In droughty Klamath Falls, watering trees may help defend against bronze birch borer

June 29, 2017
Birch lightly damaged by birch borer
If beetle infestation is not too severe, the tree might be saved. This birch shows mild symptoms. Photo by Daniel Leavell, OSU

KLAMATH FALLS, Ore. – Symptoms of a voracious beetle that attacks paper birches have been spotted for the first time in Klamath Falls, according to a horticulturist with Oregon State University Extension.

The bronze birch borer, a North American native, has been a problem in the eastern United States for some decades, said Nicole Sanchez, Extension horticulturist based at the Klamath Basin Research and Extension Center.

The slender, metallic-bronze beetle was first detected in Oregon in 2003, appearing in Portland and migrating south in the years since. “Now we’ve spotted symptomatic trees throughout Klamath Falls, with concentrated areas of impacted trees in the south suburbs,” said Sanchez.

The beetle lays its eggs on the European “paper” birch (Betula pendula), prized by landscapers and homeowners for its slender white trunk and papery bark. When the larvae hatch, they burrow into the bark and feed on the cambium layer, disrupting the flow of water and nutrients and leaving a criss-crossing network of larval “galleries” under the bark. As the beetles mature, they dig their way out through the bark. If unchecked, the larval feeding eventually kills the tree.

The bronze birch borer is part of the Buprestidae family of wood-boring beetles. It is olive to brown in color with coppery metallic wings and body. Adults are about half an inch long. Larvae are creamy white, about an inch long when fully developed, with a distinct flat head.

Both larvae and adults are hard to spot, said Sanchez. Most people don’t know they’re there until they see damage to the tree.

Because paper birch is a particularly vulnerable host, the bronze birch borer has been hard to control. But there are measures homeowners can take to slow the damage and halt the spread of the pest, said Sanchez. She has written a fact sheet to help in choosing the right cultural or chemical strategies to combat the bug.

“Many trees in Klamath Falls are already affected,” she said, “but some of them can be saved with appropriate treatment.”

One key tip: Well-watered birch trees seem to be better at resisting the bug. Sanchez observed that the worst-looking trees in Klamath Falls tended to be in the driest patches of ground, whereas those in moister areas, such as near watered lawns, were in better condition. 

“Most people don’t think about watering their mature trees,” she said. “But even mature landscape trees need water in areas with low rainfall. Birch particularly prefer moist soils; they’re not ideally suited to our climate. So your first line of defense against this pest is keeping your trees watered.”

Additional recommendations:

  • Before you do anything, assess the level of damage. Slightly or moderately damaged trees have a good chance of recovery. Severely damaged trees are probably a lost cause.
  • Before you spray, think about collateral damage to beneficial insects. Sprays, injections or drenches with systemic insecticides can be effective (although expensive), but these chemicals can also harm bees and other pollinators. Always follow label instructions, or hire a reputable licensed pesticide applicator.
  • Before you cut down a dead tree, think about how to dispose of the wood. Many pests, including bronze birch borer, lurk in downed wood. It’s okay to use it for firewood, but don’t transport it to another location, and don’t stack it near your neighbors’ healthy trees.  
  • Before you replant, consider a more-resistant birch species, or another species of tree altogether. Your Extension horticulturist can advise you.
Author: Gail Wells
Source: Nicole Sanchez