|I have an apple tree (about 6 yrs old) with a stem of about 3|
|Without seeing the trees, I can only offer educated experience: what you describe sounds like borers are attacking your fruit trees. These beetle and moth larvae chew (bore) their way into trunks, crowns, and stems of many plants. Borers holes provide entry to infectious diseases, leading to rot that can kill plants. They are difficult to control with sprays, since spraying must be timed before the pest actually enters the plant.
After leaves fall in autumn, spray horticultural oil as directed on the product label. In spring, make frequent checks of tree trunks near the soil line for evidence of borers. If you find a hole, probe inside with a stiff wire to kill the caterpillar. I would also encourage parasitic wasps to visit your garden by providing lots of flowers in bloom throughout the growing season.
You might also check with your local cooperative extension office to see what other control measures are available in California. University of California Cooperative Extension; 1553 Berger Dr. ; San Jose, CA 95112; (408) 282-3104
Best wishes with your trees!