This is the time of year in which problems can occur 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, preferring plants in the rose family (Rosaceae), such as crabapple, cotoneaster, hawthorn, mountain ash, pyracantha, and quince. Young maples (Acer spp.) are highly susceptible to both borers. Flatheaded and roundheaded appletree borers are opportunistic, tending 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 typically 12 mm in length, 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 single eggs in the crevices or slits in the bark, typically near the base of trees and shrubs. The eggs hatch into legless, creamy white larvae that are 3†to 4 mm long. The larvae bore through the bark into the cambium and then move up and down the plant--feeding within the sapwood. The larvae, which are 25†mm long when full grown, produce long, winding, tortuous tunnels that can girdle and kill large branches and young trees. Larval activity can be easily detected by the white sap flowing from cracks in the bark. Eventually, the larvae bore into the heartwood to pu-pate. When flatheaded appletree borer adults emerge, they leave a D-shaped hole, whereas holes from round-headed appletree borer adult emergence are round. The adult females of both species generally emerge in late spring to early summer and may live up to 40 days. Flatheaded appletree borer has one generation per year, whereas the roundheaded appletree borer takes 2†to 3 years to complete its life cycle.
As with all wood-boring insects, the best way to minimize or reduce problems is to avoid stress by maintaining plant health. Trees and shrubs that are properly watered, fertilized, mulched, and pruned are less susceptible to attack from both borers. Remove any dead wood from trees and shrubs, as this provides potential entry sites for the borers. Also, donít store freshly cut wood near plants because adult beetles that emerge can attack nearby trees and shrubs. 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 postemergent herbicide such as glyphosate (Roundup), glufosinate-ammonium (Finale), diquat dibromide (Reward), or pelargonic acid (Scythe) may reduce problems with both species.
The primary insecticide recommended for controlling both borer species is imidacloprid (Merit). Imidacloprid (Merit) is a systemic insecticide, so applications must to be made early enough (May to early June) that the active ingredient is present in the plant tissues when the larvae initiate tunneling beneath the bark. This results in the larvae being killed before they cause any plant injury. The insecticide does not provide control if plants are stressed.