Birches are beautiful and, in a home landscape, often short-lived. Most of their troubles come back to one thing: birches like cool, moist soil, and many yards are hot, dry and compacted. A stressed birch attracts the one pest that really matters, the bronze birch borer. Other problems look worse than they are. Here is how to tell them apart.
The short answer: thinning and dieback at the top of a white-barked birch, with raised ridges on the bark and D-shaped holes, is bronze birch borer and needs action now. Brown blotches inside the leaves in spring are birch leafminer, which is mostly cosmetic. Sticky leaves usually mean aphids. Yellow leaves with green veins on a river birch are iron chlorosis. Keeping the tree watered and mulched prevents most of the serious problems.
Which birch problem do you have?
| Sign | Likely cause | How serious |
|---|---|---|
| Upper crown thin, yellowing, branches dying from the top down | Bronze birch borer | Serious |
| Raised, bumpy ridges on bark; D-shaped holes about 1/8 to 3/16 inch | Bronze birch borer | Serious |
| Brown, papery blotches inside leaves in spring | Birch leafminer | Mostly cosmetic |
| Sticky leaves, black sooty film, curled new growth | Aphids | Minor |
| Leaves skeletonized, small white cocoons on twigs | Birch skeletonizer | Minor |
| Brown spots on leaves in wet weather | Leaf spot fungi | Minor |
| Yellow leaves with green veins (river birch) | Iron chlorosis | Moderate, slowly worsening |
Bronze birch borer
The bronze birch borer is a native metallic wood-boring beetle. The adults do little harm; the damage comes from the larvae tunneling under the bark. Iowa State University Extension and the Morton Arboretum describe the progression:
- The first sign is yellowing and thinning foliage at the very top of the tree.
- Branches then die back from the top down over successive years.
- Raised, bumpy ridges appear on the bark where larvae have tunneled underneath.
- Adults leave D-shaped exit holes when they emerge, usually in early summer.
The Morton Arboretum notes that, if left alone, an infestation can move down the tree and kill it in two to three years. Purdue University adds that trees receiving no corrective care usually die within a few years of the first dead branches.
Two things matter most: which birch you have and how stressed it is. The University of Minnesota lists European white birch and whitebarked Himalayan birch as preferred hosts, with paper and gray birch also attacked. River birch is rarely infested. The beetle is attracted to stressed trees and, according to Iowa State, does not survive well in a healthy one.
What to do:
- Water deeply during dry spells and keep a wide mulch ring over the roots.
- Prune out dead and dying branches. The University of New Hampshire advises completing this before adult beetles emerge in late spring.
- Confirm the borer before using any chemical treatment. Top-down dieback has other causes too.
- For valuable trees, a certified arborist can apply a labeled systemic insecticide at the right time. This works best before the tree has lost a large part of its crown.
Birch leafminer
Birch leafminer larvae feed inside the leaf between the upper and lower surfaces, leaving brown, blister-like blotches in spring. Gray, paper and European white birches are commonly hit. Damage looks dramatic in a heavy year, but healthy trees usually put out new leaves later in the season. Keeping the tree watered is the main defense.
Aphids and skeletonizers
Aphids suck sap from new growth, leaving sticky honeydew and, often, black sooty mold on leaves below. A strong spray of water helps on small trees, and natural predators usually catch up. The birch skeletonizer eats the leaf surface in late summer, leaving it lacy and brown. Both are usually minor on a healthy tree.
Leaf spots
In wet springs and summers, several fungi cause brown spots on birch leaves and some early leaf drop. Raking up fallen leaves and avoiding overhead watering is usually enough.
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River birch: watch the soil
River birch resists bronze birch borer but turns yellow in alkaline soil. If your river birch has yellow leaves with green veins, read why river birch leaves turn yellow.
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Keeping a birch healthy
Most birch problems are easier to prevent than to cure. Plant birches where their roots stay cool, ideally with shade on the soil and sun on the leaves, as on the north or east side of a building. Keep a wide ring of mulch over the roots, water deeply during dry spells and avoid wounding the bark with mowers and trimmers. If you are planting a new birch in a hot, dry spot, a river birch is the safer choice.
When to call a certified arborist
Call a certified arborist if more than a few branches at the top are dead, if you see D-shaped holes or bark ridges, or if a large birch near the house is declining. They can confirm the cause and tell you whether treatment is still worth it. The ISA Find an Arborist directory can help you find one.
For the next steps, see how to tell if a birch tree is dying and how much water birch trees need.
Sources
- Bronze Birch Borer, Iowa State University Extension
- Bronze Birch Borer, The Morton Arboretum
- Birch: Dead Branches in Canopy, University of Minnesota Extension
- Bronze Birch Borer (E-50), Purdue University Extension Entomology
- River Birch, University of Minnesota Extension
- Bronze Birch Borer (fact sheet), UNH Extension
Top photo: Whitney Cranshaw, Colorado State University, CC BY 3.0 us, via Wikimedia Commons