Bronze birch borer and twolined chestnut borer in Minnesota
Flatheaded borers in the family Buprestidae feed and reproduce on stressed and dying trees in forests and landscapes throughout Minnesota. Most native trees (birch, oak, honeylocust, basswood, maple and ironwood) are attacked by native flatheaded borers. In landscapes they can be pests that cause the death of valuable trees.
The two most damaging native flatheaded borers in Minnesota landscapes are the bronze birch borer (Agrilus anxius) and the twolined chestnut borer (A. bilineatus). As its name suggests, the bronze birch borer feeds on birch. The twolined chestnut borer is a pest of white and red oak species.
Most Asian and European species and cultivars of white barked birch that are planted in landscapes are very susceptible to attacks by the bronze birch borer. Highly susceptible white barked birch species include European birch (Betula pendula), Asian birch (B. platyphylla), Himalayan birch (B. utilis), and Japanese monarch birch (B. maximowicziana). Native species of white barked birch, such as paper (B. papyrifera) and gray (B. populifolia) birch have evolved with the borers and are much more resistant when the trees are healthy. Many North American native non-white barked species also have significant resistance to bronze birch borers. Yellow birch (B. alleghaniensis) and sweet birch (B. lenta) are also generally resistant to bronze birch borer unless stressed, and river birch (B. nigra) appears to be immune.
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), and black oak (Q. veluntina).
Adult bronze birch borers and two-lined chestnut borers are ¼ to ½ inch long dark colored beetles. Bronze birch borers have a bronze appearance (Fig. 1), while two-lined chestnut borers are a matte black color with two parallel yellow stripes that run the length of their wing covers (Fig. 2). The larvae of both borers are up to one inch long when fully grown, pale white with a flattened body and two pincer-like tails at their rear end. The head is mostly hidden; only the mandibles are easily seen (Fig. 3).
David Cappaert, Michigan State University, Bugwood.org
Figure 3. Buprestid larvae are pale yellow and have a large flattened head. Picture is of two-lined chestnut borer larvae in a red oak.
Bronze birch borers and twolined chestnut borers overwinter as larvae under the bark of trees in pupal chambers. They pupate in spring and start to emerge as adults in early June in Minnesota when Vanhoutte spirea and black locust are in full bloom. Emergence continues into July. The adults live for two to five weeks, feeding on leaves, mating and laying eggs in branch crotches or bark crevices of host trees.
The eggs hatch and small larvae chew through the bark to get to the nutritious phloem as well as the outer xylem tissue. As larvae feed, they create galleries that become packed with their frass. Frass is a mixture of borer feces and sawdust. Most larvae will consume enough tissue through summer and fall feeding to complete their life cycle in one season. However, in very cold climates, or where the larval resources are limiting in the tree, it may take two years to complete a generation.
Trees stressed by factors such as sustained drought or defoliation are especially susceptible to attacks by the bronze birch borer and twolined chestnut borer. In landscapes, birch trees tend to be grown in open locations where roots are exposed to heat and drying. Stress can also be caused by poor planting sites, such as compacted soils; damage to tree roots and trunks from mowing and weed management; and by construction practices including re-grading the landscape, installing new landscape components or by damaging roots when building or expanding homes. Trees under stress have a reduced ability to acquire and distribute water and carbohydrates throughout their canopy which leads to reduced tree defense against the borer larvae.
Adult beetles feeding on the leaves of trees cause little damage to overall tree health. However, the galleries created by the larvae cause significant damage to trees. These galleries disrupt the transport of water and nutrients in infested trees. The initial symptom is often branch dieback at the top of the tree (Fig. 4). If the tree continues to decline, dieback extends down into major branches and eventually into the main stem. Bronze birch borer galleries can be observed as ridges of raised, bumpy callus tissue on the bark of infested birch (Fig. 5). Twolined chestnut borer galleries cannot be seen unless the bark is removed. Infested branches and trunks usually exhibit 1/8 inch D-shaped adult emergence holes (Figs. 6 and 7). It is possible to see callus ridges on paper birch without exit holes being present.
European and Asian species and cultivars of birch are very susceptible to bronze birch borer, even if trees are healthy and vigorous. However, native species, such as paper, gray, yellow and sweet birch are more resistant to borer attack as long as they are not stressed by drought, over mature, or have some other health issue. River birch appears to be immune to this borer. Avoid planting species of trees that are highly susceptible to bronze birch borer.
Two-lined chestnut borer attacks all species of oaks and although all North American species of oaks have some resistance, it can be overcome when trees are stressed.
Since both borers are attracted to trees that are stressed and unhealthy, it is important to avoid or minimize potential stresses, such as drought and defoliation. Adding organic mulch to oaks and birch, whether they are newly planted or established trees, can also help improve their health. Mulch helps keep soil temperatures cooler as well as slow the rate of moisture evaporation. It helps create a better rooting environment through the addition of organic matter which increases the water holding capacity of the soil. Mulch is particularly helpful for birch which has a shallower root system compared to oaks.
Mature trees that are attractive to bronze birch borer and two-lined chestnut borer are usually water stressed. The most important management step that can be taken is to keep trees well-watered. While it might seem reasonable to fertilize stressed trees to make them less susceptible to borers, fertilizing them does not improve their health. Fertilizers increase water demand for the trees and can actually be injurious to them. Avoid fertilizing stressed trees.
In addition, if any changes are being made to the landscape, make sure to protect the roots of the trees that will remain on the property. Root damage caused by soil compaction or root severing due to heavy equipment will stress trees and making them more attractive and susceptible to damage from borers. Remember that the roots can extend well beyond the canopy of the tree.
Treating trees with insecticides to kill borers is only effective if the tree is in the initial stages of decline and dieback. When over 40% - 50% of the canopy has been killed by borers, the effectiveness of insecticides is greatly diminished and a treatment should not be made.
There are systemic insecticides available for treating bronze birch borer and two-lined chestnut borer. These products are transported by the tree through the xylem tissue to where the larvae are actively feeding on the phloem and outer xylem. 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 (Fig. 8) has already occurred in the spring to reduce pesticide exposure to bees. Also, do not apply systemic insecticides to the soil when bee attractive flowers are planted directly adjacent to trees.
Non-systemic chemical control of borers is difficult. To be effective an insecticide must be applied to the infested tree when the adult beetles are first active in early June. The insecticide must be sprayed on the trunk and branches where the eggs are being laid so that larvae hatching from the eggs will contact it as they burrow through the bark of the tree. Homeowners can reasonably spray small trees themselves; products containing bifenthrin or permethrin are effective, although two applications may be necessary. The first application should be applied as black locust trees bloom and the second two to three weeks later. However, it is not practical for residents to spray large trees themselves.
Commercial tree care companies have experience in managing borers and in handling and applying pesticides. They also have access to products and procedures to manage borers that are unavailable or unfamiliar to residents. Look for a licensed applicator when you wish to use a professional company and/or when treating trees in not practical for residents.
Caution: When using pesticides, follow all label direction carefully. The label is the law and the final word in how a particular product is used.