How can I control borers in apple trees?

Answer: There are two species of flatheaded borers that may invade apple trees. Chrysobothris femorata is the species endemic to the East. On the Pacific coast, C. mali fills a similar niche. Adults emerge from woodland trees in late April through early May and begin laying eggs beneath bark scales on the tree. The graft union is a favorite place for egg deposition.
Maintaining trees in good vigor is important first-line protection from flatheaded borers since a tree in good vigor will be able to drown an invading larva with sap. Drought-stressed trees are much more susceptible to borers; therefore, adequate water is essential.For all species of borers, the larvae can be removed from the trunk with a jackknife or piece of wire. Look for signs of borer damage, such as frass mixed with sawdust at the base of the tree and at the pest’s entry hole. Because the roundheaded borer may burrow deep into roots, it is important to check routinely (at least twice during the growing season (e.g., once in May and again in September) for borers, or they can extend beyond the range of manual removal.
Perhaps the best non-chemical protection from all species of borers is to wrap the bottom 12 to 18 inches of the trunk in window screen (metal, fiberglass, or nylon are all effective). Secure the top with a twist-tie, being certain to loosen and retie it at least once a year. The bottom should be snug against the ground or also secured with a twist-tie.
NCAT Horticulture Specialist Guy Ames recently wrote a blog post on this very topic, which includes detailed photos.
In addition, the following ATTRA publications will prove useful for further study:Apples: Organic Production GuideApple Insect Pests Identification SheetApple Diseases Identification Sheet