Pacific Flathead Borer

April is the time to start checking apple, apricot, avocado, cherry, peach, pear, plum and prune trees, and blueberries, for signs of the dreaded Pacific flathead borer.

Flathead borers

​Like other borers, these pests chew tunnels in wood, weakening a tree’s structure, and robbing it of important nutrients found in the inner cambium layer. Newly planted trees and trees weakened by drought, water-stress, scale insects, carpenterworms, or diseases, such as Phytophthora or Armillaria, are particularly susceptible. These weakened areas are then more likely to be attacked by other pests, such as shot hole borers. 

Pacific flathead borer feeding can also girdle a young tree, killing it. The only symptom you may see is a dark colored depression in the bark, or tiny cracks where you might see frass (bug poop), usually on the side receiving the most sunlight.

Pacific flathead borer identification

Pacific flathead borers (Chrysobothris mali) are flattened, wedge-shaped, dark bronze beetles that can be 0.5 to 0.75” long. You may see copper-colored spots on the wing covers.

Eggs are very tiny, only 0.04” in diameter, flattened, oval, and white. Larvae grow to 0.75” in length and are white, with an amber colored head. Larvae are flattened, with a widened area just behind the head, tapering towards the rear end. Pupae are also whitish and flattened, getting darker as they mature.

Pacific flathead borer lifecycle

These pests overwinter in a prepupal stage. As temperatures begin to rise, they pupate. From April through July, adult beetles emerge, usually beginning around the same time apple trees are blooming. Then females mate and begin laying eggs in the bark, favoring areas weakened by sunburn or mechanical injury from tree supports, weedwackers, and out of control lawn mowers. When the eggs hatch, larvae burrow directly into the bark and begin feeding on the sugary cambium layer, robbing your trees of the nutrients they need to grow and produce fruit. As the larvae mature, they will either create a pupal chamber in the xylem, or burrow under the bark, where they will stay until the following spring.

Flathead borers are easy to miss

Pacific flathead borer controls

Healthy trees are better able to resist and recover from Pacific flathead borer attack. This means selecting trees suitable to your microclimate, planting them at the proper depth, and feeding, irrigating, training, and pruning them properly.

Since eggs are laid in weakened bark, protect trees from mechanical injury, and be sure to whitewash exposed bark before sun damage can occur.

Birds, especially woodpeckers, will find and remove Pacific flathead borers, and carpenter ants eat both larvae and pupae. Insecticides are commonly used in commercial orchards to kill new larvae, but once the larvae are inside the tree, there is nothing you can do besides pruning out infested wood and burning it.

Now you know.