The newest research (2007 into 2008) shows that nylon footies are effective. You wrap these around the apple when the apple is the size of a nickel (late May / early June). You don't need to tie the footy on and they are not messy like clay or tanglefoot. To order footies, email email@example.com. You might also find them at Raintree Nursery.
First, thin your apples so that only the biggest apple in each cluster remains. Next, put a footy on each apple left on the tree. Place the footy on so that the apple sits in the middle. Give the open end a little twist so that it stays in place. As the apple grows, the footy expands with it. Good luck!
The apple maggot is the larva of a fruit fly. The fruit fly lands on the apple beginning in early June and lays her eggs. The eggs hatch into maggots which tunnel vigorously throughout the apple causing it to rot. This is a bad pest because the entire apple goes bad. With other pests, such as codling moth, you may see half a worm smiling back at you (where's the other half?), but most the apple is good.
The outside of the apple will either look normal and yummy, or you may see a "railroading" pattern on the apple. The apples often drop from the tree before they are ripe.
The inside of the apple has rotted thanks to the apple maggot. You may not see the culprit in the apple, but you will certainly see the damage.
The whole apple is bad and unusable. Discard the apple and don't put it in your compost pile. On the other hand ... if you catch it quickly with just a few marks inside, you could cook with it, especially to make the long anticipated apple pie.