I have a dwarf Jonagold apple tree by the front of my house which is
doing fine. I had a lot of apples off it last year.

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.

Thank you!

Priscilla
in a neighborhood of Boston, MA Zone 6 (though these days it sometimes
feels more like zone 7!)