I agree with the other two posts. We have raccoons that will come in our yard and strip our fruit trees bare in one night-- one day I am super close to harvesting, the next day, gone. My only clue will be a core on the ground. This has happened to my apples, pears, and plums a few years running. But we also have a darling little rat (!) that will eat an apple on the tree while it is still attached. Just chew away until it is gone.

Once something has taken a nibble out of the fruit, weather it is a rat or mouse, a caterpillar, or raccoon, that is when other bugs will get in. Rolly-pollys (pill bugs) and pincher bugs are both species that I have seen falling out of a bite.

Another preventative measure would be to add "tangle foot" to the base of the tree. You can pick it up at your local hardware store in the garden department. You wrap a sort of paper around the base of the tree, and cover it with this super sticky gunk that rodents get stuck on and can't proceed up the tree to your fruit. If you do that, make sure that you have no tree branches hitting a fence or another neighboring tree, or else the critters will be able to get on to the tree that way, avoiding the tangle foot altogether.