Horse chestnut scale became established in the London area in the 1960s and has since spread to northern England. Although it is often noticeable on the trunks and branches of trees and shrubs, its presence thankfully does not cause serious damage. The heaviest infestations often occur on street trees or plants growing in sheltered places, such as on a patio.
What is 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;
- Although present on suitable host plants all year round, this scale insect often goes unnoticed until early summer, when it starts depositing egg masses on the bark of the trunks and larger branches
- The eggs are embedded in a white waxy material that initially shows as a white crescent beneath the lower edge of the brown shell or scale that covers the insect and its eggs
- After the insects have died, the shells blow away, leaving circular white egg masses, 5-6mm in diameter, on the bark
Large numbers of egg masses may be considered unsightly and alarming but plants seem to cope with heavy infestations, so control measures may not be 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 in early summer.
If a pesticide has to be used, small plants can be sprayed with plant oils (Vitax Organic 2 in 1 Pest & Disease Control, Scotts Bug Clear for Fruit & Veg), deltamethrin (Bayer Sprayday Greenfly Killer), thiacloprid (Bayer Provado Ultimate Bug Killer), acetamiprid (Scotts Bug Clear Ultra) or thiamethoxam* (Westland Plant Rescue Bug Killer Ornamental Plants).
The best time to spray is in early July when the more vulnerable newly hatched scales are present.
* Please note this product is being withdrawn. The withdrawal (in effect a ban) comes into force on 30 September 2013, but there is to be a period of grace to use up these materials by 30 November 2013. After this time it will be illegal to use them, any remaining products should be taken to a manned local authority household waste site where they should be handed over to the staff. For information on waste disposal sites see www.pesticidedisposal.org. It remains legal to use other neonicotinoid-based products that are not affected by the withdrawal. This withdrawal follows concern over their effects on bees.
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 insect dies and the brown cap over the egg mass blows away.
Common name Horse chestnut scale
Latin name Pulvinaria regalis
Plants affected Horse chestnut, lime (Tilia), sycamore, maples and other Acer spp., elm, bay, magnolia, Cornus spp.
Main symptoms In early summer, white circular egg masses, partly covered by brown shells, appear on the trunks and larger branches
Caused by A sap-sucking insect