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. In the summer of 2001 I was finally desperate enough to spray my apple trees in a desperate attempt to see if I can get fruit that is somewhat free of worms. Most of my apricots and pears are worm free. Spraying should be done in the morning before insects are active. These sort of chemicals work by putting a layer, or get absorbed into the skin, of poison on the fruit.