Bronze birch borers are small beetles belonging to the order Coleoptera. They undergo complete metamorphosis (holometabolic insects). Before becoming adults, they pass through the egg, larval and pupal stage.
Eggs: Oval and flattened, about 1.5 mm long; creamy white at first, turning yellowish.
Larvae: White, elongated and flattened; 13 to 35 mm long; legless, with two small pincer-shaped spines at the tip of the abdomen.
Pupae: Similar to adults, but smaller; creamy white, turning olive-green as they mature.
Adults: Elongated body (7 to 12 mm); olive-green to black, with metallic bronze; square head, smooth back and pointed abdomen.
Bronze birch borers overwinter as larvae, hidden deep in the tree. In spring, when the sap begins to flow, the larvae wake up and start carving out tunnels under the bark, where they pupate before emerging as adults.
Adults emerge from early June to August, forming characteristic “D”-or half-moon shaped holes in the bark. Each adult lives for about three weeks. The adults mate shortly after emerging. The females then lay their eggs in crevices in the bark, usually on parts of the tree exposed to the sun.
The eggs hatch after a two-week incubation period. The young larvae burrow through the bark to reach the cambium, where they carve out meandering tunnels.
In fall, they move deeper into the tree, where they spend the winter. In our climate, it usually takes two years for the full life cycle to be completed.