One of the major pests of birch trees in the northeast is the birch leafminer. The name "leafminer" is derived from the larval habit of feeding, or mining, the plant tissues between the upper and lower surfaces of birch leaves. The trees that are most likely to be attacked are gray, paper and European white birch. The birch leafminer is not a native insect; it was accidentally introduced from Europe.

Description:

Birch leafminers are related to wasps. The adults are small, black, four-winged sawflies about 3 mm (1/8 inch) long. The name "sawfly" is derived from the saw-like egg-laying organ of the female.

Life Cycle:

The adults overwinter in the soil and begin to emerge in early to mid-May. They congregate on birches and mate; females lay their eggs in newly-developing leaves. The eggs hatch in seven to ten days and the larvae begin feeding, making mines which are small and somewhat serpentine in form. As the larvae grow, feeding increases and the serpentine mines often run together to form the characteristic blotches and blisters on the birch. The larvae mature in one to two weeks, drop to the ground, and enter the soil to pupate. New adults appear in about 15 to 20 days to start the cycle over again. During a normal year, a life cycle can be completed in five to six weeks. There are three generations per year. Only the first generation is considered destructive, because adult females prefer to lay their eggs in soft, young tissue and there are very few new leaves available late in the season.

Damage:

Leaves that are attacked soon turn brown. Affected trees, seen from a distance, have a scorched or blighted appearance that is often mistaken for a disease. Leaves examined at closer range have a blotched or blistered appearance. Under normal conditions, the tops of trees are often the most seriously affected portions, although the entire tree can be affected. A healthy tree can normally lose part or nearly all of the current crop of leaves without being seriously weakened. Repeated losses, however, year after year, will weaken the tree and may result in death.

Control:

This insect is normally controlled by parasitic wasps in its native Europe. URI researchers have established one of these European parasites in Rhode Island; the wasp has spread throughout the entire state and is providing a degree of control that is increasing annually. Before spraying insecticides against the birch leafminer, it may be worth watching your trees for a season or two to see if the pest comes under natural control.

If you do decide to treat, realize that sprays kill the parasites as well as the pests. Treat only the first generation by spraying in early May, just after the leaves unfold and adults are just beginning egg laying. Several insecticides which are registered and effective against this pest are available at garden centers. Later treatments are unnecessary in Rhode Island.

By Dr. Richard A. Casagrande and the University of Connecticut Integrated Pest Management Program, 1999