Posted: 22/06/2015 at 13:01
It's not really a good idea to go spraying insecticide around if you don't know what the problem is. You may well find you've killed beneficial insects which prey on garden pests.
I can't see any caterpillars on your photo - can you post a closeup - click on the green tree icon on the toolbar above where you type your post, and follow the instructions. You may get an error message but ignore it. We will then be able to zoom in on the photo and get a better look.
Apple trees are usually host to all sorts of insects, most beneficial or at least harmless - they may damage a few leaves, but this usually does no harm to the tree.