The PA Black Fly Suppression Program provides relief to millions of Pennsylvania residents and tourists each summer. Black flies have been a recognized pest of humans and livestock in Pennsylvania since the 1970s. Black flies have been present in Pennsylvania since pre-Columbian times. There is anecdotal evidence of pest level populations of black flies from the late 1800s through present in some areas of Pennsylvania, and that black flies were present in pest numbers in some areas in the 1950s and 1960s. In many areas the black fly populations were present for many years before the program provided service. In other areas, such as acid affected streams where discharges have been eliminated or treated, black fly populations have developed to relatively high levels in a single season and have remained at those levels, unless affected by a pollution incident. The black fly populations have built to a high stable endemic population and remain at that level.
The recent expansion of black fly problems in Pennsylvania appears to parallel improvements in water quality. Many large waterways in Pennsylvania were severely degraded by the beginning of the twentieth century because of unregulated logging, acid-mine drainage, untreated sewage, industry, and urban sprawl. Individuals began to recognize the detrimental effects these impacts were having on the Commonwealth's aquatic ecosystems. An effort was made to reverse this trend. Over the last several decades, many streams and rivers began to show various degrees of recovery, including the return of diverse assemblages of macroinvertebrates and fish. This list included the return of many pollution-sensitive organisms. One such taxon which is recovering in vast numbers is the black fly (Diptera: Simuliidae). The larvae of this group spend the immature stages of their life in fast-flowing waters, feeding on minute particles that are filtered out of the water column. The adults emerge from their aquatic habitat and many species become serious pests of both humans and livestock because of their persistent biting and swarming behavior. As many of our state's waterways continue to show signs of improving water quality many systems will support a diverse black fly fauna and black fly-human conflicts will continue to rise.
From 1981 through 1983, citizens complained about adult black flies in the Harrisburg area. They formed a citizens group, Neighbors Against Gnats (NAG). These citizens met with elected officials and industry leaders in the area to seek funding for a community wide solution to their problem. They obtained radio, television and print media time to ask for donations to establish an area wide treatment program. In 1983, with the support of NAG and the Lower Dauphin County Council of Governments, Department of Environmental Resources staff contracted to have the first experimental treatment program. The project used helicopters to apply Bti to a section of the Susquehanna River and to a portion of the Conodoguinet and Swatara Creeks. The program proved to be successful in reducing adult black fly populations for a short time. In 1985, a larger program involving 107 miles of the Susquehanna River , Conodoguinet Creek and Swatara Creek began. This project also proved successful, but by midsummer, black flies were drifting into the area from the lower Juniata River basin. The program again expanded in 1986 to include not only the lower Juniata River, but a total of 535 stream miles in 17 counties in the Susquehanna and Allegheny River basins. In 1996, the program expanded into the Delaware River basin, and in 2001 into the Schuylkill River basin.