Now is the time of year to prevent problems with wood-boring beetles such as the flatheaded appletree borer, Chrysobothris femorata, and the roundheaded appletree borer, Saperda candida. Both the flatheaded appletree borer and the roundheaded appletree borer attack a wide range of trees and shrubs; however, they seem to prefer plants in the rose family (Rosaceae) including hawthorn, quince, crabapple, cotoneaster, mountain ash, and pyracantha. Young maple trees are also highly susceptible to borer infestation. Both borers are opportunistic and tend to attack damaged or dying plants, or newly transplanted trees and shrubs. Vigorous, healthy trees are rarely attacked. Adult beetles, which feed on fruit, bark, and leaves, may infest plants growing in nurseries and landscapes.
Flatheaded appletree borer adults are typically 12 mm long, metallic, and vary in color from brown to gray. Roundheaded appletree borer adults are 24 mm long and are gray with black stripes. The adult females of both species lay single eggs in the crevices or slits in the bark, generally near the base. Eggs are laid mostly at night. Eggs hatch into legless, creamy white larvae (about 3 to 4 mm long) that bore through the bark into the cambium. They then move up and down the plant, feeding within the sapwood. The larvae are about 25 mm long when full grown. Larvae produce long, winding, tortuous tunnels that can girdle and kill large branches and young trees. Larval activity can be detected by the presence of white sap flowing from cracks in the bark. Eventually, the larvae bore into the heartwood to pupate. When flatheaded appletree borer adults emerge, they leave a D-shaped hole, whereas holes from roundheaded appletree borer adult emergence are circular or round. Females generally emerge in late spring to early summer. Adults of both species may live up to 40 days. Flat-headed appletree borer has one generation per year. In contrast, roundheaded appletree borer takes 2 to 3 years to complete its life cycle.
The key to minimizing problems with wood-boring insects is plant health. Plants that are properly watered, fertilized, mulched, and pruned are less susceptible to attack from both the flatheaded appletree borer and the roundheaded appletree borer. Remove any dead wood from trees, as it provides potential entry sites for the borers. In addition, donít store freshly cut wood near trees because adult beetles that emerge can attack nearby trees. A horticultural wrap of paper or burlap may be useful in protecting young trees and shrubs. In nurseries, clean cultivation, removing grassy weeds by mowing, or using a postemergence herbicide such as Roundup, Finale, Reward, or Scythe may reduce problems with both species.
Applications of insecticides including chlorpyrifos (Dursban), lindane, or permethrin (Astro) in late May or early June kills eggs and the newly hatched larvae before they bore into trees. Injected or systemic insecticides may not provide control, especially if plants are stressed.