Nest desertion due to harassment by black flies (Simulium annulus and S. johannseni) during incubation has been a major factor inhibiting success of the reintroduced Eastern Migratory Population of whooping cranes (Grus americana). To avoid this problem, which was prevalent in the core reintroduction area in central Wisconsin, the Whooping Crane Eastern Partnership changed the primary reintroduction area to eastern Wisconsin in 2011. However, a 2010 assessment of black flies in that area had concentrated predominantly on the southern portion of the new area. In 2017-2018, we collected the first samples of black flies in Green Lake County, including a new primary reintroduction site on White River Marsh (WRM), by sweep-netting over taxidermic crane mounts on artificial nests. In 2017, peak mean numbers of S. johannseni per sample at WRM and Grand River Marsh were 3,077 (maximum 6,838) and 891specimens, respectively. Numbers of black flies of this magnitude (and lower) collected during sampling by the same technique have been associated with nest desertion at Necedah National Wildlife Refuge in the core area. Numbers of S. johannseni were much lower in Green Lake County in 2018, and S. annulus was not abundant in either year. In contrast, an additional survey of black flies at WRM in 2021 recorded numbers of S. annulus potentially large enough to affect whooping crane nesting. Multi-year studies of black flies at WRM and other new reintroduction sites, coordinated with monitoring of whooping crane nesting, are needed to ascertain the impact of black flies and implement plans to promote success of this whooping crane population
