Horse chestnut scale

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

Quick facts

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
Timing May-August

What is horse chestnut scale?

Horse chestnut scale is a sap-sucking, limpet-like insect feeds on a wide range of trees and shrubs.

Symptoms

How to spot horse chestnut scale;

  • Although present on host plants all year round, this scale insect often goes unnoticed until late-spring or 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
  • If egg masses are found on the leaves of acers it is likely that the infestation is hydrangea scale and not horse chestnut scale
  • After the adult insects have died, the shells often blow away, leaving circular white egg masses, 5-6mm in diameter, on the bark

Control

Large numbers of egg masses may be unsightly and alarming but plants cope with heavy infestations, so control measures are not usually necessary.

Non-chemical control

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.

Chemical control

  • Use of pesticide against this insect is not usually necessary as this insect has little impact on host plant vigour
  • If a pesticide has to be used, only plants small enough to be given a full coverage can be treated
  • The best time to spray is in early July when the more vulnerable newly hatched scales are present
  • Note that dead scales can remain firmly attached to the plants. The success of any treatment can be gauged by the extent to which new growth remains free of infestation
  • Organic pesticides, based on plant oils or extracts (e.g. Vitax Organic Pest & Disease Control Concentrate) or fatty acids (e.g. Solabiol Bug free, Doff Greenfly and Blackfly Killer) or based on pyrethrins (e.g. Bug Clear for Fruit & Veg, Defenders Bug Killer, ecofective Bug Killer (also contains fatty acids)) can be used. These are contact in action, have a short persistence and thorough application to the underside of leaves is required. More frequent applications of organic pesticides may be required to deal with the scale nymphs as they hatch
  • Contact synthetic sprays containing deltamethrin (e.g. Sprayday Greenfly Killer),  lambda-cyhalothrin (e.g. Westland Resolva Bug Killer) or cypermethrin (e.g. Py Bug Killer) can give control if applied thoroughly to the underside of leaves
  • The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is absorbed into the foliage and taken in by the young scales as they feed
  • 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
  • Do not spray on or near plants in flower due to the danger to pollinating insects

Download

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

Biology

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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