Pacific Flatheaded Borer

Adult pacific flatheaded borers are generally present in May and June. When spring months are warm, they may be seen in late 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. They attack aboveground portions of the tree that have been previously injured. Beetles lay eggs on the injured area, and larvae excavate large caverns just beneath the bark and bore tunnels deep into the heartwood of the tree. Excavations are usually filled with tightly packed, 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 to die, or girdle and kill young trees.

Flatheaded borers often invade sunburned areas on the trunk of newly planted trees. At planting time protect the trunks of trees from sunburn by painting them with a mixture of white latex paint and water, or use tree wraps. Wrap or paint the tree trunk from 2 feet above to 1 inch below the soil line 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.