The bronze birch borer has been a problem for over 25 years in eastern Washington and parts of Oregon, especially among ornamental birch species. The borer is especially effective at causing birch mortality when precipitation has been low, so more infestations are likely to appear in the spring of 2013.
Vulnerable Birch Species
Imported varieties of birch are the most susceptible to borer attack. The European white birch and the white-barked Himalayan birch are the most vulnerable. Paper birch, gray birch, sweet birch and yellow birch are slightly less vulnerable. River birch is rarely attacked.
Conditions That Make a Birch Tree Vulnerable
Birch trees share a common trait—they are shallow rooting trees that prefer cool, moist growing conditions. Dry weather can injure the root system very quickly. This makes watering and mulching any birch tree very important during a drought.
Tent caterpillars or birch leafminers are stressors that can weaken a birch as well. A birch may also be infected by a leaf fungus that reduces the number of chlorophyll producing cells, even if leaf loss does not occur. In either case, the loss of the chlorophyll producing cells in the leaves weakens the tree right down to its roots.
Most trees can tolerate significant leaf loss because of their size and energy stores, yet if this happens year after year, it can lead to severe weakening of the tree—something that attracts adult bronze birch borers.
Preventative Tree Care Steps to Protect Your Trees
1. Keep your tree healthy.
This is the first strategy you must put in place. You’ll be rewarded for providing water at the right times, trimming out dead wood, and pruning to open up the tree’s interior to air. Nurture the health of soil organisms and the earthworm population. Provide organic nutrients.
Healthy birch trees have the ability to defend themselves against the birch borer larva. They will produce a callus around the larva which prevents it from expanding its feeding gallery. This protective reaction shows up as raised zig-zag patches under the bark.
2. Watch for signs of an infestation.
Repeated exposure to larva may eventually weaken the tree, so it is wise to watch for signs of an infestation. If a tree shows all of the following symptoms, it needs to be cut down and destroyed to reduce the adult borer population.
- Yellowing of foliage in the upper crown of the tree
- Dieback of the upper branches of the tree
- D-shaped exit holes in the bark, often stained with rust colored sap
A tree that has not started showing dieback and exit holes may be saved by proper care, such as pruning away infected wood, watering and mulching.
3. Prune during dormancy or before full leaf expansion.
An infected tree may contain larvae at every stage of development. Typical development in Washington and Oregon’s colder climate occurs over two years. If the tree has been weakened, this cycle may accelerate to one year.
The smaller larvae stop feeding during the winter, and then resume their burrowing through the phloem, the layer that transports nutrients from the canopy to the roots. Mature larvae pupate during the winter and immerge as adult beetles from May through late June.
When pruning takes place during dormancy or before the weather has warmed up enough for the leaves to expand fully, some of those future adult beetles and immature larvae are removed. This is especially true when dead and dying branches are pruned below the area of apparent damage.
Proper sanitation is essential after pruning. Don’t leave branches lying around. It can take some time for the branches to die, allowing some adult beetles to emerge.
Even infected trees may be saved if you take action early, before the tree has shown significant dieback. Death by the bronze birch borer is not inevitable. You can do something to reduce the risks.
These tree care tips are provided by Westcoast Tree Care Inc. Promoting the beauty and health of trees throughout Eastern Washington and Oregon, we help you maintain the safety and beauty of your home.
Westcoast Tree Care is a professional arboricultural company whose professional arborists are certified through the International Society of Arboriculture (ISA), the Pacific Northwest Chapter