We believe that you are having an infestation of Codling moths. You may not be able to see them because it is the moth's larval stage that does the damage, and they are terribly small. The moth lays its eggs on the tree or the fruit, again too small to see well. Your apples are probably falling from the tree because they are ripening.

We are sending you a fact sheet that will tell you how to treat the problem, along with links to other relative information. We suggest that you begin to treat immediately.

http://extension.colostate.edu/topic-areas/yard-garden/backyard-orchard-apples-and-pears-2-800/

Please let us know if there is anything else that we can do for you.