Pacific flatheaded borer—Chrysobothris mali
At least 69 Chrysobothris species occur in California. Of these Pacific flatheaded borer is the most common. Adults of Chrysobothris flatheaded borers (family Buprestidae) are attracted to lay eggs on diseased or stressed trees and shrubs and wounds on woody limbs and trunks. Pacific flatheaded borer can infest dozens of broadleaved, fruit and nut trees and woody ornamentals.
Damaged bark as described below is commonly the first indication of an infestation of Pacific flatheaded borer or similar species, such as flatheaded appletree borer, Chrysobothris femorata. The adults are easily overlooked because they can blend with the color of bark. Except for the tiny eggs the other life stages (larvae and pupae) occur hidden under bark.
The adult is bullet shaped, hard bodied, and 1/2 to 3/4 inch long. It has a coppery, dark bronze, or gray body and wing covers with pale mottling. The circular, disklike egg is laid on bark and is white and about 1/25 inch in diameter.
The pale yellow to whitish larva is legless. It has brown mouthparts and distinct segments and tapers toward the rear. The segment immediately behind the head (prothorax) is greatly enlarged. At maturity larvae are 3/5 to 3/4 inch long. The elongate, oval pupa is the length of adults and initially translucent white, then creamy white, then gradually takes on the color of adults.
Mated females lay eggs in bark crevices and wounds, such as injury caused by pruning cuts, trunk stakes and ties, and sunburn or where the rootstock and scion are grafted together. Each adult female can lay about 100 eggs singly on the tree. The emerging larvae chew into bark and feed and tunnel in cambium. As larvae grow they may bore deeper into wood.
Mature larvae (prepupae) overwinter under bark then pupate in spring. In mild-winter locations the beetle may be active throughout the year. Adults mostly emerge and fly from April through August, especially during June and July. There commonly is 1 generation per year. But in cooler locations more than 1 year can be required to complete 1 generation.
Pacific flatheaded borer can infest at least 70 species of trees in 21 plant families. Wet spots and cracking on bark are usually the first damage symptoms observed. Bark then becomes increasingly roughened in areas where larvae are feeding underneath. Damaged bark commonly occurs in a winding pattern that corresponds to the larval tunneling. Granular frass (excrement) may become visible on bark, but most frass remains hidden packed in larval tunnels. After pupating the emerging adult leaves a round to D-shaped hole about 1/10 inch in diameter in bark and wood.
Adults also chew twigs and the base of leaf petioles, but this injury is usually innocuous and easily overlooked. However, when adults are especially abundant their feeding can partially defoliate young trees.
Larvae are preyed upon by larvae of predatory beetles and at least four species of parasitic wasps. In California the wasp Trigonura californica (family Chalcididae) is the most common parasite of Pacific flatheaded borer. Adults of this wasp are about 5/16 inch long and black with a covering of fine hairs that can give them a silvery appearance. The harvest (grain, or itch) mite, Pediculoides ventricosus, also commonly preys on the borer larvae. Woodpeckers commonly feed on the larvae, but their flaking off of bark to reach the prey increases injury to the tree.
Prevention is the most effective method of managing flatheaded borers. Plant only species well adapted for the conditions at that location. Properly prepare the site, plant correctly, and protect roots and trunks from injuries.
Whitewash trunks of newly planted and young trees by applying indoor, white latex paint diluted with an equal amount of water. Or wrap trunks with heavy paper to help prevent sunburned bark that attracts the egg laying females. If trunks are wrapped, periodically remove wrapping and inspect bark for injuries. Learn the cultural requirements of the plants present and provide proper care to keep them growing vigorously. Appropriate irrigation is particularly important as drought stress increases tree susceptibility to bark- and wood-boring insects.
Prune out and dispose of borers restricted to a few limbs. Remove and dispose of dying trees to prevent boring insects from emerging and attacking other nearby trees. Do not pile unseasoned, freshly cut wood near woody plants as borers can continue to develop in cut wood and emerge to lay eggs on nearby hosts.
Solarize recently cut wood to kill beetles infesting it. Seal wood beneath heavy clear plastic tarps in a sunny location for several months through the warm season; after this time, any emerged borers will have died and the dry wood will no longer be suitable for borers. Tightly seal the tarp edges (e.g., with soil) to prevent any insects from escaping. Instead of one large pile, use several smaller wood piles to promote quicker drying.
Larvae can sometimes be killed by probing tunnels with a sharp wire. This method is practical only in a small infestation, and it is often difficult to know whether the wire has reached and killed the larva. Avoid further damaging bark when probing trunks.
Especially valuable trees may be protected from further attack by an early May application of carbaryl or a pyrethroid to kill the egg laying adults. Products available to homeowners are not effective and a professional applicator must be hired to make the application. Consult Pest Notes: Hiring a Pest Control Company for guidance when selecting a company for hire.
Unless trees are monitored regularly so that borer activity can be detected early, pesticide use is likely to be too late and ineffective. Do not substitute insecticide applications for proper cultural care. Most borers are attracted to trees that are already unhealthy from other causes and relying only on insecticide can allow trees to die from those other causes unless the growing environment and cultural practices are improved.
For more information see Guide to Insect Borers in North American Broadleaf Trees and Shrubs and The Pacific Flatheaded Borer (PDF) from the U.S. Department of Agriculture.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).