Scale insects
Scale insects spend most of their lives fixed in place under a waxy shell, sucking sap. They weaken plants and drip honeydew that grows black sooty mould. Citrus, bay, hydrangeas and houseplants are common hosts.
Pests
What it looks like
They look like part of the plant until you scrape one.
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Bumps on stems Oval brown, grey or white bumps along stems and veins. -
White egg sacs Cottony masses under some scales in early summer. -
Sticky leaves Honeydew coats leaves below. -
Black film Sooty mould follows the honeydew.
How to confirm it
Scrape a bump with a fingernail.
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Lift a scale
It comes away, often with a soft insect or eggs beneath.
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Follow the stickiness
Look above sticky or sooty leaves.
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Check leaf undersides
Especially along the midrib.
What to do
Remove by hand and treat the young.
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Wipe or scrape off
Use a soft brush or cloth with soapy water on smaller plants.
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Prune heavily infested shoots
Cut out the worst stems.
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Spray the crawlers
Plant oil or soft soap sprays work on the mobile young in early summer.
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Biological control
Under glass, parasitic wasps can control soft scale.
Prevent it next time
- Check new plants carefully.
- Keep indoor plants well-lit and not stressed.
- Inspect citrus regularly, especially in winter indoors.
What not to do
Do not rely on sprays against adult scales; their shell protects them.
In the growing guides
Where the guides mention it, with plant-specific advice.
Sources
- Scale insects, Royal Horticultural Society