Longhorned beetles, or roundheaded wood borers
Most any woody species can become infested by longhorned beetles (Cerambycidae), also called roundheaded wood borers. The larvae chew inner bark and sometimes the wood of limbs, trunks, and main roots. Most species of longhorned beetles primarily attack trees that were already injured or stressed.
Oval-shaped or round emergence holes in bark and cracked, stained, or oozing liquid on limbs or trunks are common symptoms of infestation by roundheaded wood borers. Exceptions include California prionus, Prionus californicus, which chews the surface of roots, then bores inside roots and pupates in soil; no bark damage or emergence holes may be evident aboveground.
Foliage may discolor and wilt, limbs may die back, and branches or entire plants may die when infested by roundheaded wood borers. There are many causes of similar damage, including aeration deficit, cold temperatures, root-disease pathogens, water deficit and other types of bark- and wood-boring insects.
To help distinguish the cause, assess whether plants are experiencing other maladies e.g., those named above. Examine unhealthy limbs for adult borer emergence holes, or cut beneath damaged or discolored bark and look for tunnels with frass (insect excrement) or the insects themselves.
Adult roundheaded borers are medium to large, elongate, cylindrical beetles. Their long antennae ("horns") are often longer than their body length. Their body color varies by species. Adults can be dull brownish or gray, brightly colored, or a mix of pale and dark colors in a banded or spotted pattern. The larvae typically are creamy white, cylindrical in cross-section, elongate, and distinctly segmented.
Species of Cerambycidae. Important introduced (exotic) species include eucalyptus longhorned borers and the Asian longhorned beetle (ALB), which is established in the eastern United States and has been spreading westward. Several native longhorned beetles resemble ALB; they all are black and white patterned. Submit any suspected ALB to the county agricultural commissioner.
The native poplar borer (Saperda calcarata) infests unhealthy aspen, cottonwood, poplar, and willow. The banded alder borer (or California laurel borer), California prionus, nautical borer (Xylotrechus nauticus), and Neoclytus conjunctus attack dying or recently killed oaks and other woody species. The roundheaded fir borer (Tetropium abietis) attacks and kills weakened firs that might otherwise have survived.
For identification help, contact your local UC ANR Cooperative Extension office or consult A Photographic Catalog of the Cerambycidae of the New World by the California Department of Food and Agriculture and Screening Aid for the Cerambycidae of the Western U.S.A. (PDF) by the Oregon Department of Agriculture and U.S. Forest Service.
Females lay eggs in bark crevices, and the emerging larvae bore into the inner bark and sapwood. Some species bore deeper into the heartwood. Larvae chew increasingly larger mines as they grow, and pack their mines with frass and coarse wood debris. After pupation, the adult leaves a round or oval hole in the surface of the bark or wood.
Longhorned beetles can take one or more years to complete one generation, but this can vary by beetle species, host quality, growing location, and time of the year. For example, during the spring and summer eucalyptus longhorned borers may complete their life cycle in 3 to 4 months; eggs laid during the fall and winter may require up to 9 months to develop into adults.
Native species of roundheaded borers attack mostly damaged or dying plants; vigorous trees are rarely attacked. Certain introduced species can be serious pests, attacking and killing trees that appeared to be healthy before becoming infested by borers. The wood-boring larvae can be serious pests because their chewing damages plants’ vascular system. Larval tunneling weakens limbs and trunks and can kill branches or entire plants.
Prevention is the primary method of preventing damage by longhorned beetles. Plant trees that are well adapted to the site conditions, provide proper cultural care, and protect trees from injuries. Appropriate irrigation is particularly important; plants are seriously damaged by irrigating too frequently or by providing too little water.
Prune off dead limbs during the season when adult borers are not active. Promptly remove dead or dying trees. Do not store freshly cut wood near trees. Debark or solarize logs beneath clear plastic in the sun to prevent beetles from emerging from the wood and attacking nearby hosts.
Except for pruning and general cultural practices that improve tree vigor, little can be done to control most boring insects beneath bark once trees are already significantly damaged or unhealthy. Larvae of the larger, shallow-boring species can sometimes be killed by probing tunnels with a sharp wire. This method is practical only on a small plant, and it is often difficult to know whether the wire has sufficiently penetrated the tunnel and killed the larva.
For certain borers, especially valuable trees may be protected from further attack by a well-timed application of a broad-spectrum, persistent insecticide to bark, or in some instances certain systemic insecticides. However, unless trees are monitored regularly so that borer activity and attack can be detected early, and the type of boring insect is identified so an effective treatment method can be selected, 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.