This time of year in Illinois, European white birch, gray birch, paper birch, and yellow birch trees growing in landscapes are susceptible to bronze birch borer, Agrilus anxius, infestations. Adult bronze birch borers are coppery-brown with a boat-shaped abdomen. The adult females are strong fliers and lay eggs in cracks, crevices, or under loose bark near the tops of birch trees. Egg-laying sites are generally located on the sunny side of trees. After approximately 2 weeks, eggs hatch into larvae that bore directly into the bark and begin making feeding galleries. These galleries can range from 3 to 5 inches in length and are packed with excrement. The larvae are ivory colored, long (up to almost 1-1/2 inches), and flat, with a head that is wider than the body. Larval feeding can result in girdling of the trunk or branches. Infested trees commonly have ridged or swollen bark where larvae have made their zigzag pattern under the bark. Bronze birch borer overwinters as late-instar larvae underneath the tree trunk. Larvae then pupate near the bark surface. They later emerge in late May to early June as adults, by chewing a D-shaped hole in the bark. The adults feed on the leaves of alder, birch, and poplar. There is only one generation per year.
The key to managing bronze birch borer is prevention. Bronze birch borers prefer stressed trees because they are unable to survive in healthy trees. Thus, proper watering, fertilizing, mulching, and pruning can reduce the potential for bronze birch borer infestations. The pruning of birches should not be done between May 1 and August 1, as this is the general flight period of the bronze birch borer, and females are attracted to pruning cuts. In addition, planting less susceptible varieties of birch such as ‘Heritage’ and ‘Whitespire,’ can lead to fewer problems with bronze birch borer.
The insecticide chlorpyrifos (Dursban) or dimeth-oate (Cygon) are recommended for managing bronze birch borer. These insecticides should be applied to the bark. This will kill larvae that hatch from eggs as well as adults that feed on the bark to provide a site for egg laying. Cygon should be applied to the bark as a band no more than 6 inches wide.