At least 330 species of flatheaded borers (family Buprestidae) occur in California. They are also called flatheaded wood borers or metallic wood borers.
Adults are flattened and elliptical or oval. Coloration varies with the species. Some are mostly gray or otherwise dark or dull colored while others are metallic blue, coppery, or green. Mature adults of most species are about 1/4 to 3/4 inch long, varying by species and larval nutrition. Adults are most active on sunny days when they may be observed running rapidly over the bark, stopping to lay eggs, or chewing on foliage or twigs of their host plants. The oval eggs are laid on bark, commonly in crevices or where bark has been wounded.
Flatheaded borer larvae are distinctly segmented, legless, and mostly whitish to yellow. They have a tiny head and brown mouthparts. At maturity, larvae can be about twice the length of adults of the species. Larvae occur under bark and are the source of the name flatheaded; the first segment behind the head and sometimes also the second and third thoracic segments are greatly enlarged (swollen) compared with the rest of the body. This characteristic readily distinguishes larvae of Buprestidae from all other types of wood-boring species. Other wood-boring insects include bark beetles (subfamily Scolytinae), longhorned borers (longhorned beetles or roundheaded wood borers, family Cerambycidae), carpenterworms (Cossidae), clearwing moths (Sesiidae), various other moths, wood wasps (horntails, Siricidae), and less commonly other types of insects.
The oblong pupae of flatheaded borers occur under bark. They are initially whitish or yellow but as they mature develop the appendages and coloration of adults. After the adult emerges, a D-shaped, elliptical, oval, or round hole is left in the bark.
Several common species are discussed and pictured at the links below:
- Bronze birch borer, Agrilus anxius, infests only birch.
- Flatheaded alder borer, Agrilus burkei, attacks only alders, especially white alder (Alnus rhombifolia) that is drought stressed.
- Flatheaded appletree borer, Chrysobothris femorata, prefers apple, maple, and poplar, but also attacks fruit and nut trees and various other broadleaved trees.
- Goldspotted oak borer, Agrilus auroguttatus, infests only oaks and in California mainly California black oak, coast live oak, and interior live oak.
- Oak twig girdler, Agrilus angelicus, attacks oaks. Hosts include California black oak, canyon live oak, Engelmann oak, interior live oak, and valley oak.
- Pacific flatheaded borer, Chrysobothris mali, infests at least 70 species of broadleaved trees.
- Western cedar borer (PDF), Trachykele blondeli, is a metallic coppery, green, and yellowish species that infests Monterey cypress, Sargent cypress, western juniper, and western red cedar.
Flatheaded borers develop through four life stages: egg, larva, pupa, and adult. Mated females lay eggs in bark crevices and wounds, such as injury caused by pruning cuts, trunk stakes and ties, sunburn, or the graft where the rootstock and scion join. The emerging larvae develop through four increasingly larger instars as they chew and feed in cambial tissue, wood, or both.
Mature larvae (prepupae) overwinter under bark then pupate in spring or early summer and emerge as adults. In mild-winter locations the beetles may be active throughout the year. Most species have only 1 generation per year. In cooler locations more than 1 year can be required to complete 1 generation.
The important damage is caused by larval feeding. Some species mine the cambium of branches or roots or trunks, bore extensively in the wood, or both. Larval galleries are flattened to oval in cross section, winding, and always packed tightly with frass (excrement). Some species make spiral galleries around small stems.
Wet spots and cracking on bark are usually the first damage symptoms observed. Bark then becomes increasingly roughened as larvae feed underneath. Damaged bark commonly occurs in a winding pattern that corresponds to the larval tunneling. Granular frass may become visible on bark, but most frass remains hidden in larval tunnels. After pupating the emerging adults leave holes in bark.
Adults chew the base of leaf petioles, twigs, and sometimes leaves, but this injury is usually innocuous and easily overlooked. When adults are especially abundant their feeding may partially defoliate young trees.
Adults of most species are attracted to lay eggs on dying, injured, stressed, weakened, or recently dead trees. Drought stress commonly results in trees being attacked by wood borers. A few species, such as goldspotted oak borer, attack apparently healthy trees.
Larvae are preyed upon by larvae of various predatory beetles and parasitoid (parasitic) wasps. The harvest (grain, or itch) mite, Pediculoides ventricosus, also commonly preys on the larvae. Woodpeckers feed on the larvae, but their flaking off of bark to reach the prey can increase 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 heavily pruned, newly planted, and young trees by applying white, indoor latex paint diluted with an equal amount of water. Alternatively, wrap trunks with heavy paper to help prevent sunburned bark that can attract the egg laying female borers. 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 because 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 nearby hosts. Avoid pruning from spring through summer when adults are active. 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 most borer species. Tightly seal the tarp edges (e.g., with soil) to prevent any insects from escaping. Instead of one large pile, create several smaller wood piles to promote quicker drying.
Larvae can sometimes be killed by probing tunnels with a sharp, stiff wire. This method is practical only in a small infestation on small trees. 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 a spring application of carbaryl or a pyrethroid to kill the egg laying adults. Effective products are available for use only by professional pesticide applicators and a professional must be hired to make the application. To get the services you want consult Pest Notes: Hiring a Pest Control Company.
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 plant care. Most borers are attracted to trees that are already unhealthy from other causes. Relying only on insecticide can allow trees to die from those other causes unless the growing environment and cultural practices are improved.
Adapted from the publications linked above, Guide to Insect Borers in North American Broadleaf Trees and Shrubs from the U.S. Department of Agriculture Forest Service, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Bark oozing from boring of flatheaded alder borer larvae.
Injury from borer larvae tunneling under bark.
Larva of Pacific flatheaded borer exposed under bark.
Adult western cedar borer.
Adult bronze birch and its emergence hole in bark.
flatheaded borer adult