Bronze birch borer

What it is

Agrilus anxius, a wood-boring beetle

Trees affected

Birch, especially paper and European white birch

Where in our area

Lawn birches on valley lots and at Tahoe, about 4,000 to 6,500 feet

Key sign

Thinning from the top, D-shaped exit holes about 3/16 inch, bark ridges

Stress link

Borers prefer stressed or weakened trees

This borer is the most serious insect pest of paper birch, according to the USDA Forest Service, and Utah State University reports European white birch severely affected by borers in much of Utah. This page covers what to look for, which trees, what an arborist checks and when pruning or removal is the answer.

What does bronze birch borer damage look like?

The crown thins first at the top, with small branches in the upper canopy attacked first, as Utah State University describes. Leaves in the top are small and sparse, and branches die. On the trunk and branches, look for D-shaped exit holes, about 3/16 inch across, where adults have chewed out. Utah State says the larvae's galleries can show as raised bumps or ridges on the bark.

If the bark is peeled back, winding tunnels packed with sawdust spiral around the branch or trunk, University of New Hampshire Extension notes. Peeling bark off a standing tree is an arborist's job.

• Dieback from the top of the crown
• D-shaped exit holes
• Raised ridges on bark
• Small, sparse leaves

Which trees and which elevations?

Utah State lists paper birch as most susceptible, with silver and gray birch also susceptible and Monarch, Japanese white and river birch more resistant. New Hampshire Extension adds that gray, western, water and sweet birch can be attacked. The birches on our lots are the white-barked kinds on the white birch page, and quaking aspen is a different tree that the borer is not a leading pest of.

Elevation is a stress factor rather than a limit. Valley lots at Reno (4,505 feet) and Carson City (4,680 feet) get hot summers and about 7 to 9 inches of precipitation a year, which is hard on birches. Utah State suggests cooler, moister conditions may keep borers down, which is why Tahoe birches at 6,243 feet and up may be better off.

Why does a birch get attacked?

The Forest Service says the borer attacks injured, overmature or declining trees, and Utah State says borers prefer stressed or weakened trees, and drought and iron chlorosis are common stresses. A birch with a hot south exposure, dry soil or lawn-root competition is in the stressed group. See iron chlorosis and drought stress and dieback.

Adults are around from late May through August, peaking in July, with usually one generation per year; the larvae under the bark do the damage.

What does an arborist check?

The arborist checks how much of the crown is dead, whether the dieback is only at the top or has reached the lower crown, whether exit holes and galleries are on the trunk and what stress factors are present. They also look at neighboring birches.

• Percent of crown dead or thin
• Exit holes and galleries on trunk or limbs
• Soil, water and sun stress
• Nearby birches
• Fall zone

When is pruning or removal the answer?

University of New Hampshire Extension advises pruning and destroying dead or heavily damaged branches by early May. In practice that means a light infestation caught early, where the rest of the tree is green. Utah State puts adult beetles in flight from late May through August, which is why the New Hampshire advice is to finish pruning by early May.

Removal is the answer when the dieback has reached more than the top of the crown or when exit holes show on the main trunk. New Hampshire notes that once a tree is heavily infested, its chance of survival is low. A dead birch drops limbs; see tree removal and stump grinding.

What does Battle Born Tree Service do about bronze birch borer?

The free estimate is where this gets decided. We assess the tree in person, and the arborist's call is one of pruning out dead wood or removal. We hold a Nevada Business License (#20131580674) and are fully insured, and two of the eight crew members are ISA Certified Arborists (WE-11728A is one person's credential). We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved; we do not spray, inject, fertilize or apply products.