What is Birch Leafminer?

Birch leafminer, also known as Fenusa pusilla, is a common pest that affects birch trees. It is a small, black fly that lays its eggs on the leaves of birch trees. The larvae then hatch and burrow into the leaves, feeding on the tissue inside. This feeding activity causes the leaves to become discolored and distorted, ultimately leading to a decline in the health and appearance of the affected tree.

Life Cycle of Birch Leafminer

The life cycle of the birch leafminer begins when the adult flies emerge in the spring. These flies are typically active from April to May, depending on the climate. The female flies lay their eggs on the undersides of birch leaves, usually near the midrib. Each female can lay up to 100 eggs during her lifespan.

After the eggs are laid, they hatch within a few weeks, and the larvae begin to feed on the leaf tissue. The larvae are small, yellowish-white worms with black heads. They create tunnels or mines within the leaves as they feed, hence the name “leafminer.”

As the larvae continue to feed and grow, they go through several molts before reaching maturity. The feeding activity of the larvae causes the leaves to turn brown and become distorted. In severe infestations, the leaves may even drop prematurely.

Damage Caused by Birch Leafminer

The damage caused by birch leafminer can vary depending on the severity of the infestation and the health of the tree. In mild cases, the leaves may show small, discolored patches or blotches. In more severe cases, the leaves may become completely brown and shriveled.

Repeated infestations over several years can weaken the tree and make it more susceptible to other pests and diseases. The overall health and vigor of the tree may decline, and it may become more prone to branch dieback or even death in extreme cases.

Identification and Monitoring

Identifying birch leafminer infestations can be relatively easy, especially during the larval stage. Look for small, yellowish-white worms with black heads inside the leaves. These larvae create winding mines or tunnels within the leaf tissue.

Monitoring the population of birch leafminer can be done by inspecting the leaves regularly during the spring and early summer. Look for signs of leaf discoloration, distortion, or premature leaf drop. If you notice any of these symptoms, it is likely that your birch tree is infested with leafminers.

Prevention and Control

Preventing and controlling birch leafminer infestations can be challenging, but there are several strategies that can help minimize the damage:

1. Plant resistant birch tree varieties: Some birch tree varieties are more resistant to leafminer infestations than others. Consider planting resistant varieties, such as Betula nigra or Betula pendula ‘Dalecarlica’.

2. Maintain tree health: Keeping your birch tree healthy and vigorous can help it withstand leafminer attacks. Ensure proper watering, fertilization, and pruning practices to promote tree health.

3. Use insecticides: In severe infestations, insecticides may be necessary to control birch leafminers. Consult with a professional arborist or horticulturist to determine the most appropriate insecticide and application method for your specific situation.


In conclusion, birch leafminer is a common pest that can cause significant damage to birch trees. Understanding its life cycle, identifying the infestation, and implementing preventive and control measures are essential for maintaining the health and appearance of your birch trees. By following these guidelines, you can effectively manage birch leafminer infestations and ensure the longevity of your trees.