disfigure leaves on some birch trees.
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| Blotch mines from the larvae of birch leafminer
disfigure leaves on some birch trees. |
Birch leafminer (Fenusa pusilla) is a common pest on many birch species in Wisconsin. It is not a native insect, but was accidentally introduced from Europe. Birch leafminer prefers gray, paper, and white birches, and is only rarely are found on black, yellow, European white, or river birches. Its activity will soon become apparent as the trees begin to leaf out beginning in mid-May (although the last two years birch leafminer activity has been very light). From a distance affected trees may have a scorched or blighted appearance that is often mistaken for a disease. The first sign of leafminer damage is a tiny water-soaked blotch on a leaf's upper surface. Eventually the tiny spot becomes a conspicuous blotch or blister on the leaf.
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| The adult birch leafminer. |
The tiny creature causing the blotched leaves is the immature stage of a sawfly in the Family Tenthredinidae. Sawflies are closely related to bees, ants, and wasps, but are plant feeders. The adult is a small, black winged insect with yellowish brown legs. It looks like a wide-waisted wasp about ¼ inch long, but it does not sting or do any damage to birch tree.
Birch leafminers overwinter as mature larvae in cells in the ground underneath trees and pupate in the spring. Adults emerge in early spring – about the same time common lilac is finishing its bloom and silver maple is dropping its seed – and begin depositing eggs individually in the soft, newly developing leaves. Vigorously growing birch trees are the most attractive to egg-laying females. They lay eggs in individual slits cut in the upper surface of developing leaves. When the eggs hatch after 7-10 days, the larvae begin to tunnel between upper and lower surfaces of host leaves. The flat larvae are whitish to light greenish-yellow with four black spots on the underside near the front. They and black fecal matter (frass) can easily be seen in the mines when infested leaves are held up to the light.
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| Larva in its mine. |
The larvae feed and develop for two to three weeks. When the larvae are mature they chew through the leaf surface, drop to the ground and build individual cells in which to pupate. In three weeks they emerge as adults to start the mating and feeding cycle over again. There are two or more generations of leafminers in Wisconsin each year (depending on the weather), but the first generation mining the first leaves of the season causes the greatest damage. When trees are defoliated by first generation leafminers, newly produced leaves are readily attacked by the next generation of leafminers. Later generations, if they occur at all, rarely cause damage because leaf surfaces are generally hardened by the time those adults appear and the females prefer to lay their eggs in soft, young tissue.
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| Mines of the birch leafminer. Photo by E. Bradford Walker, Vermont Department of Forests, Parks and Recreation, Waterbury, VT |
Birch leafminer injury first appears in late spring as irregular winding tunnels (mines) that gradually come together to form a single, large, hollowed-out blotch, or "scorched" area on the leaf surface. Mines are translucent green at first, then turn gray or brown. Eventually the entire leaf turns brown and drops off as damaged leaves dry out over the growing season. Normally the tops of trees are most affected. If populations are high enough, some trees can be almost completely browned from top to bottom. A healthy tree can usually lose part or nearly all of its leaves once without being seriously weakened. After a few seasons of continuous foliage injury, trees attacked by birch leafminers show visible symptoms of decline; sometimes they die. And these trees are more susceptible to other stress-induced problems, such as attack by bronze birch borer.
Planting resistant varieties such as river birch (Betula nigra), dahurian birch (Betula davurica), Monarch birch (B. maximowiczana), B. schmitii and B. castata can reduce problems from this insect.
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| Birch leafminer damage. |
Chemical control of the first generation is most important where this insect has been a pest in past seasons. If the first generation is controlled, damage for that season will be greatly reduced. Insecticides will provide good control only if adequate tree coverage is obtained. A residual insecticide can be used at budbreak to prevent adult egg laying. Systemic insecticides, such as acephate (Orthene) and dimethoate (Cygon), that kill larvae in newly developing mines, are particularly effective for birch leafminer control. Imidacloprid - Bayer Advanced Line for Gardeners is also a good systemic, but the slow uptake makes fall application more effective. Sprays to kill the larvae should be applied about the time of egg hatch, when small mines can be seen when the leaves are held up to the light. If the egg-laying period is extended, a second application seven to 10 days later may be necessary. Spray again in late June to early July to control the second generation if early-season damage is unusually high. Regardless of the product you choose to use, read the label and mix and apply according to the directions.
– Susan Mahr and Phil Pelletteri, University of Wisconsin - Madison
Brand names are listed only for reference. Such references are not intended as an endorsement by the University of Wisconsin - Extension over similar brands.
- Birch Leafminer – Ohio State University Extension Factsheet HYG-2035-91
- Birch Leafminer – University of Minnesota Extension bulletin FS-06134