The birch leaf miner is a tiny wasp-like sawfly that feeds between the surfaces of birch leaves. Repeated attacks over several years may weaken a tree, making it more susceptible to damage from other insects.
Identification and Life Cycle
The birch leaf miner overwinters as mature larvae in the soil beneath a tree. In early spring, adults emerge, mate and lay eggs which are inserted into the new foliage. In 7 to 10 days, eggs hatch into larvae which feed for 2 to 3 weeks between the leaf surfaces. Larvae and black fecal matter can be seen when infested foliage is held up to light. Mature larvae cut holes through the leaf and drop to the ground, where they build a resting (pupal) cell. First generation adults emerge 2 or 3 weeks later and the cycle then repeats.
Description of Damage
At the beginning of summer, pale green spots appear on the surface of leaves, caused by larvae feeding within them. As summer progresses, these spots become larger and turn brown and papery. If an entire leaf is affected, it may drop prematurely.
Controlling birch leaf miner is an annual challenge for many gardeners. In early April and mid-June, apply a systemic insecticide such as Bayer Advanced Tree and Shrub to the soil around the tree, making sure to water thoroughly. In late April and July, when the tree’s leaves are half-expanded, an insecticide spray such as Sevin or Ortho Systemic Insect Spray can help control young larvae. Apply these products according to label instructions.