This native insect is a serious pest of ornamental birch (Betula spp.), one of the more popular group of trees found in landscapes today. Grown as a specimen, white-barked birch is often sited in heavy clay soils or grown under other adverse conditions, making it stressed and more susceptible to borer attacks than it would be in its natural environment.
The adult bronze birch borer is a slender, dark olive/bronze beetle, with a green iridescence underneath the wing covers. The males are about 3/8-inch long, while the female is 1/2-inch long. The 3/4-inch larva is ivory with a light brown head that is slightly tucked into the first thoracic segment. The legless borer has two brown, pincer-like structures found at the tip of the abdomen.
The bronze birch borer produces one generation of 75 eggs per year. The adult female deposits her eggs in the cracks and crevices of susceptible birch trees in late May or June. The larvae hatch about ten days later, boring into the wood of the host tree and feeding on interior tissue of the bark. They continue to mine their intricate feeding tunnels actively until fall, then form a cell at the end of a tunnel where they overwinter. Larvae pupate the following spring and emerge as adult beetles through telltale “D-shaped” holes cut into the bark. Adults feed on the tender young foliage for about a week before laying their eggs and can be seen moving around the sunny sides of trees.
While all species of birch can be attacked, white-barked birch varieties are more susceptible than others. The following chart compares susceptibility of the most common varieties:
Scientific Name / Common Name / Susceptibility
Betula jacquemontii / white-barked Himalayan birch / High
Betula pendula / European white birch / High
Betula pendula ‘Youngii’ / European white weeping birch / High
Betula lenta / sweet birch, black birch / Moderate
Betula lutea / yellow birch / Moderate
Betula papyrifera / paper birch, white birch / Moderate
Betula populifolia / gray birch / Moderate
Betula populifolia ‘Whitespire’ / Whitespire birch / Moderate
An early warning sign of bronze birch borer damage is a yellowing and thinning of foliage in the upper crown of the tree. These symptoms are more evident in hot, dry weather.
The adult bronze birch borer is attracted to trees under stress, but does not cause direct damage to the tree. The wilting occurs because of the labyrinth of tunnels created during the larval stage of the borer’s life cycle. The tunnels girdle the trunk or branches, reducing the flow of plant sap within the tree. Externally, these tunnels can be identified on the bark of the tree as a raised or rippled 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 usually 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. Note that discolored leaves may not always be the result of bronze birch borer attacks. Another birch pest, the birch leafminer, can also cause foliage to turn brown. To positively identify the infestation, check for the “D-shaped” holes in the bark made by the emerging adults in the spring.
The most effective method of managing this pest is through proper cultural practices. First, if you are considering planting a birch in your landscape, select one of the resistant varieties. Next, make sure you site the tree properly. Birches prefer cool, moist, shaded situations and are not well suited to the open, sunny, exposed locations typical of most new-home subdivisions. Trees under stress are more susceptible to attack, so maintaining a vigorous, healthy tree will help prevent any infestation. Fertilize the tree if it is lacking in nutrients, and 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 are attracted to fresh pruning sites. Avoid composting infested wood.
Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current recommendations. Use pesticides safely and wisely; read and follow label directions. The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.