Birch leaf miner (Fenusa pusilla) is a common pest of birch trees in our area. It invests trees in the early spring and eats between the layers of the leaf causing them to turn brown and drop off early.

The main hosts of this insect are the paper, gray, European white and cutleaf birches. Black, yellow and river birch seem to be less susceptible to attack.

Birch leaf miner adults are a black sawfly about 1/8 inch long. They emerge from the ground in early May when the leaves are about half to fully expanded. Often, the adults may be seen hovering around the tree or moving around on the foliage.

Females insert their eggs into the upper surface of the leaf. Eggs hatch about 7 to 10 days later and the larvae begin to feed or "mine" in the tissue between the upper and lower leaf surfaces. This produces the characteristic blistering or translucent spots on the leaves.

Initially, larvae feed singly, but as they develop, nearby mines on the same leaf may join together. This creates larger hollow areas in the leaf. The larvae feed for about two weeks and then chew an exit hole. They drop to the ground and pupate for several weeks.

A second adult generation emerges from the ground in late June to early July but damage is usually restricted to the newly developing leaves on the top of the tree. A third generation may occur in late August. Mature larvae spend the winter in the ground and emerge the following April.

Even with total defoliation during extremely high populations, the leaf miner rarely causes the death of birch trees. However, a heavy attack that causes the loss of 50% or more of the leaves causes a stress to the tree. Stressed trees are weakened and are much more susceptible to attack by the bronze birch borer which may eventually kill the tree.

 

Cultural Controls: Keeping birch trees in good vigor through proper fertilization and watering is the first line of defense. Birches do not tolerate drought well so watering during the hot, dry periods of the summer may be the single most important cultural approach.

Chemical Treatment: The most serious damage is done by the first generation so this is the time to implement a chemical control alternative. If the first generation is controlled, subsequent rounds do little additional damage.

A series of preventative sprays may be effective for controlling damage on valuable specimens. About early to mid-May as the new leaves unfold from the buds, observe the leaves. When small "mines" begin to appear in the leaves, make an application of an insecticide labeled for this problem. Two applications at 10 day intervals are usually sufficient.

There are also systemic insecticides that may be applied to the root zone or injected directly into the tree. Check with a commercial pesticide applicator for these options.

Commercial spray companies may soil inject a product called Merit in the fall for control of leaf miner.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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