Apple tree borers tunnel into the trunks of apple trees and eat their wood. The threat they pose to the tree largely depends on the type of borer. Dogwood and apple bark borers are the larvae of moths. These borers cause no immediate threat to the tree, but a severe infestation can kill a tree if left untreated for years. Beetle-larvae borers like the round-headed and flat-headed apple tree borer, however, feed voraciously on the live wood of an apple tree. Even one feeding larvae can kill a young tree.
Identify the culprits. Borers can be identified by the small holes they leave in trees. The larvae are hard to spot because they spend their lives inside the apple tree's wood. When actively feeding, however, they push tell-tale sawdust castings out of the hole.
Physically remove the borers in late summer when borers are most active. Find active borer holes. These are marked by the sawdust castings found at their interest. Stick a length of wire (a straightened wire coat hanger works well) into the hole until you impale the single larva inside. Pull the larva out if possible.
Discourage further infestation. Paint the exterior of the trunk with white latex paint each May to deter female borer moths from laying eggs and suffocate any borers currently feeding on the wood. Remove weeds from around the trees. This will encourage woodpeckers and nuthatches to land on the tree and eat the borer larva.