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Bronze birch borer Courtesy Canadian Forest Service

Of the number of pests which are attracted the birch trees, the bronze birch borer (Agrilus anxius) is the most important as it is both lethal and difficult to control.  As an adult, the borer is a small bronze coloured beetle up to 2 inches (5cm) in length.  The damage, though, is not caused by the beetle itself but by the larvae which bore into the phloem and cambium layers after emerging from their eggs on the bark.  The borers' tunneling weakens and kills trees by interrupting the flow of sap.  The entire lifecycle of the borer is one year from egg to beetle.

Evidence of borer infestation is a progressive thinning of the crown of the tree beginning at the top.  Trees generally die after about two or three years, that is, after two or three infestations by the larvae.  If caught early enough, there are insecticides available to prevent new infestations of the borer but these will not kill larvae already active in the tree.  Insecticides such as dormant oil control only the overwintering egg stage.  Trees with advanced crown loss will not benefit from insecticide use and should be removed.

The good news is that healthy, well-situated, well-maintained birches are more resistant to the borer.  Females prefer to lay eggs in the sunlight and are less attracted to trees whose trunks are shaded.  The insects are also attracted to tree wounds and so care should be taken to avoid damaging the bark of trees.  Finally, trees under stress are more likely to be successful borer targets so it is important to ensure that birch trees receive adequate water.

In addition, not all birch varieties are equally susceptible to the bronze birch borer.  The European white birch and gray birch are considered most vulnerable, the paper or canoe birch less so and the river birch seems to be relatively resistant.

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