Only a half-inch long as an adult, the bronze birch borer started establishing itself in the Portland area about a decade ago. Arborists say the small black beetle has since made itself at home and is not leaving.
"The problem continues to get worse," says Peter Torres, the owner of Multnomah Tree Experts. "A lot of birches have died or partially died this summer, and many more will probably succumb next summer."
Homeowners have noticed.
"My husband and I have lived in the same house in Southwest Portland near Raleigh Hills for 42 years," Sally Coulter wrote in an e-mail. "We walk our dogs around the neighborhoods. On today's walk we noticed many large, mature birch trees either dead ... or showing dead portions at their top branches."
It's a problem that's not going away, says Terrill Collier, a certified arborist and owner of Collier Arbor Care. The only answer is prevention, which requires annual soil drenches of pesticide.
Although Torres owns a company that doesn't apply pesticides, he agrees.
"I tell people, if they want to save their trees, to call a pesticide applicator and chat with them." says the board-certified arborist. "If it's an important tree, they may want to protect it."
You better get with it, though. Once a tree starts to show symptoms, the chances of saving it decline with every dying branch.
If you choose not to have pesticides applied, keeping your trees healthy is key. Birch borers are drawn to unhealthy trees. Though so often planted in full sun in areas such as parking strips that typically have poor soil and no irrigation, birches prefer some shade, must be watered during dry periods and should be mulched to keep their shallow root system cool.
Another thing to think about: If you choose to plant a birch, consider a river birch, which are the most resistant to the borer. Though they're not white-barked, they're beautiful just the same. Their shaggy, red-brown bark peels in layers. Another option is to look for newer cultivars such as 'White Satin' and 'Whitespire Senior.'
HOW TO RECOGNIZE
First indication is yellowing, sparse foliage in the upper portions of the tree (crown). If the infestation is serious, branches and twigs begin to die. Decline and death usually take several years but can occur in a single hot, dry year.
A distinctive 1-inch D-shaped hole in the bark is made by the adults when they emerge. It's often accompanied by rust-colored sap.
Adult beetles, which feed on birch leaves but don't do much damage, mate in spring, usually May and June.
Females lay eggs in bark crevices, cracks and wounds.
Within 10 days to two weeks, larvae (white, segmented, legless grubs) hatch and bore into the tissue under the bark and start eating, creating tunnels (galleries) as they go.
The tunnels girdle branches, reducing the amount of food transported from the tree canopy to the roots. Eventually, roots begin to die, the tree is unable to take up enough water, and it begins to die.
Larvae keep feeding until fall. They overwinter in cells they build in their tunnels.
Larvae pupate into adults in spring, emerge from the tree and the cycle starts again.
SUSCEPTIBILITY TO BRONZE BIRCH BORER
European white birch (Betula pendula)
Young's weeping birch (Betula pendula 'Youngii')
White barked Himalayan birch (Betula utilis var. jacquemontii)
Paper birch, white birch, canoe birch (Betula papyrifera)
Gray birch (Betula populifolia)
Yellow birch (Betula alleghaniensis)
River birch (Betula nigra)
'Heritage' river birch (Betula nigra 'Heritage')
'Whitespire Senior' gray birch (Betula populifolia 'Whitespire Senior'*)
'Dura-Heat' river birch (Betula nigra 'Dura-Heat')
'White Satin' hybrid
*Originally misidentified as a Japanese white birch (Betula platyphylla var. japonica), it is still often mislabeled. The name 'Whitespire' was supposed to be applied only to those trees vegetatively propagated (seed-grown trees are not genetically identical to the original tree, which is now 53 years old and free of bronze birch borer). However, the name has been widely applied to seed-grown trees, too, so 'Whitespire Senior' is now used to designate cloned trees.
Sources: U.S. Department of Agriculture; University of Minnesota Extension Service; Dick Bocci, Carlton Plants; Pete Brentano, Brentano's Tree Farm; J. Frank Schmidt & Son nursery
Choose a birch that is resistant (see above).
Avoid compacted soils or areas of high traffic. Birch trees have shallow roots that can be easily damaged.
Maintain your tree properly. Birch trees need to be watered in dry weather, at least one deep watering per month from late June through September, says Steve Zetterlund, a certified arborist and supervisor of plant health care for General Tree Service. To check the moisture level, Zetterlund recommends sticking a large screwdriver into the soil near your birch. If it comes out with powdery soil, it's time to water.
Fertilize if needed. General Tree Service applies a 27-10-12 slow-release fertilizer once a year in fall. Zetterlund says the "slow-release" part should be taken seriously. "You don't want to push growth and then have no nitrogen the rest of the year," he says. "You want a low amount throughout the year."
Don't prune birch trees May through August when adult borers may be in flight. Research shows females are attracted to fresh cuts.
If you have a birch tree that's highly susceptible, you can use a soil drench once a year as a preventive. If you choose to do it yourself rather than have a professional service, be sure to
By the time a homeowner notices a problem, it's usually too late. Once a tree has 30 percent dieback, it's unlikely to recover and treatment is expensive, says Terrill Collier of Collier Arbor Care. Severely infected trees should be removed.
TIPS FOR HIRING AN ARBORIST
- Be sure the firm is bonded, licensed and insured.
- Ask if they are certified by the International Society of Arborists.
- Ask for references and check them.
- Get written bids or estimates from more than one tree care firm.
- Check the Pacific Northwest Chapter of ISA website (pnwisa.org) for a membership directory and more information.
In Portland and most other municipalities, a permit is required to plant, prune or remove a street tree. For information and an application, call 503-823-4489 or go to the city of Portland website, tiny.cc/j36wv.