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Flathead Borer


Flatheaded wood borer larva.

Exit holes of adult flatheaded wood borer.

Adult emerald ash borer, Agrilus planipennis.

Pacific Flatheaded Borer

Scientific name: Chrysobothris mali


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. They can also attack freshly cut timber before it is dried. Larvae that tunnel into sapwood and heartwood can frequently damage logs and wood products. 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. Adults aid in wood decomposition by introducing yeasts, bacteria, and wood-rotting fungi that lead to tree rot and checking in the wood. In some instances, these processes occur within a couple of years. However, older limbs can be weakened to such an extent that other borers such as the shothole borer can attack successfully. The most obvious sign of a flatheaded wood borer attack is the wide, meandering galleries under the bark with tightly packed, fine boring dust. Holes that penetrate into the wood are most likely due to wood borer larvae. Emerging adults leave oval, cleanly cut exit holes


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 watersoaked 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.

Melanophila spp.Several species of wood borers are attracted to fire. In fact, species of the genus Melanophila possess specific pit-sensing organs that detect infrared radiation produced by forest fires. As a result, these beetles are often seen by firefighters laying eggs on recently burned trees. Metallic wood borers can be responsible for biting firefighters. Melanophila spp. have been known to build up their numbers in fire-damaged hosts and emerge to attack adjacent, otherwise healthy, trees. Such "outbreaks" are generally short-lived.


Emerald Ash BorerThe emerald ash borer, Agrilus planipennis Fairmaire, is an exotic flatheaded borer that kills live ash trees but is not currently found in the Rocky Mountain Region (fig. 5). It was accidentally introduced into the Lake States and is responsible for killing millions of ash trees. The beetle can be found in live and recently dead ash trees. There is great potential for it to spread to other states by moving beetle-infested ash (e.g., moving infested firewood and nursery stock). Several native species resemble the emerald ash borer, and identification should be confirmed by a specialist.






Adult emerald ash borer, Agrilus planipennis.
Common name    
(trade name) Amount to Use

A. WHITE INTERIOR LATEX PAINT# 50% paint and water mixture

COMMENTS: Paint trees at time of planting. Be sure paint extends below ground level. This treatment will prevent sunburn, which can reduce attack by flatheaded borer. Insecticide can be mixed into the paint for further protection