Since presence of bronze birch borer was confirmed in Klamath Falls in summer 2017, the destructive beetle continues to impact trees in the Klamath Basin. Birches — shallowly rooted trees suited to locations with ample moisture — are ill suited to our area. Drought increases birch tree susceptibility to BBB and speeds tree death. Wood boring beetles are most attracted to stressed trees.
Drought is now extreme. Gardeners are carefully considering how often, how much, and which plants they water. Water conservation must be weighed against the value of a mature tree (shade, aesthetic value, wildlife benefit) and the cost of not treating for BBB. Recommendations for specific amounts of water needed by mature trees are sparse: ten gallons of water per mature tree per week is one suggestion from OSU forestry colleagues.
Minimize other stress to birches: avoid pruning June thru August. Freshly cut wood attracts adult BBB to lay eggs. In 2020, OSU developed a model to predict when adult BBB are active and laying eggs. Adult activity in the Klamath Basin is predicted for June through August, with peak activity around the first of July.
BBB spend most of their lives, 1-2 years, inside birch trees feeding on the cambium layer. This is when the greatest damage occurs, though it is difficult to detect from outside the tree. The cambium is the interface between the actively growing part of the tree and the older wood. BBB feeding in the cambium disrupts the tree’s delivery of water and nutrients to outer branches. Fecal matter left behind the beetle as it tunnels through the cambium further disrupts flow of water and nutrients.
The first sign of BBB in mature trees are single dead branches high in the canopy. From this early indicator or damage to complete death of the tree takes about two years in the Klamath Basin, if left untreated. Most homeowners do not detect BBB damage at this early stage. A single dead branch can be attributed to many causes, there are no adult insects to see, and the larvae doing the damage are under the bark, unseen.
As damage progresses, it is easier to identify, and harder to treat. More branches die out, some falling out of the tree. As the wood of dead branches dries and shrinks, bark pulls away from the wood, making the tunneling patterns of the larvae easier to see. Yellow to brown staining on the branches, is sometimes visible at the base of the branch where it meets the main trunk. This location is where many eggs are laid: the staining may be the tree’s response to the hatching larvae entering the tree.
After two years (one in warmer climates), adult BBB emerge from under the bark, making a hole in the branch or trunk that resembles a sideways capital “D.” The hole, about a quarter inch in length, and half that high, is diagnostic: unlike random dead branches or discoloration of bark, it cannot be attributed to other causes. These holes are often high in the tree where they go unnoticed. Often by the time emergence holes are evident, it is too late to save the birch.
Like any wood boring beetle, management of BBB is a true challenge. Sprays of pesticide into the canopy are only helpful during the June-August period when adults are active. In a large tree, this application must be done by a professional for adequate coverage. The products listed for this type of treatment break down quickly in sunlight, so one treatment is not enough to last the two-month window of adult activity.
Trunk injection is an effective, long lasting option available only through a licensed professional. The cost of this type of treatment varies with the size of the tree, and can remain viable for several years. The only effective “DIY” options are systemic soil drenches, pesticides applied at the root zone and taken up through the tree. Injections and soil drenches are dependent on regular watering after the pesticide application to insure that the product is drawn up into the tree. These products come with an element of risk to pollinators that come into contact with birch pollen following application (though birches are wind pollinated).
While it is never an easy decision to remove a valuable landscape tree, this may be the best solution to some BBB infestations in the Klamath Basin. Older trees planted when water was more abundant will struggle when water is lacking, and may be nearing lifespan end. Treatment can be costly, and one treatment will not suffice over the lifespan of the tree-especially when nearby trees on other property are not treated. When canopy loss exceeds 40%, it is unlikely that any treatment will be effective.
— Nicole Sanchez is horticulture faculty at OSU’s Klamath Basin Research and Extension Center. For more information on this or other gardening topics, contact Sanchez at 541-883-7131 or Nicole.firstname.lastname@example.org.