Hello:

That is an aphid, called woolly apple aphid. They form a protective waxy covering on their bodies, and as you know, aphids feed in large groups, so the wzxy coverings blend together to look like white fuzz, or sheep's wool. The coverings make them pretty impermeable to most insecticides.

As this time of year, you could mix up a batch of insecticidal soap and 2% horticultural oil in water (both can be purchased at a garden center or box store). Use a hand spray bottle to spray each colony on the twigs to the point of dripping.

These aphids have actually been in your tree all summer. They start becoming active around mid to late June. So next year, you should watch the tree limbs for the cottony aphids. They will first show up in crack and crevices, pruning cuts, and any other hiding places where they have spent the winter.

Here is some more information (starting on page 5): http://extension.usu.edu/files/publications/factsheet/aphids-apple.pdf.