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Birch leafminer damage Courtesy Canadian Forest Service	Advanced leafminer damage Courtesy US Forest Service

A troublesome pest for a birch tree is the Birch Leaf Miner (Fenusa pusilla) . Like the lethal bronze birch borer, it is the larvae of the birch leaf miner which causes damage by burrowing into birch leaves in search of nutrients. The first indications of birch miner infestation are small green spots on the leaf surface, usually noticeable in May or June, which develop into ugly brown splotches. A birch leaf miner infestation rarely kills birch trees on its own but it is disfiguring and can weaken a tree making it susceptible to borer attack.

The leaf miner season begins in late April to mid May with eggs laid in slits on the upper surfaces of birch leaves. The larvae, small, flat whitish grubs (see image at left) in appearance, emerge in late May or June to begin feeding on leaves. In June or July, the larvae drop onto the ground to pupate.  The adults, small black fly-like insects called a sawflies, emerging two weeks later. There can be up to four generations per season depending on the climate.

Birch leaf miners can be effectively controlled by systemic insecticides. Consult a tree care specialist for details on the kinds of insecticide appropriate for your area. And, as always, be sure to follow directions carefully.

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