Oak twig girdler—Agrilus angelicus
This flatheaded borer (family Buprestidae) infests and kills twigs of tanoak and true oaks, especially coast live oak in Southern California. Other hosts include California black oak, canyon live oak, Engelmann oak, interior live oak, and valley oak.
The presence of oak twig girdler is indicated by scattered patches of whitish brown leaves throughout the canopy. Leaves are dead but have not been chewed and exhibit no surface scraping.
Except for adults, all life stages occur hidden beneath bark. Adults are dark coppery brown beetles about 1/4 inch long. Eggs are long, narrow, and whitish. Larvae are white to pale yellow with brown mouthparts. The larval body has distinct constrictions between each segment and enlarged segments immediately behind the mouthparts.
To confirm oak twig girdler as the cause of damage peel back bark at the junction between dead and live twigs. Look for tunnels that spiral around the twig and are packed with dark, granular frass (excrement). A tunnel may also contain a beetle larva or pupa.
Various other insects and plant pathogens can cause similar, scattered dieback on oaks. These include plant pathogens that cause oak branch canker and dieback and oak twig blight. Less-common boring insects that cause similar damage include at least two roundheaded borers, Aneflomorpha lineare and Styloxus fulleri (family Cerambycidae), and a false powder post beetle Scobicia suturalis (family Bostrichidae). These insects look distinctly different than the adults and larvae of oak twig girdler.
The adult oak twig girdler emerges around June in coastal areas and May at inland locations. The eggs are laid singly on young twigs. The hatching whitish larva chews inside and for 3 to 6 months creates a linear mine several inches long. It then mines spirally and girdles the twig, causing foliage terminal (outward) from the mining to die and turn brown. During the next growing season the larva mines a foot or more down the branch toward the trunk, causing more foliage to die. It then bores back outward in the center of the branch, pupates just under the surface, and emerges as an adult. The development from egg to adult requires about 2 years.
Oak twig girdler larvae chew and feed inside small branches, causing the shoot terminal and attached leaves to die. Although this damage can be unsightly, oak twig girdler does not significantly impair tree health.
Provide trees with proper cultural care and good growing conditions. Drought-weakened oaks are especially prone to twig girdler attack and other pest problems. Because urbanization often reduces the natural availability of soil moisture, even drought-adapted trees may warrant infrequent, deep watering.
Prune infested branches to restore the oak’s aesthetic quality. At least six species of parasitic wasps attack oak twig girdler; however, their importance in biological control has not been documented. Because damage by this pest does not affect tree survival and insecticide application kills natural enemies, no further management is recommended.
For more information see A Field Guide to Insects and Diseases of California Oaks.
Adapted from the publication above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).