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Scales are limpet-like insects that feed by sucking sap from a wide range of plants, including houseplants, greenhouse plants and many fruit and ornamental plants grown outdoors. There are more than 25 species of scale insect in the UK Scale insects can weaken 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 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.
You may see the following symptoms:
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)Biological control suppliers (Adobe Acrobat pdf document)
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.
Beech scaleBrown scaleChemical labels explained Chemicals: using a sprayer Chemicals: using safely and effectively Chemicals: storing and disposing safely Cushion scaleDiaspid scale Euonymus scaleFluted scaleHemispherical scaleHorse chestnut scaleHydrangea scaleJuniper scaleMealybug Mussel scaleOleander scaleProtect your garden RHS statement on pesticides in horticulture Scurfy rose scaleSoft scaleViburnum scaleWisteria scaleWoolly vine or currant scaleYew scale
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