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Adult scale insects are usually covered in waxy shell-like cover. They feed by sucking sap from a wide range of plants, including house-plants, greenhouse plants and many fruit and ornamental plants grown outdoors. There are more than 25 species of scale insect in the UK. Some scale insects can weaken host plants and many excrete a sticky substance (honeydew) on foliage, which allows the growth of black, sooty moulds.
Scale insects on bay. Image: RHS, Horticultural Science
There are several different species of scale insects that can suck the sap from garden and glasshouse plants. They range in size from less than 1mm to over 1cm in diameter. Many species excrete a sticky, sugary substance, honeydew. Some species also produce white, waxy egg masses on stems and the undersides of leaves, this can be mistaken for mealybug or woolly aphid.
Scale insects can be found on a wide range of ornamental plants, fruit trees and bushes grown out of doors. Several species of scale insects are confined to house-plants, those growing in greenhouses or other sheltered places.
You may see the following symptoms:
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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 waxy 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.
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Cotton stringy scale
Horse chestnut scale
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Scurfy rose scale
Woolly vine or currant scale
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