Bronze Birch Borer

This native North American insect occurs on birch from Newfoundland to British Columbia and south to West Virginia, Ohio, Colorado, Idaho and Oregon. The larvae have been recovered from most birch species but European white birch, water birch, paper or canoe birch, and yellow birch seem to be the most preferred hosts. It is a serious pest of ornamental birch plantings.

Adults are slender, dark olive-bronze in color with a bright green iridescence underneath the wing covers. Males are usually about 3/8-inch long, while females may reach ½ inch long. Larvae are white, slender, legless with a light-brown head capsule, and up to ½ inch long. Adults begin to emerge through the bark in early to mid-June and may continue for five to six weeks. They feed on the foliage for at least a week before beginning  to lay eggs in bark crevices or beneath loose bark. The bronze birch borer is attracted to the sunny side of trees for feeding and egg laying. There is only one generation per year.

Damage

Attack usually begins in crown branches (1/4 to 1 inch in diameter), progressing  downward year after year. Initial damage is usually discovered when the top part of a birch tree suddenly wilts and dies. This happens when a larva bores in the trunk and larger limbs, often girdling them. Tree injury is caused by larval tunneling in the inner bark or cambium. Careful examination of branches and the trunk usually reveals raised ridges or bumps, commonly referred to as "gouting" of the branches. These are ridges formed when a larval tunnel is closed in by callous tissue of the tree. The galleries range to approximately 5 feet in length and are packed with a dark brown sawdust-like frass. Often, D-shaped holes are found in the bark and these may be stained with rust colored sap. These holes are emergence holes made by adult beetles. 

Injured trees or those weakened by drought are most susceptible to attack. In healthy, vigorous trees the larvae do not complete development and the galleries heal over. Heavy attacks and continued reinfestation results in most of the branches dying from the top down. Eventually the trunk is girdled and the entire tree dies. 

Control

Select an appropriate location for your birch tree when planting. Ideal locations for birch trees are sites where the soil will remain cool and moist, but where the tree will receive full sunshine on its leaves most of the day. East and north sides of homes are generally best.

Avoid planting susceptible (white-barked) birch species; European white birch and its cutleaf cultivar are most susceptible. River birch is resistant to the bronze birch borer. 'Heritage' is a relatively resistant white-barked cultivar of river birch that is readily available in clump form and as single stems.

Pruning should not be done from May 1 to July 1. This is during the borer flight period and it has been shown that female borers are attracted to fresh pruning wounds.

It is possible to prolong the life of susceptible trees by reducing stress. Deep watering may be helpful during dry periods. Trunk injury, as by lawn mowers, can also weaken the tree and provides an attractive site forbronze birch borer to lay eggs.

Bronze birch borer-infested firewood should be burned before May to stop adults from emerging. 

Chemical control is difficult once larvae have bored into the trunk. Use foliar and bark insecticide sprays to control egg-laying adults or newly-hatched larvae before they enter the bark. Apply the first spray thoroughly to the bark and foliage in early June when Kousa dogwood blooms. Since the adult flight period may last for six weeks or more, two additional sprays at three-week intervals are suggested. Most effective insecticides for the bronze birch borer are restricted to professional applicators. Homeowners may find permethrin available at some garden centers.

Adapted from: University of Vermont Cooperative Extension,1999; Ohio State University Extension; University of Minnesota Extension, 1997