When my customer first saw these holes, they thought that a borer insect was invading their tree. This is actually damage done by a sapsucker (woodpecker). The sapsucker really did some damage here. Our natural trees can usually overcome damage left buy these birds. Yet our ornamentals such as our hollies, or our small trees can incur severe stress and disease. Damage to the vascular system of the tree can also cause a reduction in uptake of nutrients to the plant. The best way to control sapsuckers is to apply a repellent to the trunks of your trees. I have used a mixture of petroleum jelly and cayenne pepper together, and applied on my trees. It seems to have done the job.
