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Bronze birch borer and twolined chestnut borer

Quick facts

  • Buprestid beetles (metallic wood-boring beetles) are common in forests and landscapes in Minnesota.
  • Two common species in Minnesota:
    • The bronze birch borer (Agrilus anxius) feeds on birch.
    • The twolined chestnut borer (A. bilineatus) feeds on white and red oak.
  • They prefer to feed and reproduce in stressed, dying trees
  • Foliage at the top of the tree canopy starts to die back first and work its way down the tree.
  • Raised ridges can be seen on the trunk and branches of infested trees.
  • Keeping trees healthy helps to reduce their attractiveness to native borers.
  • Insecticides can help protect valued trees.

How to identify bronze birch borer and twolined chestnut borer

Adults

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

Larvae

  • 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.
Bronze-coppery beetle with 6 legs
Bronze birch borer adult
Bluish-black beetle with to yellow lines running down its wing covers
Twolined chestnut borer adult
2 pale yellow larvae with a flattened head
Twolined chestnut borer larvae

Biology

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Damage caused by bronze birch borer and twolined chestnut borer

Trees under stress are more susceptible to attacks by these beetles. Stressed trees are less able to get and move water and food (carbohydrates) to the canopy. This leads to a reduced ability to defend against borer larvae.

Stress factors

  • Sustained drought.
  • Prolonged defoliation.
  • Poor planting sites, such as compacted soils; birch grown in open locations where roots are exposed to heat and drying.
  • Physical damage to roots and trunks from construction damage or lawn mower injury.
  • Construction practices, such as re-grading the landscape.

Adult beetles feeding on the leaves of trees do not affect tree health. But larvae create destructive galleries under the bark that disrupt the transport of water and nutrients.

Identifying infested trees is challenging

Look for these symptoms:

  • Wilting or yellowing of leaves and dieback starting at the top of the tree.
    • There can be other problems that can cause similar symptoms in the canopy so dieback is not automatically due to borers.
  • As the tree continues to decline, dieback extends down into major branches and eventually into the main stem.
  • Sawdust packed S-shaped galleries under the bark.
    • Bronze birch borer galleries create raised ridges in the thin bark due to calluses forming over the galleries.
    • Twolined chestnut borer galleries can only be seen when the bark is removed.
  • 1/8-inch D-shaped exit holes on trunks and branches. They can be hard to see.

How to protect your trees

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Jeffrey Hahn, Extension entomologist

Reviewed in 2018

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