If you have noticed wilted, yellowing foliage in the upper crowns of your birch trees, accompanied by a dieback of branches, your trees may be infested by the bronze birch borer (BBB). This half-inch, olive/brown/coppery colored beetle, though increasingly common in the Pacific Northwest, is rarely seen, but given the opportunity will attack and eventually kill susceptible trees.
The best solution is to prevent the beetle from getting a foothold by selecting resistant trees, planting them in the right places and keeping them healthy. If you do see early signs of infestation, don’t despair: with proper management, it may be possible to save your trees — or you can plant alternatives that are just as lovely and more resilient.
Natives of North America, BBBs begin their life cycles as eggs laid by an adult beetle in late spring on the bark of a birch tree. After about 10 days, the eggs will hatch into 1-inch long, creamy white larvae, which will burrow into the bark and begin eating tunnels through the cambium layer — the part of the tree under the bark that transports water and nutrients between its roots and branches. The winding tunnels, called “galleries,” disrupt this vital flow, and portions of the tree begin to weaken and die.
After one to two years of development, larvae pupate inside the tree and emerge as adults through a characteristic “D”-shaped hole, usually above chest height on the trunk, beginning the cycle again. Eventually, if this process is left unchecked, the tree will be effectively girdled, unable to sustain itself, and the entire tree will be killed.
All birch species are susceptible to the BBB, but some more so than others. Stressed trees are the most vulnerable, as are non-native species. Species native to North America that have evolved with the beetle are more resistant.
According to the United States Forest Service, the most severely attacked are the European white birch (Betula pendula), and the whitebarked Himalayan birch (Betula jacquemontii). Paper birch (B. papyrifera), grey birch (B. populifolia), sweet birch (B. lenta), and yellow birch (B. alleghaniensis), are described as “moderately susceptible.” The native river birch (B. nigra) is the most resistant and “rarely attacked.”
To keep birches healthy, they should be planted where their roots stay cool, shaded, and moist, though they prefer their crowns to receive sun. Birch roots are shallow (18 inches deep) and dry out quickly, so they should be mulched with a 2- to 4-inch layer of wood chips, leaves or other organic material to the dripline (but not within 6 inches of the trunk to prevent rot) in order to moderate soil temperature and conserve water. Most importantly, birches should be watered regularly during dry weather, especially when it’s hot. Inadequate water is the biggest source of stress to the trees. They will also appreciate regular fertilization. Be careful when applying herbicides around birches. Their shallow roots can absorb the chemicals, which can weaken the trees and make them vulnerable to attack.
If you do find that your trees are under attack, take action. Make sure your trees are getting enough water and nutrients, as already discussed. Prune out and dispose of all affected branches (but avoid pruning in April-June when adult BBBs may be active — adults are attracted to fresh cuts).
Pesticides can be applied to control the beetle infestation. Systemic pesticides — often applied either as a soil drench or injection into the bark or soil — are taken into the tree and kill the larvae feeding there. Tree injections must be performed by licensed pesticide applicators — contact an International Society of Arboriculture (ISA) certified arborist for assistance. Homeowners can do soil drenches on their own; just be sure that the product is listed as controlling birch boring beetles in ornamental trees, and read and follow label directions carefully. Even if they contain the same active ingredient, products that are not formulated for trees may be ineffective and damage the tree.
Timing of pesticide application is important. Some systemic pesticides for controlling birch borer, such as imidacloprid, dinotefuran and acetamiprid, are neonicotinoid compounds — a class of chemicals implicated in the demise of honeybees. Therefore, soil drenches containing such chemicals should only be done after the trees have flowered in the spring, and avoided when flowers planted under the trees, or even nearby weeds, are in bloom to prevent poisoning bees feeding on blossoms.
Non-systemic pesticides for controlling BBBs also are available. These are applied to the tree trunk and lower limbs in the late spring to kill the adults as they emerge from their exit holes, or to kill larvae as they emerge from the eggs. While non-systemic can prevent reinfestation, they will not control larvae already feeding inside the tree. Note that as with systemic pesticides, some of these chemicals (such as carbaryl, or Sevin) are also toxic to bees.
Any chemical control measure will need to be repeated at least annually to manage an infestation. All must be used in strict accordance with label directions to protect the trees, people, pets and the environment.
If management efforts fail, and your trees succumb to the BBB, take heart. There are a number of alternatives to susceptible birches that will grace almost any landscape and provide many of the same desirable features, such as attractive bark, an open, airy feel, and good wildlife value.
First, remove and destroy all affected trees, including trunks and branches. This prevents the BBB from spreading to other trees in the area. Never transport BBB-infested wood to other locations, such as for use as firewood, since this may spread the beetles elsewhere.
Next, remember the mantra, “right plant, right place,” when making new tree choices. This means making sure that the planting site you have in mind matches the needs (soil, water and light requirements) and growth habits (height and width at maturity, growth rate, for example) of the trees you select.
If you wish to replant birches, consider paper birch (Betula papyrifera) or river birch (Betula nigra) — the two species native to North America. As mentioned, birches require a cool, moist environment, which may be more difficult to maintain as our area becomes hotter and dryer with climate change, so even resistant birches may not be the best choice for the long run.
Other resilient, beautiful trees to consider include paperbark maple (Acer griseum), red alder (Alnus rubra), eastern redbud (Cercis canadensis), eucalyptus species, crape myrtles (Lagerstroemia), Persian ironwood (Parrotia persica), quaking aspen (Populus tremuloides), and stewartia. All of these trees are recommended by Roy Farrow, horticulture supervisor at the University of Washington Botanic Gardens at the Washington Park Arboretum. He reviews the specific benefits of each in the excellent online article, “Alternatives to Borer-Susceptible Birches.”
