Although native North American parasitoids do attack the birch leafminer, they haven't had much impact on this introduced sawfly's populations. In the 1970's European ichneumonid wasps were introduced in New Jersey, Delaware, Maryland, Pennsylvania, Massachusetts and Quebec. Lathrolestes nigricollis was one of two species that became established in several places and is now widespread throughout southern New England.
The effect of this parasitoid on the density of the birch leafminer in Massachusetts was evaluated over a 15 year period (1979-1995). At the original release site there was a decline in leaves mined by the first generation of the pest from 50-54% to under 3%.
By 1992, L. nigricollis had spread from a release site in western Massachusetts to the eastern side of the state, Rhode Island, Connecticut, eastern New York, and southern Vermont. Although birch leafminer densities are now acceptably low at sites close to the original release site, this is not true in all locations. The farther from the original release sites, the lower the parasitism and the higher the damage. Some sites, particularly roadside trees, still have high levels of mined leaves despite the presence of the parasitoid in the general area. This suggests that either it takes a while for this parasitoid to exert its full effect, or additional natural enemies may be needed.
This parasitoid might eventually spread naturally to the Midwest, or could be released here.
Van Driesche, R. G., R. Childs, R. A. Casagrande, and L. Tewksbury. 1997. Establishment, distribution, and impact in southern New England of Lathrolestes nigricollis (Thompson), an introduced parasitoid of the birch leafminer Fenusa pusilla (Lepeletier). Can. Entomol. 129: 601-611.
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