I have had problems with aphids, pear slugs as well as catipilars. It also works on catipilars but not as well. Soap is a contact poison for many insects and it leaves no residue. Do NOT spray the trees in hot weather. Pick a cool time and rinse the trees after.
Pear slugs are a translucent to dark slug that eats the leaves of trees.
A solution of vegatable oil with a bit of soap makes an insecticidal soap spray. Aphids seem to primarily go after my plum trees or apple trees and they prefer new leaves over all else. Lime sulphur sprays must be put on before the leaves open. Pesticides are a severe health hazard causing both mental and physical retardation in children. Spray this after the flower pedals have dropped or else you will kill insects which pollinate the trees.