Horse chestnut scale insect

From April to July, tree trunks and branches in our street become mottled with Horse Chestnut scale insects. The bright white, powdery substance under the 'scale' is the ovisac, loaded with thousands of eggs. The scale is the dead body of the female, which has invested her effort into the production of the ovisac, after months of feeding on tree sap, and whose function is now protect the ovisac. The Horse Chestnut scale insect, Pulvinaria regalis, is an invasive species from Asia found in the early 60s and described from specimens in Paris in 1968. It is a generalist species, found mainly in urban environments and feeds on a diverse array of deciduous trees, and has a liking for Sycamore and Maples, Lime and Bay leaf tree.
  Scale insects do not look much like insects, do they? What we called scale insects are adult females, with flattened shiny bodies hiding tiny legs underneath, and piercing mouth parts like a shieldbug (scale insctsct are homopterans). Pulvinaria regalis has some mobility before egg laying. Both the first nymph instar (called crawlers) and adult males look more like a typical insect.
 Horse Chestnut scale insects hatch in summer and move onto the underside of leaves, where they feed.  Then, before leaf fall, they move back to the tree trunks and branches. In spring, mature males and females appear. Mature males are winged but do not feed, so they spend their short lives looking for females and mating. Females crawl to leaves, mate and start feeding on the tree sap and when fully mature and ready to lay, they move onto tree trunks to lay their ovisac. Although they can appear in such large numbers, they do not obviously cause much harm to mature trees, although saplings or weak trees may suffer from loss of sap. Scale insects, in particular eggs and nymphs, are the main food source for some ladybird species such as Exochomus quadripustulatus, the Pine Ladybird, no wonder they are also common on our street.

More information
GB non native fact sheets