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Horse chestnut scale became established in the London area in the 1960s and has since spread throughout England and into Wales. Although it is often noticeable on the trunks and branches of trees and shrubs, its presence does not cause serious damage. The heaviest infestations often occur on street trees or plants growing in sheltered places.
Horse chestnut scale
Horse chestnut scale is a sap-sucking, limpet-like insect feeds on a wide range of trees and shrubs.
How to spot horse chestnut scale;
Large numbers of egg masses may be unsightly and alarming but plants cope with heavy infestations, so control measures are not usually necessary.
Mature trees are too large to be sprayed effectively. On small plants, such as Japanese maples and bay trees, it may be feasible to scrape off the scales and egg masses when seen.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Horse chestnut scale has one generation a year.
Eggs hatch in late June-July and the scale nymphs crawl on to the underside of leaves, where they feed by sucking sap.
In autumn, they move to the bark and overwinter. At that time they are difficult to see, as they are flat, oval scales about 1mm long and closely matching the colour of bark. They remain on the bark and resume feeding in spring.
They are fully grown in May, when the scales become brown and circular in shape, with a diameter of 4-5 mm. Eggs and a white waxy material are deposited under the shell or scale that covers the insect.
After egg-laying is complete, the adult insect dies and the brown cap over the egg mass detaches.
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