At a glance
- What they look like: Small brown, tan, or white bumps firmly attached to stems and leaves
- First sign often: Sticky honeydew on leaves or surfaces below the plant; sooty mold
- Two types: Soft scale (easier to treat) and armored scale (harder; waxy covering resists sprays)
- Best treatment: 70% isopropyl alcohol on a cotton swab applied directly to each scale
- Also effective: Neem oil or horticultural oil spray to catch crawlers
- Isolate immediately: Crawlers spread to nearby plants quickly
What scale insects are
Scale insects are sap-sucking insects that, in their adult settled form, look almost nothing like insects. Once a scale insect settles on a plant and begins feeding, it covers itself with a protective coating and becomes largely immobile. The result is small oval bumps on stems and leaves that are easily confused with part of the plant itself, especially on plants with naturally textured bark or stems.
Scale is one of the most commonly overlooked houseplant pests because of this camouflage. Many infestations are not noticed until they are quite advanced, at which point the plant may already be significantly stressed.
Soft scale versus armored scale
Soft scale insects (family Coccidae) have a waxy or cottony coating but retain a soft body underneath. They excrete large amounts of honeydew, the sticky sugary liquid that drips onto leaves and surfaces below the plant and attracts sooty mold. Brown soft scale is one of the most common houseplant species, appearing as flat, oval brown bumps on stems. Soft scale is somewhat easier to treat because the protective coating is penetrated more readily by oil and soap sprays.
Armored scale insects (family Diaspididae) produce a hard, waxy cover that is physically separate from the insect's body. This armor is difficult for oil and soap sprays to penetrate. Armored scale does not produce honeydew, so it is often not noticed until the plant is visibly stressed. Common armored scale species include oyster shell scale and San Jose scale. The rubbing alcohol method is most effective for armored scale.
How to confirm it is scale
Scrape one of the suspicious bumps with your fingernail or a toothpick. Scale insects detach cleanly, leaving a small wound mark on the stem. Underneath soft scale you will find a soft, pale or yellowish insect body. Underneath armored scale the shell may be empty (the insect inside has matured or died) or may contain a flat, disc-like body.
Also check for honeydew: if nearby leaves or surfaces below the plant feel slightly sticky, and if you see a black powdery coating developing on any leaves (sooty mold feeding on honeydew), soft scale is very likely present even if you have not yet spotted the insects themselves.
Step 1: isolate the plant
Move the affected plant away from all others immediately. While adult scale do not move, juvenile crawlers are mobile and can spread to neighboring plants quickly. Inspect every plant that was near the affected one for early signs of infestation before returning them to a shared space.
Step 2: alcohol treatment
Dip a cotton swab or soft cloth in 70% isopropyl rubbing alcohol and apply it directly to each visible scale insect. The alcohol penetrates and dissolves both soft and armored scale coverings and kills the insect on contact. This is the most effective treatment for settled adult scale, which are resistant to spray treatments.
Work through the entire plant systematically: every stem, every leaf junction, the undersides of leaves, and the base of the plant where the stem meets the soil. Remove dead scale with a soft brush after treatment. Repeat every 5 to 7 days for 4 to 6 weeks.
Step 3: neem oil or horticultural oil spray
After manual removal, spray the entire plant with neem oil solution or horticultural oil to catch mobile crawlers that the alcohol treatment may have missed. Crawlers are the vulnerable stage: they have not yet settled and formed their protective covering, so they are killed by oil contact. Spray all surfaces including the undersides of leaves, and repeat weekly.
Horticultural oil (also called dormant oil or mineral oil) is particularly effective against scale at all life stages because it suffocates insects by coating their breathing pores. Follow label directions for concentration; too high a concentration can damage leaves, particularly in hot weather.
Systemic insecticide for severe infestations
For very severe infestations that do not respond to alcohol and oil treatment, a systemic insecticide containing imidacloprid can be applied as a soil drench. The plant absorbs the chemical through its roots and distributes it through the sap; scale feeding on the sap are killed. Systemic treatments should be a last resort because they affect all insects that contact the plant, including beneficial ones, and residues persist in the plant tissue for an extended period.
Plants commonly affected by scale
Scale insects are particularly common on ficus (including fiddle leaf fig and rubber plant), citrus, orchids, hoyas, schefflera, ferns, bay trees, and palms. Any houseplant can be affected, but plants with smooth stems and waxy leaves seem to be preferred by many scale species. Plants under stress from poor light, overwatering, or underfeeding are more susceptible.
Prevention
Inspect new plants carefully before bringing them indoors, particularly the stems and undersides of leaves. Quarantine new plants for 2 to 3 weeks before placing them near established plants. Regularly checking stems during watering and cleaning routines allows early detection when infestations are still small and easy to eliminate manually.
Keeping plants healthy and well-nourished reduces their vulnerability. Scale, like mealybugs, seems to prefer plants that are stressed or receiving excess nitrogen fertilizer, which makes the sap more attractive.