Very little as I understand it. The chap who owns the airfield I'm based on gave it up in the late 80's / early 90's. The only reason for doing it was that a conventional tractor and spray trailer could destroy up to 10% of the crop that it was trying to protect. With the advent of very narrow wheeled specialised spraying machines the loss rate came down to a much more acceptable figure and expensive aerial crop spraying was doomed in the UK.