Q What is horse chestnut scale?

A Scales are sap-sucking insects covered in a tough shell-like covering. Horse chestnut scale (Pulvinaria regalis) is a conspicuous scale that feeds on a range of trees and shrubs.

Caption: Scales are unsightly but the tree seems to cope with them

Q How do I recognise horse chestnut scale?

A The most obvious feature is the white egg masses scattered, sometimes in large numbers, on the bark of trees and shrubs. The mature scales are dark brown, circular, flattened and about 4mm across.

Q What plants does horse chestnut scale attack?

A This species of scale was first found in Britain in 1964 on horse chestnut, and is believed to have come from Japan. It has since been found feeding on a wide range of trees and shrubs including bay, dogwood, lime, magnolia, maple, skimmia and sycamore. Street trees can play host to big colonies as the warmer microclimate of the city favours their growth.

Q What damage does horse chestnut scale do?

A Though unsightly, the scales do not appear to have any effect on the overall health of infested trees. They don’t produce honeydew, with its attendant sooty mould, so are far less of a nuisance than some other scale insects.

Q Could I mistake horse chestnut scale for any thing else?

A There are many other scale insects that attack some of the same plants. However, these do not usually leave uncovered egg masses, and most of them produce honeydew.

Q What is the life cycle of horse chestnut scale?

A The eggs are laid in May or June and protected by a mass of white, waxy strands. The females then die and usually drop off the tree leaving the egg masses. These hatch in June or July and the young nymphs, known as crawlers, move onto the leaves to feed until late summer. They then return to the bark and develop a scale covering ready to overwinter. These immature scales are pale brown and oval.

Q How do I control a horse chestnut scale attack?

A The pests appear to do little damage, but can be scrubbed off with soapy water if unsightly.