Scale insects are among the more stubborn houseplant pests to eradicate because the adult insects are protected by a hard shell (in armoured scale) or waxy coating (in soft scale) that resists many spray treatments. The most effective approach is a combination of manual removal of visible adults, rubbing alcohol spot treatment, and repeated applications of neem oil or insecticidal soap to target the mobile crawler stage (young scale before they develop their protective coating). Persistence over four to six weeks is essential: a single treatment will not eradicate an established scale infestation because eggs and crawlers are not killed by treatments that reach only the adults.
Identifying Scale Insects
Scale insects appear as small, flat, oval bumps firmly attached to stems and the undersides of leaves. Armoured scale (including brown soft scale and oleander scale) have a hard protective cover that is separate from the insect body underneath. Soft scale (including hemispherical scale) have a softer coating and produce significant quantities of sticky honeydew. Both types vary from tan to brown to grey depending on species. They are often mistaken for part of the plant's stem or a dried-up part of the leaf, particularly in early infestation stages.
Check the main stem, leaf undersides along the midrib, and stem junctions. Scale insects are almost always static once they reach the adult feeding stage: they settle on one spot and feed continuously. The crawlers (mobile juveniles) are tiny, almost invisible to the naked eye, and move from the adults to settle new feeding sites.
Manual Removal
The first step in treating a scale infestation is manually removing as many visible adult scales as possible. Use a cotton bud or soft cloth dampened with rubbing alcohol (isopropyl alcohol at 70% concentration) to wipe each scale off the stem, or use a soft toothbrush to dislodge them. The alcohol both softens the attachment and kills the insect on contact. Work systematically along each stem, checking both sides of each leaf and every stem junction.
After manual removal, apply a rubbing alcohol solution with a cotton bud directly to any remaining scales and to areas where scales were present (to kill any crawlers or eggs left behind).
Treating the Crawler Stage
Adult scale under their protective covering are resistant to most spray treatments, but the crawler stage is vulnerable. Neem oil solution or insecticidal soap spray applied thoroughly to all stem and leaf surfaces kills crawlers on contact. Apply every five to seven days for four to six weeks to ensure successive crawler generations are killed as they hatch. Missing weekly applications allows new crawlers to settle and develop their protective coating, resetting the cycle.
Systemic Insecticide
For persistent or severe scale infestations, a systemic insecticide (applied as a soil drench or compost granule) is absorbed by the plant through the roots and transported into the sap. Scale insects feeding on the sap ingest the insecticide and are killed at all life stages. This is particularly effective against soft scale on the undersides of leaves where spray coverage is difficult. Follow product instructions and note that some systemic insecticides are harmful to bees and pets; use in a well-ventilated room and keep away from flowering plants. We would only recommend this on rare occasions for severe infestations.