Birch Beetles
- Adult bronze birch borer beetles are 0.38 to 0.5 inches long. They have elongated bronze- or olive-green bodies and are iridescent green below their wing covers. The females lay approximately 75 eggs each year. The eggs hatch into 0.5- to 1-inch larval insects with white bodies, flattened heads, two brown, pincer-like parts on their last segment, and dark-brown mouth parts. These insects have a 2-year life cycle in cold climates, and a 1-year life cycle in warmer areas.
- The female beetle lays her eggs in the cracks and crevices of susceptible birch tree bark during the summer, and the larvae hatch within 2 weeks. They burrow into the tree and feed on the tissue between the outer bark and the inner wood, creating elaborate tunnels inside the tree. They pass through 4 instars, or larval stages, as they mature. Larvae of all developmental stages stop feeding during cold weather and overwinter in the tree. Mature birch borer larvae pupate in spring, while younger insects resume feeding on birch wood. After they finish pupating, the newly emerging adult beetles create D-shaped exit holes in the bark from May to July depending on the climate and temperature. Adult beetles live for around 3 weeks.
- Bronze birch borers attack stressed trees. The larvae devour the inner tissue all the way around infested branches. This process, called girdling, prevents water and nutrients from reaching the area beyond the girdled tissue. As a result, the leaves turn yellow and fall from the tree, and the branches die. Infested branches can show considerable dieback by late summer. As the infestation grows, the larvae work their way down the tree and into the main trunk. Severely infested trees usually die within 3 to 4 years.
- You can control birch beetle infestations by planting resistant trees. The University of Wisconsin Extension notes that Betula nigra "Heritage" is a highly resistant cultivar. Birch trees suffer the least amount of temperature stress if you plant them in moist soil with partial shade. Water trees during periods of drought, and fertilize them as needed to maintain their vigor. Prune diseased branches and remove severely infested trees. Spray trees with permethrin to kill young birch borers before they can burrow into the tree.
