Out back I have a small dwarf (not necessarily redundant -- it's a SMALL dwarf) 3-in-1 (used to be 4-in-1) apple tree that I am bringing back from several years of neglect. While the leaves on the Jonagold out front (maybe 40 feet away) are fine, the leaves of the 3-in-1 are now munched down to lace due to the action of some pest. The area of leaves from nearby maples which is closest and above the 3-in-1 are also lace from some pest. I presume the same pest. Other leaves on the nearby trees are fine. In other words, it's a very localized phenomenon: on the little apple and up onto one side of the foliage of the nearby maple.
When I inspect the tree, I see no pest on the 3-in-1 -- no insect, no snails, no slugs. Obviously there is plenty of time when I am not inspecting the tree.
My question is twofold:
1. a) How do I determine what is eating the apple tree leaves? b) Does precisely what pest is doing it affect what I should do to remedy the situation?
2. As an organic gardener, what can I do to repel the pest and/or help the tree?
I am an experienced urban organic gardener in some ways but naive in matters I haven't done much of yet. I am very new to fruit trees.
Priscilla in a neighborhood of Boston, MA Zone 6 (though these days it sometimes feels more like zone 7!)