Birch Leafminers are sawflies, which are closely related to bees and wasps, and the most common insect pest of birch trees in North America. Its preferred host is paper, gray European white and cut leaf birches with black, yellow and river birches being less susceptible. Tree mortality is rare but heavy attacks by this miner stresses the tree by reducing the leaf area available for photosynthesis. Birch trees that lose more than half of their foliage are more susceptible to attack from other insects, especially bronze birch borer.
Adults emerge in May, depending on temperature and humidity and are small, black and fly like. The adults can easily be seen hovering around host trees or moving around foliage. Females deposit their eggs singly in slits cut in the central areas of young leaves, usually near the tips of branches. The eggs hatch into legless, worm-like larvae and feed or “mine” in the tissue between the leaf surfaces producing blistered or translucent spots on the leaves. The areas on the leaves that are consumed turn brown. This browning is caused by the outer layers of the leaf drying out after the leaf miner larva has consumed the green tissue between the outer layer on the leaf.
The larvae feed for about two weeks, enlarging the mine until most of the leaf is damaged. Mature larvae chew an exit hole in the leafmine and drop to the ground where they enter the soil layer to develop into pupae. A second adult generation emerges in late June and a third generation may occur in mid August. Mature leafminer larvae overwinter in the soil below the tree and pupate again the following spring.
Just remember to keep a mindful eye on your trees, shrubs and plants. If you see something that doesn’t look “normal”, contact a Branch Tree & Landscape Service certified arborist.
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