Pacific Flatheaded Borer

The Pacific flatheaded borer overwinters in the prepupal stage. Pupation occurs in spring, and beetles begin to emerge in April. Adult beetles are flattened and vary in length from 0.5 to 0.75 inch. They are dark bronze with coppery spots on the wing covers, giving a coppery sheen to the elytra. The head and thorax are almost as wide as the abdomen, forming a wedge shape. Adult emergence continues until July, which accounts for the various- sized larvae found during summer. After emergence, female beetles mate and lay eggs on the bark, usually in sites exposed to the sun or weakened limbs. Eggs are circular, flattened, whitish, and about 0.04 inch in diameter. Upon hatching, larvae bore through the bottom of the egg directly into the bark. Almost all of the larval period is spent feeding in the cambium layer of the bark. When the larva reaches maturity, it either bores into the xylem and constructs an oval pupal chamber or pupates just under the bark where it stays in the prepupal stage until the following spring. Mature larvae are about 0.75 inch long and whitish in color. The body is flattened, and the head is amber colored. The larvae have a broad flat area behind the head, and the body tapers toward the posterior. The typical beetlelike pupa is also quite flat. It is creamy white when first formed but darkens as the time approaches for emergence to an adult.

The Pacific flatheaded borer prefers weak or injured trees. Newly planted or grafted trees are particularly susceptible to attack because of the stress caused by planting and the possibility of sunburn on the bark and at the bud union. The larvae feed in the cambium and can completely girdle and kill young or newly grafted trees in a short period of time. Limbs of older trees (particularly if sunburned) are also attacked, but they rarely die from flatheaded borer attack. However, older limbs can be weakened to such an extent that other borers such as the shothole borer can attack successfully.

Maintaining healthy trees and preventing sunburn are the keys to preventing damage by Pacific flatheaded borer. Painting the trees with white wash or a 50:50 mixture of white interior latex paint and water will help prevent sunburn and possibly inhibit egg laying. Avoid pruning during summer, and prune trees so that scaffolds are shaded to prevent sunburn. Remove horizontal scaffolds when pruning/thinning young trees.

Protect newly planted or newly grafted trees from sunburn by painting the trunk and graft with white interior latex paint or using tree wrappers around the trunk. If paint is used, be sure to mix it with water; undiluted latex paint can kill young trees. Thin the latex paint to a mixture of one-half water and one-half latex paint and paint the trunk from 2 inches below ground level to 2 feet above.

In older orchards, monitor for flatheaded borers by looking for water-soaked areas on the bark. As injury progresses, the bark will split, exposing the frass-filled feeding galleries. Paint the exposed upper sides of scaffolds particularly on the north and east side of older trees to prevent sunburn and subsequent flatheaded borer attack.