Bronze birch borer and twolined chestnut borer
- 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
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 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.
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.
- 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
For bronze birch borer
Avoid planting species of trees that are easily targeted by bronze birch borer.
Most Asian and European varieties of white barked birch are affected by the bronze birch borer, even when they are healthy.
White barked birch species that can be attacked include:
- European birch (Betula pendula)
- Asian birch (B. platyphylla)
- Himalayan birch (B. utilis)
- Japanese monarch birch (B. maximowicziana)
Native species are more resistant to borer attack as long as they are not stressed by drought, over mature or have some other health issue.
- White barked birch - paper (B. papyrifera) and gray (B. populifolia) birch
- Non-white barked species
- Yellow birch (B. alleghaniensis)
- Sweet birch (B. lenta)
- River birch (B. nigra) seem to be immune
For twolined chestnut borer
Twolined chestnut borer attacks all species of oaks. All North American species of oaks have some resistance, but can suffer damage when trees are stressed.
The twolined chestnut borer will attack both native and introduced oaks that are under stress.
In Minnesota, this includes:
- White oak (Quercus alba)
- Swamp white oak (Q. bicolor)
- Bur oak (Q. macrocarpa)
- Northern pin oak (Q. ellipsoides)
- Red oak (Q. rubra)
- Black oak (Q. veluntina)
Trees that are stressed from drought or defoliation may suffer more damage due to feeding by flatheaded borers. Stress affects the ability of a tree to acquire and distribute water and carbohydrates throughout their canopy.
Stress can be caused by:
- Exposing roots to heat and drying.
- Poor planting sites, such as compacted soils.
- Damage to tree roots and trunks from mowing and weed management.
- Construction practices like re-grading the landscape, installing new landscape components or by damaging roots when building or expanding homes.
To minimize potential stresses:
- Add organic mulch to oaks and birch to improve their health.
- Mulch keeps soil temperatures cooler and slows the rate of moisture evaporation.
- It increases the water holding capacity of the soil and creates a better rooting system.
- Mulch is helpful for birch which has a shallower root system.
- Always keep trees well-watered.
- Avoid fertilizing stressed trees. Fertilizers increase water demand for the trees and can be injurious to them.
If changes are being made to the landscape, make sure to protect the roots of trees.
- Root damage caused by soil compaction or root severing due to heavy equipment will stress trees.
- Remember that the roots can extend well beyond the canopy of the tree.
Treating trees with pesticides to kill borers is only effective if the tree is in the initial stages of decline and dieback.
Pesticides are not effective when more than 40 to 50 percent of the canopy has been killed by borers.
Imidacloprid is applied as a liquid drench to the soil around the trunk of the tree (professional applicators can also apply it as a soil injection or a trunk injection).
Dinotefuron is applied as granules to the soil directly around the tree (professional applicators can also apply it as a bark spray, soil drench or soil injection).
CAUTION: Apply these products to birch and oak trees only after flowering in the spring to reduce pesticide exposure to bees. Do not apply systemic pesticides to the soil when bee attractive flowers are planted next to trees.
Non-systemic control of borers is difficult. Pesticide is effective if applied to the infested tree when the adult beetles are first active in early June.
Spray pesticide on the trunk and branches where the eggs are being laid. When the larvae hatch from the eggs, they will come in contact with the pesticide as they burrow through the bark.
Products containing bifenthrin or permethrin are effective.
- Two applications may be necessary.
- The first application should be applied as black locust trees bloom and the second two to three weeks later.
- Homeowners can spray small trees themselves.
- Contact professional tree care companies for larger trees.
Contact a professional when you are dealing with larger trees. Commercial tree care companies have experience in managing borers and in handling and applying pesticides. They have access to products and procedures that are unavailable to homeowners.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Reviewed in 2018