Now is the time of year to be aware of potential problems in nurseries and landscapes from the flatheaded appletree borer, Chrysobothris femorata, and/or the roundheaded appletree borer, Saperda candida. Both are wood-boring beetles (Order: Coleoptera) that attack a wide range of trees and shrubs. They tend to prefer plants in the rose family (Rosaceae), including crabapple, cotoneaster, hawthorn, mountain ash, pyracantha, and quince. Young maples (Acer spp.) are particularly susceptible to attack from either borer species. Flatheaded and roundheaded appletree borers are opportunistic, with a tendency to attack damaged or dying trees, or newly transplanted trees and shrubs. These borers rarely attack healthy, vigorously growing trees and shrubs. Adult beetles, which feed on fruit, bark, and leaves, may infest plants growing in nurseries and landscapes.
Flatheaded appletree borer adults are about 12 mm long, metallic in appearance, and vary in color from brown to gray. Roundheaded appletree borer adults are about 24 mm long and are gray, with black stripes on the wing covers. Adult females of both species lay eggs individually in the crevices or slits in the bark, typically near the base of plants. Eggs hatch into legless, creamy white larvae 3 to 4 mm long. The larvae burrow through the bark into the cambium and then move up and down the plant, feeding within the sapwood. Larvae, which are 25 mm long when full-grown, create long, winding, tortuous tunnels that may girdle and kill large branches and/or young trees. The presence of larval activity can easily be detected via the white sap flowing from cracks in the bark surface. Eventually, the larvae bore into the heartwood to pupate. When flatheaded appletree borer adults emerge, they leave a D-shaped hole, whereas adult emergence holes of the rounded appletree borer are (you guessed it) round. The adult females of both species generally emerge in late spring to early summer and live up to 40 days. Flatheaded appletree borer has one generation per year in Illinois; rounded appletree borer takes 2 to 3 years to complete a life cycle.
The best way to minimize or reduce problems with both borer species, as with all wood-boring insects, is to avoid stress by maintaining plant health. Trees and shrubs that are properly irrigated, fertilized, mulched, and pruned are less susceptible to attack from both borers. Remove any dead wood from trees and shrubs because this provides potential entry sites for the borers. Additionally, avoid storing freshly cut wood near plants, as adult beetles that emerge can attack nearby trees and shrubs. A commercially available horticultural wrap of paper or burlap may be useful in protecting young trees and shrubs. In nurseries, removing grassy and broadleaf weeds by mowing or using a postemergent herbicide may reduce problems with both species.
The insecticide recommended for controlling both flatheaded and roundheaded appletree borer is imidacloprid (Merit). Imidacloprid is a systemic insecticide, so applications must be made early enough (May to early June) so that the active ingredient is present in the plant tissues when the larvae begin tunneling beneath the bark. The larvae are then killed before they cause any plant injury. Imidacloprid does not provide any control if plants are stressed.