Pear and Cherry Slug are the larvae of a Sawfly, who chooses the foliage of the target trees to lay eggs.
Once these eggs hatch, the slimy, glossy black larvae are visible on the leaves, and their damage can be significant – foliage can be skeletonised very quickly by these voracious pests.
Slugs eat the leaves, and cause leaves to dry up and turn brown
You can treat initially by picking or squashing or hosing off.
If there are too many, then dust with wood ash or chalk, flour or powdered clay (make sure you stand up-wind)
All of these desiccate the skin surface so that the slugs shrivel and die.
You can also use lime powder or lime dissolved in water.