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 is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

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, 5-6 mm 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
• Encourage predators in the garden, some ladybirds, parasitoid wasps and some birds will eat scale insects. Larvae of the pine ladybird Exochomus quadripustulatus are known to be natural enemies of horse chestnut scale
• On small plants, such as Japanese maples and bay trees, it may be feasible to scrape off the scales and egg masses when seen

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 1 mm 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.