Royal Horticultural Society

What are scale insects?

Scale insects are sap-sucking insect pests that weaken growth and can infest a wide range of plants. Many species excrete a sticky, sugary substance, called honeydew, on the leaves and stems on which they feed.

A wide range of ornamental plants and fruit trees grown out of doors can be attacked. Several species of scale insects are confined to houseplants or those growing in greenhouses or other sheltered places. Vegetables are usually not affected.

Symptoms

You may see the following symptoms:

• Scales or shell-like bumps on plant stems and the underside of leaves. These are the outer coverings of scale insects.
• Heavy infestations may result in poor growth.
• Some species of scale insect excrete honeydew, which accumulates on the upper leaf surfaces. Under damp conditions this can be colonised by a black non-parasitic fungus known as sooty mould.

Control

Non-chemical control

Biological control can be attempted during the summer in greenhouses with a parasitic wasp, Metaphycus helvolus. This attacks two species of scale insect, soft scale (Coccus hesperidum) and hemisperical scale (Saisettia coffeae).
The parasitic wasp is available from some mail order suppliers of biological controls.

Chemical control

The shell or scale protects the older insects from insecticides, so spraying is more effective against the newly hatched nymphs. With scales on outdoor plants there is usually one generation a year and in most species the eggs hatch in late June to July.

Scales in greenhouses or on houseplants breed throughout the year so all stages in the life cycle may be present at the same time. Scale insects can remain attached to the plant long after they are dead but new growth should be free of scales once they have been brought under control.

• Deciduous fruit trees and roses can be treated with Growing Success Winter Wash (contains plant oils) on a mild dry day during December to control the overwintering scale nymphs.
• Ornamental plants in leaf can be sprayed with thiacloprid (Provado Ultimate Bug Killer), which is available as a spray concentrate and a ready to use spray. The ready to use spray can be used on some fruits, including apple, plum, gooseberry, currants, raspberry and other cane fruits.
• Other systemic insecticides for use on ornamental plants only are acetamiprid (Scotts Bug Clear Ultra) and thiamethoxam (Westland Bug Attack). The latter is for container-grown ornamental plants only.
• Sprays based on natural substances include fatty acids (Bayer Organic Pest Control, Doff Greenfly and Blackfly Killer or Greenfingers Organic Pest Spray) or plant oils and extracts (Growing Success Fruit & Veg Bug Killer, Scotts Bug Clear for Fruit & Veg and Vitax Organic 2 in 1 Pest & Disease Control). These pesticides have very little persistence and so may need several applications during the scale's egg-hatching period, but they can be used on fruit trees and bushes.

Links

Biological controls

Biology

More than 25 different species of scale insects are pests in the UK and these include Pulvinaria, Diaspis, Parthenolecanium, Unaspis, Coccus species and others. They suck sap from the leaves and stems of their host plants. They are mostly 1-6mm (1/16-¼in) long, although wisteria scale, Eulecanium excrescens, can be up to 10mm (3/8in), and vary in shape and colour.

All species have a shell-like covering over their bodies. The eggs are often laid under the protection of this shell but with the cushion scales (Pulvinaria species) the eggs are deposited outside the scale under a mass of white waxy fibres.

The adults are sedentary but newly hatched nymphs crawl actively over the plant surface and spread the infestation.

Scale insects in greenhouses can breed continuously throughout the year but those species that infest outdoor plants mostly have one generation a year.