bronze birch borer

Bronze birch borer is a common, native, small to medium size, metallic wood boring beetle. It occurs across the northern United States and southern Canada.

Bronze birch borer is considered a secondary pest of birch trees. It attacks trees that have injury to the stem or roots, or are weakened or stressed by drought, old age, insect defoliation, or soil compaction. It occasionally also attacks healthy trees. It is a serious economic pest in Missouri, where there have been extensive plantings of non-native silver birch, a European species. However, bronze birch borer is native, and as such it is considered a normal component of any ecosystem dominated by birch.

Adults are active in Minnesota from late May to August. They are found in deciduous forests and woodlands, in parks, and in urban and residential areas, anywhere birch trees are found. They feed on the leaves of birch (Betula spp.) and poplar (Populus spp.) trees. The larvae feed on the inner bark (cambium), creating serpentine tunnels (galleries) that interrupt the flow of food. They eventually girdle and kill the branch or tree. In Minnesota, they attack paper birch and yellow birch, but they are not known to attach river birch. They have been reported attacking quaking aspen, eastern cottonwood, and bigtooth aspen, all in the genus Populus, but these reports are likely misidentifications of the similar looking granulate poplar borer.

Adults are 3⁄16″ to 9⁄16″ (5 to 13 mm) in length. Males are smaller than females. The body is somewhat bullet-shaped, rigid, narrow, elongate, and slightly flattened. It is black with metallic bronze reflections. The amount of metallic reflection varies with the angle and strength of the light, so that the overall color can change as the observer shifts positions.

The head is black with bronze reflections. It is flat and it is slightly tucked beneath the first segment of the thorax (prothorax). The eyes are kidney shaped. The antennae are short and have 11 segments. Segments 1 through 3 are cylindrical. Segments 4 through 11 are sawtoothed. The mouthparts are projected downward. On the male the upper part of the face (frons) has green reflections. On the female the frons has copper bronze reflections.

The upper plate covering the prothorax (pronotum) is wider than long. It is black with bronze reflections. There is a distinct ridge on each side of the pronotum at the shoulder region (prehumeral carina) near the base of the elytron.

The wing covers (elytra) are colored similar to the pronotum but are usually darker. They are slightly narrowed before the middle and tapered beyond the middle toward the tips. The tips are separated, broadly rounded, and finely toothed. Toward the tip the inner margins are slightly elevated.

On the male, the fourth segment (tibia) on the front and middle legs has a tooth at the tip.

Total length: 3⁄16″ to 9⁄16″ (5 to 13 mm)

Deciduous forests and woodlands, parks, urban and residential areas

Late May to August

Paper birch and yellow birch

Leaves of birch (Betula spp.) and poplar (Populus spp.) trees

Sources

27, 29, 30, 82, 83.

The counties in light green have only records prior to 1980.

Biodiversity occurrence data published by: Minnesota Biodiversity Atlas (accessed through the Minnesota Biodiversity Atlas Portal, bellatlas.umn.edu, 6/5/2025).

Hallinen, Marie J; Steffens, Wayne P; Schultz, Jennifer L; Wittman, Jacob T; Aukema, Brian H. (2021). The Jewel Beetles of Minnesota. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/218928.

Common

Subgenus
The genus Agrilus is a very large group of jewel beetles. Attempts to classify it into subgenera exist, but these are often based on regional groupings of species rather than a universally agreed-upon system. These divisions are often regional and not universally accepted.

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6/11/2024

Location: Albany, NY