Bronze birch borer (BBB) is a beetle in the Buprestidae family. It is olive to brown in color with a coppery metallic reflection (Figure 1). Adults are about a half-inch long (females tend to be slightly larger). Males and females have slightly different head color. Larvae, the immature and most destructive form of the pest, are creamy white, about an inch long when fully developed, and have a distinct flat head (Figure 2, page 2). Both larvae and adults are difficult to find. Signs of the insects’ damage are easier to detect than the actual insects, which spend most of their lives inside the tree. Wilted leaves near the top of the tree are often the first identifiable sign of the borer’s presence, followed by twig and branch dieback.
Larvae hatch from eggs laid on the surface of the birch tree and bore into branches or the tree trunk. Larvae feed in the cambium layer of the tree. The cambium is between the bark and wood of the tree (Figure 3, page 2), and is where the tree’s nutrient and water transport systems are located. Extensive feeding by bronze birch borer larvae disrupts the flow of water and nutrients in the tree and kills roots, branches, and eventually the entire tree. Because it is under the bark, this damage is impossible to see without removing the bark (Figure 4). After growing to about one inch in length, larvae pupate inside the tree (Figure 3). “Rumpled” or wrinkled looking bark is another sign of BBB presence; it is found on small trunks or at the base of branches where they join the trunk (Figure 5, page 3). Cambrial feeding at branch bases cuts off water and nutrients to the branch, causing the individual branch to die back. Larval development takes 1 to 2 years, depending on the climate and length of the growing season.
Adults emerge out of the pupal casings and feed on the leaves of the tree but rarely cause extensive damage. The D-shaped exit hole made by the adult emerging from the tree is another sign of the insect’s presence (Figure 6, page 3); it is sometimes accompanied by brown staining on the bark. The D-shaped emergence holes are usually found above chest height on the tree and often are undetected until well after damage in the canopy is apparent.
An illustrated scale can help homeowners assess tree damage and the likelihood that treatment will prove beneficial. The scale ranges from 1 (healthy tree) to 5 (dead/dying tree) (Figure 7, page 4). Trees can be preserved if the damage is caught and treated early. Generally, trees with level-1 to level-3 damage have a good chance of recovery. With level-4 damage, the tree is less likely to recover, but recovery is still possible. However, a tree with level-5 damage is dead and needs to be removed. Landscape practitioners report that recovery is possible if treatment is applied while trees are actively growing and less than 50 percent of the tree canopy is affected.
Different birch species have varying levels of susceptibility to bronze birch borer. For future birch plantings in areas where BBB is present, select a resistant type. See the U.S. Forest Service’s “Forest Insect & Disease Leaflet 111” (http://oregonstate.edu/dept/kbrec/sites/default/files/bronzebirchborer_0.pdf) for more detailed information on birch species’ susceptibility to BBB. Native birch species are the most resistant to this insect.
The transport of firewood is one of the main ways BBB is moved to new locations. Purchasing locally sourced firewood can reduce the possibility of introducing BBB into a non-infested area.
BBB cannot kill healthy trees and is most likely to attack old, stressed, or drought-stricken trees. While many homeowners may not place a high priority on watering mature trees, this can be a key strategy for preventing BBB damage in dry areas. Birch trees tend to be shallow-rooted and respond well to watering.
Removing trees too damaged to recover disrupts the BBB life cycle and can slow the spread of the pest. Removal is also the only option for trees with level-5 damage. The success of your treatment plan may be affected by the actions (or inactions) of your neighbors regarding control of this pest. If you find that BBB is a problem in your landscape, treatment is not only beneficial to your trees but also to trees in your neighborhood.
Systemic insecticides can be effective tools for homeowners against BBB. Depending on the application method and product, you may have to hire a licensed pesticide applicator. Tree injection products, for example, are only available to professional applicators. Read the pesticide label before purchasing a product to ensure it is intended for use on ornamental trees, including birch. Similar products with the same active ingredients that are not formulated for trees will not be effective and could even damage your tree further.
Carbaryl is a non-systemic insecticide that can be applied as a spray to the trunk and lower limbs of trees; it should be applied during the active growing season. Carbaryl and other non-systemic products are only useful when they come into contact with BBB eggs and newly hatched larvae and, therefore, must be timed to adult emergence (typically a 2- to 3-week window that will vary each year due to weather and temperature fluctuation).
Systemic insecticides such as imidacloprid or dinotefuran, depending on the pesticide label, can be applied as a tree injection, soil injection, or soil drench. They are effective in controlling larvae feeding inside the tree. Standard, commercial practice for BBB in European Birch (the birch species most affected in the Upper Klamath Basin) is trunk injection of emamectin benzoate every 3 years, or trunk injection or soil drench of imidacloprid every year. Soil drenches should be followed by regular watering because it can take a large tree several months to fully draw in these products. In areas with no summer rains, watering is necessary for the tree to draw the pesticide from the soil.
Birch trees are wind pollinated. However, because pollen from birch catkins may be attractive to bees, you should apply systemic insecticides only after flowering has occurred in the spring to reduce possible pesticide exposure to bees. Also, do not apply systemic insecticides to the soil (spray, soil drench, or injection) when flowers that are attractive to bees are blooming underneath treated trees. Always follow pesticide label instructions. The label is the law.
It is not known at this time how much pesticide is expressed in birch pollen the year of application or in the year following application of various systemic pesticides. Depending on the application method and rate, imidacloprid and certain other pesticides may persist into the following year. Dinotefuran usually does not persist in trees as long as imidacloprid and, therefore, may be less likely to be expressed in pollen in the spring. Homeowners may consider using dinotefuran, particularly when fall applications are necessary.
For more information or guidance on managing BBB in your area, contact horticulture faculty at your local Extension office. The following are some useful publications.
Bronze birch beetle was first detected in Oregon in 2003, appearing in Portland and migrating south in the years since. Recently, it was detected in Klamath Falls (2017), where the timing of BBB adult spring emergence is not yet known. However, in areas with a climate similar to Klamath Falls, adults emerge in May and June. Because the exact timing of the BBB’s life cycle is not known in Klamath Falls, the use of non-systemic products, such as Carbaryl, may not be as effective because the chemical has to come in contact with the eggs and newly hatched larvae. Watering, however, is a key preventive strategy. In one Klamath Falls neighborhood severely impacted by BBB, observation suggested a correlation between irrigated yards and healthy trees, while trees in non-irrigated yards were the most severely affected.
Efforts to trap adult BBB in Klamath Falls after detection in 2017 were not successful. Additional trapping efforts are planned over the next several years, along with the use of predictive modeling, to determine the likely emergence time frame for BBB adults in the Upper Klamath Basin. Understanding the timing of adult emergence will increase treatment options for homeowners with birch trees.