Scale insects

What are scale insects?

There are several different species of scale insects that can attack cultivated plants. These sap-sucking insects can weaken the growth of a wide range of plants. They range in size from less than 1mm to over 1cm in diameter. Many species excrete a sticky, sugary substance, honeydew, on the leaves and stems on which they are feeding. Some species also produce white, waxy egg masses on stems and the undersides of leaves, this can be mistaken for mealybug or woolly aphid

A wide range of ornamental plants, fruit trees and bushes 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.

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 insect
• Heavy infestations may result in poor growth
• Some species of scale insect excrete honeydew, which accumulates on the upper leaf surfaces. This can be colonised by a black non-parasitic fungus known as sooty mould
• Some scale insects deposit their eggs under a covering of white waxy fibres in early summer

Control

Non-chemical control

• Biological controls can be attempted during the summer in greenhouses
• Parsitic wasps Metaphycus helvolus, Encyrtus spp. and Encarsia citrina can be used against soft scale (Coccus hesperidum) and hemisperical scale (Saisettia coffeae)
• The small black ladybird Chilocorus nigrtus is sometimes available for diaspid scale control
• These controls are available from mail order suppliers of biological controls

Chemical control

• The shell or scale gives some protection to adult scale 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 usually 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 plant oil winter tree wash (e.g. Vitax Winter Tree Wash) on a mild dry day during December to control overwintering scale nymphs
• The synthetic pyrethroid insecticides deltamethrin (e.g. Sprayday Greenfly Killer), lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) and cypermethrin (e.g. Py Bug Killer) are contact sprays that can be used on ornamental plants and a range of edible plants listed on the product packaging, check the label and manufacturer’s instructions
• Ornamental plants in leaf can be sprayed with the systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra). Some formulations of this spray can be used on some fruits, including apple, cherry pear and peach, check the label and manufacturer’s instructions
• Sprays based on natural substances and considered organic include fatty acids (e.g. Solabiol Bug Free, Doff Greenfly and Blackfly Killer), plant oils and extracts (e.g. Origins Bug Control, Growing Success Winter Tree Wash) or natural pyrethrum (e.g. Defenders Bug Killer, Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer (also contains fatty acids)). 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 all fruit trees and bushes
• Do not spray plants in flower due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Biological control suppliers (Adobe Acrobat pdf document)

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 (less than ¼in) long, although wisteria scale, Eulecanium excrescens, can be up to 10mm (about ½in), and vary in shape and colour.

All species have a shell-like covering over their bodies when mature. The eggs are often laid under the protection of this shell but with the cushion scales (eg 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.

See also...

Beech scale
Brown scale
Chemical labels explained
Chemicals: using a sprayer 
Chemicals: using safely and effectively
Chemicals: storing and disposing safely
Cushion scale
Diaspid scale
Euonymus scale
Fluted scale
Hemispherical scale
Horse chestnut scale
Hydrangea scale
Juniper scale
Mealybug
Mussel scale
Oleander scale
Protect your garden
RHS statement on pesticides in horticulture
Scurfy rose scale
Soft scale
Viburnum scale
Wisteria scale
Woolly vine or currant scale
Yew scale