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Bronze birch borer and twolined chestnut borer

Quick facts

  • Flatheaded borers in the family Buprestidae feed and reproduce on stressed, dying trees in forests and landscapes throughout Minnesota.
  • Most native trees (birch, oak, honeylocust, basswood, maple and ironwood) are attacked by native flatheaded borers.
  • The two most damaging flatheaded borers in Minnesota are:
    • the bronze birch borer (Agrilus anxius) - feeds on birch
    • the twolined chestnut borer (A. bilineatus) - feeds on white and red oak
  • Foliage at top of the canopy starts falling off.
  • Twigs and then branches dieback starting at the top, gradually moving down.
  • Raised lumps or ridges can be seen on the trunk and branches.

How to identify flatheaded borers

Bronze-coppery beetle with 6 legs
Bronze birch borer adult beetle

Adult flatheaded borers

  • 1/4 to 1/2 inch long, dark colored beetles.
  • Bronze birch borers are bronze colored.
  • Two-lined chestnut borers are matte bluish-black with two parallel yellow stripes running down their wing covers.

Larvae

  • Larvae of both borers are up to one inch long when fully grown.
  • Pale white.
  • Flattened body and two pincer-like tails at the rear end.
  • The head is mostly hidden but the mandibles are easily seen.
Bluish-black beetle with to yellow lines running down its wing covers
Twolined chestnut borer adult beetle

Life cycle of flatheaded borers

Flatheaded borers live through the winter as larvae under the bark of trees in pupal chambers.

  • They transform into pupae in spring.
  • Start to emerge as adults in early June in Minnesota when Vanhoutte spirea and black locust are in full bloom.
  • They can keep emerging until July.
  • The adults live two to five weeks, feeding on leaves, mating and laying eggs in branch or bark crevices.
  • The eggs hatch and small larvae chew through the bark to feed.  
2 pale yellow larvae with a flattened head
Twolined chestnut borer larvae
  • While feeding, the larvae create galleries that become packed with their frass (a mixture of borer feces and sawdust).
  • Most larvae consume enough tissue through summer and fall to complete their life cycle in one season.
  • In very cold climates, or where the larval resources are limiting in the tree, it may take two years to complete a generation.

Damage caused by flatheaded borers

Adult beetles feeding on the leaves of trees do not affect tree health.

Galleries in tree wood created by larvae disrupt the transport of water and nutrients in infested trees.

  • The first symptom of damage is branch dieback at the top of the tree.
  • If the tree continues to decline, dieback extends down into major branches and eventually into the main stem.
  • Bronze birch borer galleries look like ridges of raised, bumpy callus tissue on the bark of infested birch.
  • S-shaped galleries made by bronze birch borer can be seen under the bark.
  • Twolined chestnut borer galleries cannot be seen unless the bark is removed.
  • 1/8-inch D-shaped exit holes might be seen on trunks and branches.
  • On paper birch, you may see ridges but no exit holes.

How to protect your trees from flatheaded borers

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Jeffrey Hahn, Extension entomologist

Reviewed in 2018

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