Pacific Flatheaded Borer

Pacific flatheaded borer adults are generally present in May and June. When spring months are warm, they may be seen as early as March or early April. The adult beetle is about 0.4 inch long with a dark bronze body and coppery spots on the wing covers. A fully grown larva is light-colored, with a prominent, flat enlargement of the body just behind the head. There is one generation each year.

Pacific flatheaded borers are attracted to diseased or injured limbs, such as those affected by sunburn, scale insects, bacterial canker, or major pruning cuts. The beetles lay eggs in the injured area. Eggs hatch and the larvae excavate large caverns just beneath the bark and bore tunnels deep into the heartwood of the tree. Excavations are usually filled with finely powdered sawdust. Injury by this borer will cause the sap to flow, and the affected area will appear as a wet spot on the bark. Later, these areas may crack and expose the mines. Feeding by Pacific flatheaded borers may cause a portion of the bark on older trees to die, or it may girdle and kill young trees. This borer can be particularly damaging to new grafts in established orchards.

Flatheaded borers often invade sunburned areas on the trunk of newly planted trees. At planting time, wrap or paint the tree trunk above and 1 inch below the soil line with white, water-based paint or whitewash to protect the trunk from sunburn and flatheaded borer invasions. In older trees the best way to avoid infestations is to keep the trees sound and vigorous. Prune out all badly infested wood and burn or remove it from the orchard before the growing season starts. Spraying for this insect is not recommended.