Birch Leaf Miner

Birch trees are frequently attacked by an insect called Birch leaf miner. This pest does not usually kill the tree, but repeated attacks over several years may weaken it and make the tree more susceptible to damage from other insects. Damage by this pest also makes the tree unattractive and reduces its ornamental value.

The adult is a small, black, wasp-like sawfly that lays eggs in slits in the leaf surfaces. The larvae hatch from the eggs, burrow into the leaves and begin feeding. Damage is usually first seen in June as pale green spots that appear on the surface of the leaves. Affected leaves can be separated to reveal small, whitish larvae along with their black droppings. As the summer progresses, these spots become larger and turn brown and papery. If the entire leaf becomes affected it may drop prematurely. Mature larvae emerge several weeks later and drop to the ground to pupate. The adults emerge approximately two weeks later and the cycle repeats itself.

Where birch leaf miner is a recurring problem, treatment is necessary every year. Contact insecticides are not effective for controlling leaf miner since the insect feeds inside the leaf. Systemic insecticides containing the active ingredient dimethoate are registered to control birch leaf miner. Dimethoate is sold under various trade names.

For the most effective results, the dimethoate should be applied after the leaves on the birch have fully opened usually by late May but this may vary with weather conditions. Applications done earlier than this time will not provide effective control for the entire season. Only one application per year is recommended.

As a precaution it is not advisable to use this treatment on newly transplanted birch trees as they may be injured.