My friend Ronni wrote me, “Got a dilemma for you. Is there something attacking birch trees locally? I note not only is our birch looking sickly, but many around us are looking the same or are dead. Our once robust tree is only 17. Can I save it, or is it a goner?”
Unfortunately, this decline is common with birches in the arid West. Thousands have died starting with the droughts in the 90s through today. They aren’t hardy trees in a desert. They need water 12 months out of the year, which they rarely get, since our precipitation is so sporadic. Without adequate water, birches become stressed. When stressed, they are susceptible to attack by bronze birch borers, a metallic wood-boring beetle. European white birch and weeping birch, most commonly planted here, are highly susceptible to the borers. River birch is less susceptible.
The worm stage (larvae) of the adult lives and feeds under the bark. This cuts off the flow of water and nutrients throughout the tree. The leaves cease to produce the sugars the tree needs to survive. The initial symptoms of an infestation appear in the upper crown of the tree. Leaves are often yellow and sparse, and dieback occurs on smaller branches. The dieback progresses downward through the tree each year. Other signs of borer infestation are D-shaped holes on the trunks and branches, which can be hard to detect if they occur high in the tree. These are the exit holes produced when an adult eats its way out from under the bark.
To keep birches healthier, apply water a foot from the trunk and out two to three times the radius of the canopy to a depth of 15 to 18 inches. If they are in a lawn, remove the lawn under the drip line of the tree, so that the tree doesn’t have to compete with the grass for moisture. Keep the soil moist most of the year, watering more when it’s windy or hot. Water at least monthly in the winter.
It is difficult to control borers with an insecticide spray. Soil-applied products containing the active ingredient imidochloprid can be put on according to the label at the base of the tree, but must be applied at the right time of year to be effective. However, imidochloprid may play a role in the decline of bees.
For more information on the bronze birch borer, see http://www.unce.unr.edu/publications/files/nr/2002/FS0238.pdf.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.