Pest description and crop damage Adult is an olive-brown beetle with a blunt head and a tapering body about 0.5 inch long. Larvae are flatheaded borers. They grow to about 1 inch long and are creamy to white in color with a head wider than the body. The larvae bore into the branches or trunk after hatching and bore winding galleries along the cambial layer (between the wood and the bark). The galleries may heal, with swelling showing on the outside of the tree (lumpy bark), or may girdle and kill branches or trunks. Leaves may be yellow (chlorotic) as a result. The adults may feed on leaves, but cause little damage. Bronze birch borers attack trees weakened by age, environmental stresses, or previous insect attacks. Susceptible birch species include B. papyrifera, B. pendula, and B. populifolia. Ornamental cutleaf varieties of birch seem particularly susceptible to attack by this borer.
Biology and life history The insect overwinters as a larva in the tree. As the weather warms in spring, the larvae resume feeding. They pupate in the tree, then the adult insect chews a D-shaped hole in the bark to emerge, starting in late spring. They lay their eggs in bark cracks or under flaps of bark. The larvae hatch and bore immediately through the bark to feed on the vascular tissues. The life cycle takes 1 to 2 years to complete.
Scouting and thresholds Entry holes and borer damage greatly affect marketability, action thresholds are very low. Prevention is key.
Borers are less likely to attack healthy trees. Remove nearby infested trees immediately. Keep trees vigorous with proper watering, fertilizing, and other cultural practices. Plant resistant varieties and species, which include B. lenta, B. nigra (including Heritage), and B. platyphylla.
Chemical Control of Nursery Pests
Apply materials to the trunk and lower limbs. Avoid contacting foliage. Apply generally in late spring or when adults are active. Preventative treatments with a systemic Neonicotinoid have worked in some areas.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 272).
PNW Nursery IPM: Bronze birch borer (http://oregonstate.edu/dept/nurspest/bronze_birch_borer.htm)