Saturday, 21 June 2008

Bored to Death!

There are few trees that can rival white birch for its four season beauty in the landscape.

There are also few trees that can be so heartbreakingly disappointing when they suddenly fail. Growing up my parents had a large European white birch that gracefully dominated the front yard of our house. Every spring we would dutifully treat it to prevent bark borers.

One by one the birches in our neighborhood were removed after the slow process of dying back from the top down. Last year we took the birch down, not because of borers, it had gown so large that it no longer fit the landscape and was mechanically unstable.

The nemesis of most white birch trees is the bronze birch borer, Agrilus anxius, this North American beetle has caused extensive die offs of native birches in the past.

Most white barked birch trees are susceptible. European variety's are so prone to attack that they are not recommended for planting. In the wild birch trees prefer moist north facing slopes that allow them to have full sun on the crown of the tree and shade to keep the roots cool. This is a far cry from conditions most birches face in urban and suburban landscapes. Most birches in peoples yards are under stress. Stress makes birch trees attractive to bark beetle infestation.

The beetles lay their eggs on the rough patches of bark normally near the attachment point of branches. Eggs hatch and the larva tunnel into the cambium layer excavating zig zag tunnels around the tree cutting off the flow of sap as they destroy the trees vascular system.

Healthy trees may be able to wall the larva off by forming a callus layer around the insect and preventing it from excavating the tree tissue. More often the tree is stressed and slow to respond to the invasion and loses the battle. The callus tissue forms a lumpy mound over the larval tunnel that can be seen on the outside of the bark. The process from egg to adult can take a year or more and is dictated by the weather. Cold conditions slow the process down.

The result is active adult emergence and egg laying throughout the summer months starting in early June.

If you see the tops of your birch trees dying back you may have birch borers. Close examination of the dead branches will probably reveal raised tunnels under the bark can dark “D” shaped exit holes on the bark. Pruning out infested branches to below areas with any visible activity will be helpful. There are treatments available that can be applied to the bark or systemically to the root zone to limit the impact of the borers. Heavily infested trees should be removed from the area to prevent infestation of adjacent trees. Maintaining vigor and reducing stress factors present will be of great benefit. Properly mulching the area below the tree will help reduce root temperature and increase the vitality of existing plantings. Acting before you have any signs of borers will be of greater benefit than trying to catch up once they are present. The long term key to preventing birch decline is to chose resistant species and maintain the vigor of those that are susceptible. Contact your certified arborist for recommendations on birch trees to plant as well as maintenance regimes for your existing birches.

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