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.