This insect usually enters the fruit at the stem end or along the suture of the fruit. It feeds just under the skin. Usually you will see some “frass” or brown excrement at the entry point.
In Nevada our experience has been that we don’t usually start catching this insect in our monitoring traps until around May. If you have had wormy fruit in the past, later in the season, it would be important to apply either Bt (Dipel® or Thuricide®) or spinosad) now to the entire tree or trees, paying particular attention to covering the fruit with the spray.
I add a surfactant (an additive that helps cover the fruit better) to the spray when mixing it in the sprayer. These organic pesticides will set up a protective barrier to this insect and keep it from getting inside the fruit. Follow label directions. Repeat applications as the label directs.
A good website on peach twig borer can be found here: