REGIONAL— Ever wonder why birch trees frequently end up with unsightly brown blotches on their leaves? It’s a common occurrence in the North Country and the most common cause is the larval stage of an insect known as the birch leafminer.
The adult form of this insect is a tiny, black, fly-like insect that lays a single egg at a time in slits that it makes in birch leaves. The eggs hatch into tiny worm-like insects which feed between the surfaces of a birch leaf, creating a hollowed-out portion of leaf, which eventually turns brown. If you spot this at the right time, and hold a leaf up to light, you can often see the wiggling larva, and its droppings, inside the leaf. After about two to three weeks of feeding, the larva chews its way out from inside the leaf and drops to the ground, where it digs into the soil before entering the pupal stage. Depending on the time of year, the pupa will either develop into an adult in about two weeks or it will overwinter and come out as an adult in late May or early June, just about the time that birch leaves are forming.
While the damage from leafminers can be unsightly, particularly for birches growing in your yard, they typically don’t do any permanent damage to birch trees, particularly when the trees have sufficient water.