|Van Driesche, Roy|
Submitted to: Florida Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2009
Publication Date: 6/1/2009
Citation: Casagrande, R., Van Driesche, R.G., Mayer, M., Fuester, R.W., Gilrein, D.O., Tewksury, L. 2009. Biological Control of Fenusa pusilla (Birch Leafminer) (Hymenoptera: Tenthredinidae) in the Northeastern United States: A Thirty-Four Year Perspective on Efficacy. Florida Entomologist 92(2):243-247 Interpretive Summary: The birch leafminer is an accidentally introduced sawfly from Europe that attacks most species of birch, but gray birch and paper birch are the most susceptible. Defoliation by the birch leafminer is unsightly and may predispose the trees to attack by the bronze birch borer. Defoliation also permits excess heating of the soil by direct sunshine, resulting in high temperatures that can kill roots and lead to birch dieback. The larvae feed on tissue between the leaf surfaces, creating irregular brown, dry patches on the leaves. Full-grown larvae chew exit holes in the leaves and then drop to the ground, where they hibernate in the soil. First generation adults emerge in late April or early May. There may be 3-4 generations per year. Because the birch leafminer is not a pest in Europe where it originated, a search for natural enemies was made there and the most promising candidates, four species of small parasitic wasps, were imported and released in the northeastern U.S. and adjacent parts of Canada during the 1970’s and 1980’s. One species of wasp became established, and preliminary studies of its impact on the pest were conducted in the 1980’s and 1990’s. ARS and cooperating scientists made surveys in seven states (MA, CT, RI, NY, PA, NJ, DE) in 2007, compiling data on the birch leafminer population levels (expressed as % of leaves mined in spring) and parasitism. Survey results show that the pest has declined to barely detectable levels in five states (MA, CT, RI, NY, PA) but that in southern NJ, the pest remains abundant (ca 50% leaves mined) despite substantial parasitism levels. We conclude that the project has been completely successful in much of the northeastern USA, but that there is a southern limit to efficacy in central New Jersey and points south. The reason for this is not known, but possible causes are discussed.
Technical Abstract: Parasitoid releases against Fenusa pusilla (Lepeletier) (Hymenoptera: Tenthredinidae) in eastern North America began in 1974, with releases in eastern Canada, followed by others in the Middle Atlantic States and New England. Of four parasitoids released, only one – the ichneumonid Lathrolestes nigricollis (Thompson) – established and spread widely. Studies of its preliminary impacts were made in several locations in the 1980s and 1990s, but full impact of the parasitoid on host density was not yet achieved in that period. Here we report results of surveys in seven states (MA, CT, RI, NY, PA, NJ, DE) in 2007 documenting the current birch leafminer levels (as % of leaves mined in spring) and parasitism. Survey results show that the pest has declined dramatically to barely detectable levels in five states (MA, CT, RI, NY, PA) but that in southern NJ, the pest remains abundant (ca 50% leaves mined) despite significant parasitism levels. Survey results, in context with previous evaluations made when populations were still declining, show that the project has been completely successful in much of the northeastern USA, but that there is a southern limit to efficacy in mid-New Jersey. Possible reasons for lack of control in this area, in contrast to high levels of control elsewhere, are discussed.