Source: The University of
University of Connecticut
Extension. Photos: Cheryl
Moorehead, Whitney Cranshaw,
Colorado State University,
Fenusa pusilla (Lepeletier), are among the most common insects affecting gray, paper, river, and European white birches. The birch leafminer is not a native insect but one accidentally introduced from Europe. The leafminers feed inside the leaves, forming what is known as blotch mines. Partial or whole areas inside the leaf are consumed in blotch mines, which makes them different than serpentine leaf mines, which form meandering lines through the leaves.
Normally a healthy tree can lose part or nearly all of the current crop of leaves without being seriously weakened; but repeated losses, year after year, will weaken the tree, resulting in death, or will make it susceptible to the attacks of other insects, especially the bronze birch borer, which will hasten the death of the tree.
The facts about Birch Leafminer APPEARANCEThe adults are small, black, four-winged sawflies about 1/8-inch long. The name "sawfly" is derived from the saw-like egg-laying organ of the female. These insects are related to wasps. The larva are legless, worm-like insects. LIFE CYCLEThe adults overwinter in the soil and begin to emerge in early to mid-May when the leaves first start expanding. They congregate about birches, mate, and females lay their eggs in newly-developing terminal leaves. In seven to 10 days, the eggs hatch and the larvae begin feeding. The immature leafminers feed for about two weeks, then drop to the ground to develop into pupae. The leafminers remain as pupae about two to three weeks. The second-generation adults emerge in about 15-20 days to start the cycle again and lay eggs in newly developing leaves. The larvae again feed for about two weeks and then drop into the soil. They pupate and remain there until the following spring. DAMAGE
These immature larvae feed individually between the leaf surfaces, creating kidney-shaped mines. Early mines appear as light green or whitish discolorations on the leaves. At first, the mines 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. The areas of leaves that are consumed turn brown. Because people often do not see the early signs of birch leafminer feeding, it often appears the birch has suddenly dried up or become diseased.
Despite its diseased appearance, healthy, mature birch are not seriously injured by
first generation birch leafminers if feeding affects 30 percent of the leaves or less. For most birch, leafminer damage does not exceed 40 percent of the leaves. In some cases when trees are well watered, birch can tolerate up to 60 percent leaf damage. Birch are treated most often to protect the trees appearance and not because birch leafminers seriously damage them. CONTROLManagement of birch leafminer is usually unnecessary for the health of the tree, although treatment may be desired to protect its appearance.
The best time to manage birch leafminers is when larvae first hatch inside the leaves and begin to feed. Treatment at this time also minimizes damage to birch. However, this stage is inconspicuous and easily overlooked. It is important to anticipate when birch leafminers first begin mining leaves. Watch for evidence that they are in your trees before you begin control measures. Contact your local extension for recommended materials for your area.
One method to estimate
occurrence of first mines is to use plant phenology, i.e. a particular plant event. For birch leafminers, first mines appear approximately 10 days after birch bud break (when birch start to leaf out but before leaves are fully expanded). Although the exact number of days can vary, it should be fairly consistent each year.