Horse chestnut scale is a sap-sucking, limpet-like true bug, it feeds on a wide range of trees and shrubs. It is one of many types of scale insects encountered by gardeners.

Scale insects are sap sucking true bugs belonging to several families in the Hemiptera. Typically the adults are immobile having a flattened or raised appearance, with no visible legs. They often look like a ‘scale’ on a leaf or stem, many species produce a white wax often covering egg masses. There are more than 100 species found in Britain, 26 of which have been introduced. More than 25 species can be found in gardens or on houseplants.
 

How to spot horse chestnut scale;

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

• Some may consider large numbers of egg masses to be unsightly but plants cope with large populations and control measures are not necessary

• On small plants, such as Japanese maples and bay trees, it may be feasible to scrape off the scales and egg masses when seen

• Use of pesticide against this insect is not necessary as it has little impact on host plant vigour

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.