The bronze birch borer (Agrilus anxius) has been a serious pest of ornamental birches for many years. The first sign of borer attack is usually the die back of the uppermost branches of the tree. Over the following 2 or 3 years, the tree will gradually decline and die. Vein-like ridges or swollen bands occur in the bark and trunk. The presence of "D" shaped holes indicates where the adult borer emerged from the wood.

European white birch (Betula pendula) and its cutleaf varieties are especially susceptible to borer attack. Paper or canoe birch (Betula papyrifera), yellow (Betula alleghaniensis) and gray (Betula populifolia) birch are less susceptible. River birch (Betula nigra) appears to be resistant to borer attack.

 Adult borers emerge from infested trees in early to mid-June and may continue to emerge for another 5 to 6 weeks. The adult beetles are attracted to the sunny side of trees for feeding and egg laying. They lay their eggs in cracks and crevices and in areas that have been recently injured in some manner. The young hatch in about two weeks and begin to tunnel into the wood.

The tunnels behind active feeding larvae become packed with excrement and wood particles which turn a dark brown. They feed through the following fall and spend the winter in the tree. In April or May, they pupate and emerge as adults. There is one generation per year in the Midwest.

Resistant Trees: Perhaps the best control is to plant resistant birches. European white birch is extremely susceptible to this pest. As a non-native species, this tree often becomes stressed during the hot, dry summers. This makes them more attractive to the borer. The native gray birch is also susceptible when planted in urban/suburban sites.

 Paper birch seems to have fewer problems with borer. It is a white birch species but has a slightly different growth habit than European white birch.

 River birch is considered resistant to borer. However, it does not have the white bark but does have a salmon-pink papery bark. Also, this tree tends to get larger than the white barked types. The variety, 'Heritage', has lighter bark which more closely resembles the paper birch.

 Several white birch species are touted as resistant to the borer but most have not been tested widely.

Cultural Controls: Proper fertilizing and watering can help minimize the attack of borers. Deep watering during hot, dry weather is very important.

Also, it is important to keep a circle of mulch around the trunk of trees in the landscape. This prevents damage from lawnmowers and weed whips that can be prime entry points for the borers.

Chemical Treatment: Chemical control is difficult, if not impossible, once the larvae have entered the trunk itself. Current strategy is to spray the foliage and bark with an insecticide to control the egg laying adults or newly hatched larvae before they enter the bark.

Apply the first spray to the bark and foliage about the first week of June. Two additional sprays at 3 week intervals are needed to cover the 6 week flying period of the adults.  

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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