When the tree is healthy, it pushes the borer out, or kills it by drowning it in sap. If the tree is struggling and weak, its ability to withstand an attack is decreased so it slowly succumbs to the insects' pressure. Massive assaults on a healthy tree can also be fatal. Trees that have been invaded by borers have small exit holes in the bark where the insects left the tree, he says. These often have sawdust and sap associated with them.
Some borers only attack small limbs and branches, while others go right for the trunk.
Once a tree is invaded by borers, the pesky critters are hard to eradicate, he explains. The best defense is to keep trees as healthy as possible. Water them deeply and infrequently, while making sure water does not sit around the base or trunk of the tree. Also, keep all lawn mowers and weed eaters away from the trunks of trees. If they are growing in the lawn, remove all the grass around their base in as large a circle as feasible.