Scale insects
Coccoidea
also called Scale, Brown scale, Mussel scale
Immobile brown or white bumps fixed to stems and leaf undersides, sucking sap and weakening growth. Wipe off with a damp cloth or dab with horticultural soap; watch for the sooty mold that follows.
How to know it
Waxy limpet-like bumps 2–6 mm across clamped to stems, leaf undersides and the ribs of leaves — brown and domed, white and mealy, or shaped like a tiny mussel shell. They do not move; lift one with a thumbnail and there is a soft yellow insect and a raft of eggs beneath.
What you'll see
- Rows of small immobile bumps along stems, leaf veins and the undersides of evergreen leaves.
- Sticky honeydew on the leaves and on whatever sits below the plant.
- Black sooty mold coating the honeydew until the foliage looks smoke-damaged.
- Slow decline: sparse yellowing leaves, short growth, and dieback of the worst-covered branches.
- On bay, camellia and citrus the damage is chronic rather than sudden — a plant that never quite thrives.
Where it comes from
Eggs hatch beneath the mother’s scale into mobile "crawlers", the only stage that travels. Within days each settles, pushes in its stylet and secretes the waxy shield it will live under for the rest of its life. One or two generations a year outdoors, continuous under glass, and the empty shields stay stuck to the bark long after the insect has gone.
Thrives on woody evergreens in warm sheltered positions, on anything under glass, and on houseplants where no weather ever knocks the colony back. Drought-stressed and pot-bound plants suffer most.
Dealing with it
- Wipe the scales off with a damp cloth or a soft brush — on a bay or a citrus this is genuinely the best control there is.
- Time any spray for the crawler stage in early summer, when the insects are exposed and soft; a shield is armor against everything else.
- Use insecticidal soap or plant-oil sprays, thoroughly, on stems as well as leaves, and repeat after ten days.
- Prune out and burn branches that are crusted beyond rescue.
- Wash off the sooty mold afterwards so the leaves can work again.
Keeping it out
- Inspect the stems and leaf undersides of every new plant before it joins the others.
- Keep woody plants watered and unstressed; a vigorous plant carries light scale without complaint.
- Encourage ladybirds and parasitic wasps, both of which work scale colonies hard outdoors.
- Ventilate glasshouses well and avoid the permanently warm, still corners where colonies build.
- Scrape old scale shields off bark in winter so you can tell live infestation from history.
What it goes for
Bay, citrus, camellia, ivy, orchids and most woody houseplants; outdoors on apples, currants, ceanothus and horse chestnut.
- Abiu
- Acer
- Alocasia
- Anthurium
- Apple
- Arborvitae
- Areca Palm
- Asparagus Fern
- Atemoya
- Australian Grass Tree
- Australian Native Ginger
- Avocado
- Banksia
- Bay laurel
- Bird of Paradise
- Bird's Nest Fern
- Black currant
- Blackberry Jam Fruit
- Boston Fern
- Bottlebrush
- Breadfruit
- Breath of Heaven
- Bromeliad
- Cabbage Palm
and 141 more in the catalog
Trouble it brings with it
Who hunts it for you
- HoverflySyrphidae Bee-mimicking flies whose legless larvae graze aphid colonies clean while the adults pollinate. A ribbon of shallow, nectar-rich flowers kept in bloom all season recruits wave after wave of them. Grow to recruit it: Calendula,Chamomile,Fennel,French marigold,Phacelia,Sweet alyssum
- Ladybird (lady beetle)Coccinellidae The gardener’s favorite ally: adults and their alligator-shaped larvae each eat thousands of aphids, plus scale, mealybugs, and other soft-bodied sap-suckers. Grow flat, open flowers for the adults and never spray a colony that ladybird larvae are already working. Grow to recruit it: Calendula,Cilantro,Cosmos,Dill,Fennel,Yarrow
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com