Hello coalhouse, am i right in saying the leaf is Beech, if so then it will be the Beech wooly aphid, if you like you can spray it but one of the beech hedges on a job i do has this every year and does not seem to be a problem, i trim the beech in mid june time, and it takes most of the wooly aphid off, the new growth for the remainder of the season does not suffer that badly, the overall health of the hedge always looks ok, thats assuming its Beech, if not then i still beive it to be some type of wooly aphid, julien.

thanks julien didn't know what tree it was ,as planted loads of seeds and nuts found on an autumn walk.