Scale insects (armoured and soft scale)
Diaspididae (armoured scale) and Coccidae (soft scale)
Small flat bumps that stick to stems and leaves and suck sap. Look for sticky leaves, black sooty mould and ants on the plant.
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Identify from a photoBackground
Scale insects are sap-sucking insects that spend most of their life fixed in one spot under a protective waxy or shell-like covering, which is why they are so often mistaken for a plant disease or a growth on the bark rather than an insect. Armoured scale forms a hard, separate shell over the insect's body. Soft scale forms a softer, waxy dome and, unlike armoured scale, excretes sticky honeydew as it feeds.
That honeydew is usually the first thing a gardener notices: sticky leaves, a shiny film on paths or nearby plants underneath the infested one, and a black sooty mould that grows on the honeydew and can cover enough leaf area to weaken the plant on its own. Ants are commonly seen farming the scale for the honeydew and will actively defend it from predatory insects, which is one reason scale outbreaks can build up quickly once ants move in.
Citrus is the crop most gardeners associate with scale, but soft and armoured scale species also turn up on tomato, capsicum, and a wide range of ornamental and fruiting plants. Young crawlers, the mobile first life stage, are tiny and pale, sometimes described as looking like small white or grey specks moving along a stem before they settle and form their covering.
How to identify
- Small (2 to 6mm), flattened, shell- or dome-like bumps on stems, twigs, and the underside of leaves, usually brown, grey, white, or yellowish, that do not move when touched.
- Sticky honeydew on leaves, stems, or the ground and plants beneath the infested one.
- Black sooty mould growing on the honeydew, giving leaves and stems a dirty, blackened look.
- Ants travelling up and down stems, especially near clusters of scale.
- Yellowing leaves and dieback on heavily infested branches, since scale removes sap the plant needs.
Life cycle
Several overlapping generations a year in warm Australian climates. Eggs hatch into tiny mobile crawlers, the only mobile life stage, which move a short distance, settle on a stem or leaf, and form their protective covering within a few days, after which they stay fixed in place for the rest of their life feeding on sap.
Weather triggers
- Temperature: 16-34°C
- Humidity: >55%
- Warm, humid conditions favour rapid buildup, and plants already stressed by drought or poor drainage carry heavier infestations.
Peak season (southern hemisphere)
Red = active season · Dark red outlined = this month
Affected vegetables & crops
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Climate zones at risk
Organic & low-impact control
- Spray horticultural (white) oil to smother crawlers and soften the waxy covering on light infestations; two or three applications a week or two apart usually catches the next wave of crawlers.
- Prune out and dispose of heavily infested stems rather than trying to treat them.
- Wash sticky honeydew and sooty mould off leaves with a soapy water solution once the scale itself is under control, since the mould does not go away on its own.
- Control ants around the base of the plant with an ant bait; removing the ants lets ladybirds and parasitic wasps get to the scale.
- Encourage ladybirds and parasitic wasps, both natural predators of scale, by avoiding broad-spectrum insecticide use nearby.
Chemical control
- A registered systemic insecticide for scale on citrus and edible crops, applied per label, for infestations horticultural oil has not brought under control.
- Spot-treat affected branches rather than the whole plant where possible, to protect beneficial insects elsewhere on the tree.
Always read product labels. Registrations change.
Prevention
- Inspect new plants closely, including the underside of leaves and along stems, before adding them to the garden.
- Keep citrus and other susceptible plants well watered and fed; stressed plants carry heavier scale infestations.
- Keep ant nests near susceptible plants under control before a scale problem starts, not after.
- Avoid heavy nitrogen fertilising, which produces soft new growth scale favours.
Sources
- Queensland Department of Agriculture and Fisheries, scale insects in the home garden
- NSW Department of Primary Industries, citrus pests and their management
- Agriculture Victoria, scale insects
Accuracy confidence: medium. We update this library as new extension guidance is published.
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