Common name: Bronze birch borer
Scientific name: Agrilus anxius
Hosts: Birch trees (Betula species)
This native insect is a serious pest of ornamental birch trees (Betula spp.), especially some of the white-barked species. Grown as specimen trees, white-barked birches are often sited in heavy clay soils or grown under other adverse conditions, making them stressed and more susceptible to borer attacks than they would be in a more favorable environment. While all species of birch can be attacked, many white-barked birches are more susceptible than other species. The following lists species of birch by their level of susceptibility.
Highly susceptible to bronze birch borer
- Betula utilis (Himalayan birch)
- Betula pendula (European white birch)
- Betula pubescens (moor birch)
Shows some resistance to bronze birch borer
- Betula alleghaniensis (yellow birch)
- Betula lenta (sweet birch)
- Betula papyrifera (paper birch)
- Betula populifolia (gray birch)
Highly resistant to bronze birch borer (rarely affected)
- Betula nigra (river birch)
- Betula populifolia ‘Whitespire’ (Whitespire Senior gray birch)
An early warning sign of bronze birch borer damage is yellowing and thinning of foliage in the upper crown of the tree. This is followed by dieback of branches in the upper portion of the tree. The adult bronze birch borer is attracted to trees under stress. The adult itself does not damage the tree, but does lay eggs on the bark. The eggs will then hatch into larvae that tunnel under the bark. The wilting occurs because of the tunnels created by the larvae. The tunnels damage the tissue that moves water through the tree. The flow of water is reduced or cut off completely, leading to dieback of the upper branches. Externally, these tunnels can be identified on the bark of the tree as a raised or bumpy surface.
By late summer, the foliage will turn brown and drop from the branches, with symptoms progressing down the tree to the main trunk. Infestations often begin in 3/4” to 1” diameter crown branches, moving down the tree over successive years. If left uncontrolled, an infestation of the bronze birch borer will move downward and eventually kill the entire tree in two to three years. As adult beetles emerge in spring they leave “D-shaped” holes in the bark.
The bronze birch borer overwinters under the bark, in a cell they construct at the end of their tunnel. The larvae pupate the following spring and emerge as adult beetles through D-shaped holes cut into the bark. The adult bronze birch borer is a slender, bronze beetle. The males are about ⅜ inch long, while the female is ½ inch long. The adults will feed on the leaves for a few days, but the damage is very minor. After emergence, the adults mate and then lay their eggs on the bark. The adult female deposits her eggs in the cracks and crevices of susceptible birch trees in late May or June. The bronze birch borer produces one generation per year.
The larvae hatch about ten days later, boring into the wood of the host tree and feeding on interior tissue of the bark. The ¾ inch larva is ivory with a light brown head, the head is tucked into the first section of the body, which is flattened. This makes the larva look like it has a flattened head. The larva is legless. They continue to mine their intricate feeding tunnels actively through summer into fall, then form their overwintering cell at the end of the tunnel.
Cultural management:
The most effective method of managing the bronze birch borer is through proper cultural practices. First, select one of the more resistant species. Make sure the tree is properly sited. Birches prefer cool, moist, partially shaded situations and are not well suited to open, sunny, exposed locations. Trees under stress are more susceptible to attack, so maintaining a vigorous, healthy tree will help reduce the risk of infestation. Provide sufficient water, especially during dry periods. Mulch over the root zone to a depth of 2 to 4 inches to help moderate soil temperatures, retain moisture, and reduce trunk injury caused by lawn mowers. Identify declining and dead branches for pruning. Prune in fall when the adult borers are not active because they can be attracted to fresh pruning cuts. Remove infested wood from the landscape..
Chemical management:
There are insecticides that can be used in the management of trees infested with bronze birch borer. Contact the Plant Clinic (630-719-2424 or plantclinic@mortonarb.org) for current recommendations. Use pesticides safely and wisely; read and follow label directions.
For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).