What about peach trees?

The worms are coming from eggs being laid in the fruit. You need to spray to keep the adults off. I use Bonide Orchard Spray, with is an organic choice for both insect control and fungicide. Generally, you start spraying at petal fall (don't spray when it's in bloom - you'll interfere with pollination), then spray about every 7-10 days for the next 2-3 months.