JoAnne Skelly: Bronze birth borer an enemy of birch trees

I’m sad. I just had an old birch tree removed. After babying the sickly thing for years, I finally gave in and had it cut down. The tree was here when we moved in 30 years ago and it didn’t look great then. It never really grew. Why did it die? Bronze birch borers had infested the tree and it never recovered.

Bronze birch borers are common pests of birch trees in Nevada. The borers are attracted to stressed trees, which birches almost always are here. In their native habitat, they grow near water or in moist, cool soils on shady sites, something rare in our area. Even if we water excessively through most of the year to keep a birch healthy, it’s difficult to water enough in the winter. Our soils freeze, and few people are willing or able to either turn on their irrigation system or drag hoses around weekly. Nature rarely provides sufficient precipitation to keep a birch happy.

Unhappy birches mean happy borers. Birch trees weakened by drought or in poor health are susceptible to a borer attack. The worm stage (larvae) of the adult feeds primarily on plant tissue under the bark. This cuts off the flow of water and nutrients to the tree, eventually killing roots. Because nutrients and water cannot flow through the tree, the leaves cease to produce sugar and starches. Initially, symptoms of an infestation appear in the upper crown of the tree. Leaves often are yellow and sparse, then dieback occurs on branches three-fourths of an inch in diameter or larger. The dieback progresses downward each year. Other evidence of an infestation observed on the trunks and branches are “D” shaped holes. Rust-colored sap oozing and staining the bark may also be seen, along with swelling and bumps. The “D” shaped holes are produced when the adult beetle eats its way out from under the bark. Holes are small, one-fifth of an inch in diameter, and found near the base of infested limbs and the trunk. Because they are small, they can be easily overlooked.

If you are thinking of planting trees, avoid European white or weeping birches, Betula pendula varieties. Choose river birch, Betula nigra and varieties, instead. If you have birch trees, make sure they get ample water all year long. There are insecticides containing imidachloprid listed for bronze birch borer management, but there is controversy about whether these chemicals kill bees.

For detailed information, read “The Bronze Birch Borer” from University of Nevada Cooperative Extension, 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 skellyj@unce.unr.edu.