How can I eliminate borers in my birch tree? —John Palmblade, Hastings, NE
Prevention is the best medicine for bronze birch borers. They are attracted to stressed trees, which are often planted in poor environments. Birch are shallow-rooted trees that thrive in cool, moist soil, but we tend to plant them in exposed sites where they get lots of sun and heat and have to compete with grass for moisture.
Remove the grass in a large circle around the trunk of the birch. Mulch the area with about 3 inches of shredded bark or wood chips, leaving a space of 2 or 3 inches between the mulch and the trunk for good air circulation. Then water the tree regularly with soaker hoses or sprinklers, covering the area out to the end of the branches—and beyond, if practical.
Prune birch only minimally, and don’t prune in spring and early summer when egg-laying female borers are attracted to the scent of newly wounded wood.
Find out from your cooperative extension service which insecticides you can apply to the bark and foliage of infested trees. Insecticides won’t kill existing borers, but should prevent newly hatched larvae from entering the tree. You’ll have best results if you hire a professional to spray the tree with power equipment.
One insecticide, Bidrin, may be injected into the tree to kill existing borers. It’s a restricted-use chemical, which means only licensed pesticide applicators can use it. Remember, insecticides are valuable only when you know the tree’s cultural needs and reduce stress by improving its environment.