What are the brown bumps on my plant's stems and leaves?

Scale insects are the pest people most often fail to recognise as an animal at all. They look like a blemish on the bark, they never move once settled, and by the time the plant is sticky and blackened the population has been building for months. They are common on bay, camellia, citrus, wall shrubs, ivy, orchids and a long list of houseplants.

Step by step

1. Confirm they are insects.Flat or domed oval bumps, 2 to 5mm, brown, cream or grey, arranged in rows along stems and along the underside of leaf midribs. Push one sideways with a fingernail — a scale lifts off leaving a slightly damp mark and often a mass of tiny eggs beneath, which bark does not.
2. Look below them for the corroborating evidence.Leaves that are shiny and sticky with honeydew, a black sooty film growing on that honeydew on the leaves and any surface beneath, and ants running up and down the stem. Any one of those on a plant with bumps confirms it.
3. Understand why sprays fail.The waxy shield covers the insect completely and is not penetrated by contact treatments. Spraying an adult scale achieves nothing at all, which is why gardeners conclude the product does not work.
4. Remove them physically, which is what kills them.A fingernail, an old toothbrush, a soft cloth, or a cotton bud dipped in surgical spirit. Work along every stem, into the leaf axils, under the leaves and along the midribs. It is tedious and it is the effective treatment.
5. Find the crawler stage if you want to treat.Eggs hatch under the shield into crawlers — minute, pale, mobile and unprotected, looking like a dust of moving specks on the stems, usually in early summer. That is the only vulnerable stage. Tape a strip of white card around a stem, sticky side out, and check it for movement to find the timing.
6. Wash the plant thoroughly afterwards.Warm water and a soft cloth on every leaf removes the honeydew and the sooty mould, which are not the problem but do block light. On a houseplant, a shower over the sink is efficient.
7. Deal with the ants.Ants farm scale for honeydew and drive off the predators that would otherwise reduce the population. A grease band or a sticky barrier around the stem or pot breaks that relationship and makes everything else more effective.
8. Repeat, and keep repeating.Check and clean every two weeks for at least a couple of months. Scales you missed and crawlers that settled after the last pass are what bring the infestation back, and this is where most attempts fail.
9. Prune out badly infested growth.On a wall shrub, an ivy or a bay tree that is heavily colonised, cutting out the worst stems removes more scale in five minutes than an hour of scraping. Bag the prunings.
10. Improve the growing conditions.Scale takes hold on stressed plants, particularly ones that are dry at the roots, pot-bound or in poor light. A bay tree in a pot that has not been potted on for years is the classic case, and repotting is part of the treatment.

Tips

• Look at plants in winter as well. Scale is easiest to find on a deciduous plant with the leaves off, and dealing with it then prevents the spring build-up.
• Outdoors, parasitic wasps and ladybirds control scale reasonably well if ants are not protecting it. That is why an ant barrier is worth as much as any treatment.
• Horticultural oil-based winter washes exist for dormant deciduous fruit and ornamentals and smother overwintering stages. Check the label lists your plant.
• A plant covered in dead, empty scales still looks infested. Clean them off so you can tell whether anything new is arriving.

Common mistakes

• Spraying an insecticide at adult scale — The shield is impermeable to it. Physical removal, or timing at the crawler stage, are the only things that work.
• Treating the sooty mould — It is growing on the honeydew, not on the plant, and it disappears once the insects producing the honeydew are gone. Wash it off for appearance only.
• Cleaning the plant once and considering it done — The crawlers hatch over a period of weeks. Anything less than repeated passes over a couple of months leaves a founding population.
• Ignoring the ants — They actively defend the scale and remove the natural control. The barrier is a two-minute job with a large effect.

Questions people ask