Advice
RHS Help & Advice
Scale insects (many
species)
Plants affected
A wide range of ornamental plants and fruit trees grown out of doors can be attacked. Several species of scale insects are confined to plants growing in glasshouses or other sheltered places.
Symptoms
Scales or shell-like bumps can be seen on stems and the underside of leaves. Heavy infestations may result in poor growth. Some species of scale insect produce a sticky liquid called honeydew that is excreted by the scales and is deposited on the foliage. Under damp conditions this can be colonised by a black non-parasitic fungus known as sooty mould.
Cause
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 long (wisteria scale, Eulecanium
excrescens, can be up to 10mm) 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 (right). The adults are sedentary but the newly hatched nymphs crawl actively over the plant surface and spread the infestation.
Chemical control
Deciduous fruit trees and roses can be treated with Growing Success Winter Wash on a mild dry day during December to control the overwintering scale nymphs.
Ornamental plants in leaf can be sprayed with imidacloprid (Provado Ultimate Bug Killer), which is available as a spray concentrate and an aerosol. Ornamental plants and some fruits (see product label) can be sprayed with another systemic insecticide thiacloprid (Provado Ultimate Bug Killer Ready To Use). 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.
The shell or scale protects these 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-July.
Scales in glasshouses 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.
Organic controls
These 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.
Biological control
can be attempted during the summer in glasshouses with the
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.
Further information
Brown scale
Horse chestnut scale
Hydrangea scale