipers and pines (needle/juniper scale), and most indoor bonsai.
Treatment
Physically remove visible scale with a soft brush or a cotton bud dipped in (diluted) isopropyl alcohol. Then spray horticultural oil or insecticidal soap thoroughly, repeating every 7–10 days for several weeks to catch newly hatched crawlers and stages protected by shells. Systemic insecticides help heavy infestations. Control ants and clean off sooty mold.
Prevention
Inspect bark, axils, and leaf undersides regularly; quarantine new plants; keep trees vigorous and airy; treat at the crawler stage when most vulnerable.