Scale insects are among the most overlooked garden pests, because they barely look like insects at all. They sit motionless on stems and leaves, covered by a hard, shell-like scale, and they are often mistaken for a bump on the bark or a bit of dirt. By the time you notice them, there are usually dozens, and the plant is already weakened, sticky with honeydew and black with sooty mould. They are a particular problem on fruit trees, shrubs and houseplants, and they are stubborn — the hard scale protects them from sprays, so control takes a different approach.
This guide covers how to identify scale insects, what damage they cause, how they live, and how to control them organically.
Identifying scale insects
Scale insects are small, usually 2–5 mm long, and they are covered by a protective scale or shell that is often brown, grey or white. They sit motionless on stems, branches and the undersides of leaves, sucking sap. There are two main kinds: armoured scale, which has a hard, separate shell that can be lifted off, and soft scale, which has a waxy, soft covering that is part of the insect's body.
Look for:
- Small, round or oval bumps on stems, branches and leaf undersides — brown, grey or white, and hard to the touch
- Sticky honeydew on leaves and anything below the plant, often with black sooty mould growing on it
- Leaves that yellow, wilt or drop, and branches that die back
- Ants on the plant, farming the scale for honeydew
- Tiny, mobile crawlers on the leaves in spring or summer — the young stage, before they settle and form their scale
Scale insects are sometimes confused with other pests. Mealybugs are similar, but they are soft, cottony and can crawl slowly, while scale are hard, shell-like and motionless. Aphids are soft-bodied and cluster on shoot tips. If the pest is a hard, immobile bump on a stem, it is scale. If you are unsure, our pest identifier helper can help you work through the symptoms, and your local extension service can confirm an identification.
The scale insect lifecycle
Scale insects reproduce steadily. The female lays eggs under her scale, where they are protected. The eggs hatch into tiny, mobile crawlers — the only stage that moves — which spread out over the plant, settle down, and begin feeding. Once settled, they lose their legs, form their scale and stay in one place for the rest of their lives, feeding and, in the case of females, laying eggs.
There are usually one or two generations a year outdoors, with the crawlers appearing in late spring or early summer — the critical time for control, because the crawlers have not yet formed their protective scale and are vulnerable to sprays and predators. Indoors, scale can remain active all year, with overlapping generations.
Because the adults are protected by their scale, and the eggs are hidden underneath it, the crawler stage is the weak point in the lifecycle. Control that targets the crawlers, timed for when they are active, is far more effective than spraying the adults.
Damage signs
Scale insects damage plants by sucking sap, which weakens the plant and slows growth. Their feeding causes leaves to yellow, wilt and drop, and branches to die back. In heavy infestations, the plant may be seriously weakened, and young plants can be killed. Their honeydew coats leaves and encourages sooty mould, which blocks light and looks unpleasant.
Scale attack a wide range of plants, but they are most common on fruit trees — citrus, apples, plums — and on ornamental shrubs such as camellias, hollies and roses, and on houseplants, especially citrus, orchids, palms and ferns. On fruit trees, a heavy infestation can reduce the crop and weaken the tree over several years.
Organic-first control, in order
Work through these steps in order, starting with the gentlest.
1. Inspect regularly. Check the stems, branches and undersides of leaves at least once a month, and more often in spring and summer when the crawlers are active. Look for the bumps, for honeydew and for sooty mould. Early detection is essential, because a small colony is easy to deal with and a large one is not.
2. Scrape them off. For a small infestation, scrape the scale off with your thumbnail, an old toothbrush or a blunt knife. This is slow, but it is the most reliable method for a few plants, and it removes the adults and their eggs at once. Drop the scraped scale in a bucket of soapy water. Prune out badly infested branches and destroy them — do not compost them in a cold heap.
3. Target the crawlers. The crawlers appear in late spring or early summer, and they are the vulnerable stage. Watch for them — tiny, mobile, pale insects on the leaves and stems — and spray at that time. A strong jet of water can knock crawlers off, and insecticidal soap or horticultural oil, applied thoroughly, will kill them. Timing is everything: spray when the crawlers are active, and repeat after a week to catch the ones that hatch later.
4. Encourage beneficial insects. Ladybirds, lacewings and parasitic wasps all eat scale, and they are the best long-term control outdoors. The parasitic wasps that attack scale are tiny and harmless to people, and they are highly effective — if you see small holes in the scale, the wasps have been at work. Plant flowers that attract predators, leave some habitat, and avoid spraying anything that kills them.
5. Deal with the ants. Ants farm scale for honeydew and protect them from predators. If ants are carrying crawlers to new growth, a sticky barrier around the trunk or stem can break the partnership, and the predators will then do their job.
6. Least-toxic sprays, last. If the steps above have not brought numbers down, horticultural oil — applied in the dormant season for fruit trees, or in the growing season at a dilute rate — can smother scale and their eggs. It must be applied thoroughly, covering the stems and branches, and it should not be applied in hot sun or when the plant is stressed. Insecticidal soap can also be used on the crawler stage. Always read and follow the label, use only products approved for the crop in your area, and wash produce before eating. Never use a broad-spectrum insecticide: it kills the predators and parasitic wasps that control scale.
If you are unsure which product is appropriate, or when the crawlers are active in your area, check your local extension service for recommendations.
Prevention
Prevention is mostly about inspection and plant health. Check new plants carefully before bringing them home or into the greenhouse — scale often arrive on a new purchase, hidden on a stem. Isolate new plants for a week or two and check them before placing them among your other plants.
Keep plants healthy with steady watering and moderate feeding, and avoid overcrowding — good airflow makes plants less attractive. Prune fruit trees and shrubs to keep the canopy open, which improves airflow and makes it easier to inspect and treat. Clear away old plant material where scale can shelter.
For houseplants, keep them in good light with steady watering, isolate any plant with scale, and check the stems and leaf undersides weekly. See also our guide to fungus gnats, another common indoor pest.
When is it a serious problem vs cosmetic?
A few scale on a large, healthy tree or shrub are cosmetic — scrape them off and let predators do the rest. It becomes a serious problem when numbers are high and climbing, when leaves are yellowing and dropping, when branches are dying back, when honeydew and sooty mould cover the plant, or when a young tree or a houseplant is affected. In that case, act quickly: scrape off what you can, prune out the worst branches, and time a spray for the crawler stage.
Scale on a mature fruit tree are a problem you can manage over a season. Scale covering every stem of a young citrus in a pot are a serious problem that needs immediate action. The difference is numbers, the age of the plant and the rate of increase — and regular inspection is how you tell which one you have.
Frequently asked questions
What do scale insects look like? Small, round or oval bumps on stems and leaves — brown, grey or white, hard to the touch, and motionless. They look like a bump on the bark or a bit of dirt, not like an insect.
Are scale insects harmful to people or pets? No. They do not bite people or pets and are not poisonous. They are a plant problem, not a health problem.
Can scale spread to other plants? Yes — the crawlers walk to nearby plants, and they can also be carried on hands, tools, clothing and the wind. Isolate infested plants and wash your hands and tools after handling them.
When do scale crawlers appear? Usually in late spring or early summer, though the timing varies by species and climate. Watch for tiny, mobile, pale insects on the leaves and stems, and check your local extension service for the timing in your area.
Can I use washing-up liquid on scale? It is sometimes recommended as a home remedy, but washing-up liquid is not formulated for plants and can damage leaves, especially in sun. Insecticidal soap or horticultural oil, made for the purpose and used at the label rate, are safer and more reliable.
The bottom line
Scale insects are hard to see and hard to spray, because their shell protects them — so control means scraping them off by hand, pruning out the worst growth, and timing sprays for the crawler stage, when they are vulnerable. Encourage predators, deal with the ants that protect them, and use horticultural oil only if the gentler steps have not worked. For more pest guidance, see the guides hub and the garden tools.