JARS v63n3 - Tips for Beginners: Azalea Stem Borers

Tips for Beginners: Azalea Stem Borers
Cynthia Dickinson, Master Gardener
Warren County, North Carolina

Reprinted from The Garden Path newsletter, October 2008

Azalea stem borers are slender long-horned beetles about 1.25 to 1.6 cm (1/2 to 5/8") long with yellowish brown heads and thoraxes. The wing covers are yellowish gray with dark outer margins, and there are two black spots on the thorax. The azalea stem borer is found throughout the eastern United States wherever azaleas and related plants grow. Azalea stem borers infest azalea, rhododendron, blueberry and mountain laurel. Infested twigs wilt and die as the larvae bore downward inside. At this stage, symptoms may resemble those from aerial phytophthora blight. Later in the season, infested stems often break off at the base leaving the plant lopsided and unattractive. Small plants may break off completely.

Adult azalea stem borers emerge from mid-May through June. The eggs have been described as "yellow" and are inserted under the bark between two rows of holes chewed through the bark about 1.25 cm (1/2") apart. The larva hatches and bores down the twig into the stem and eventually all the way to the crown of the plant. The stem is greatly weakened at the base. The larvae then bore down into the roots where they spend the winter. Coarse sawdust-like frass (insect poop) is expelled through holes in the bark of the stem and at the base of the plant. Infested twigs wilt as the larva bores downward inside.

Larvae pupate the following spring. Cutting off and burning infested stems as soon as they are noticed in the growing season are recommended for control. If shrubs have been reinfested year after year, it may be helpful to protect the plants with a pyrethroid insecticide such as permethrin or cyfluthrin. This should give adequate control if applied in spring, mid-May and, again, in early June, after the new growth has emerged and hardened off.