Pacific Flatheaded Borer

Pacific flatheaded borer adults are generally present in May and June and are occasionally found in pheromone traps used to monitor other pests. When spring months are warm, adult beetles may be seen as early as March or early April. Adult beetles are about 0.4 inch long with a dark bronze body and coppery spots on the wing covers. Beetles lay eggs in injured or weakened areas on the tree and larvae bore into the wood. A full-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, mechanical injuries, or major pruning cuts. Larvae excavate large caverns just beneath the bark and bore tunnels deep into the tree's cambium tissues. Excavations are usually filled with finely powdered sawdust. Injury by this borer will cause 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 to die, or may girdle and kill young trees or scaffold limbs.

Flatheaded borers often invade sunburned areas on the trunk of newly planted first-year trees. Wrap or paint the tree trunk from 2 feet above to 1 inch below the soil line with white, interior, water-based paint or whitewash to protect the trunk from sunburn. One treatment may not be sufficient, especially on the side of the tree trunk exposed to the sun. In older trees the best way to avoid infestations is to keep trees sound and vigorous. Prune out all badly infested wood, and shred it or haul it to the dump before the growing season starts. Protect sunburned limbs with white latex paint. Insecticide treatments are not recommended for this insect.