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Welcome to Pennsylvania's Black Fly (Gnat) Suppression Website

The goal of the Pennsylvania Black Fly Suppression Program is to reduce adult black fly (gnat) populations to tolerable levels during the spring and summer recreational season using environmentally compatible methods. The program involves monitoring and treatment of approximately 1,800 miles of 48 Pennsylvania rivers and streams. DEP biologists and student interns conduct black fly monitoring, laboratory identification of samples, data entry, treatment operations and management of aerial spray contracts. Treatments are done by helicopter or hand applications, using Bacillus thuringiensis israelensis (Bti), a naturally occurring soil bacterium, to selectively target only four black fly species in the Simulium jenningsi species group that bother people. The program currently conducts control work in 37 participating counties with over 6.6 million residents, providing relief from adult black flies so residents and tourists can fully enjoy outdoor recreational activities in Pennsylvania.

Not sure if it's a black fly? Black flies are often referred to as gnats and are much smaller than the common house fly and closer to the size of a fruit fly. Black flies do not enter buildings or covered areas in any significant numbers. The gnats have the annoying habit of swarming around the heads of people outside on warm days and disrupt outdoor activities. Swatting them is useless as they are quick to return, often flying into people's eyes and sometimes delivering a painful and itchy bite. If you are encountering an issue with black flies (gnats) and would like to report it, you can follow the link and provide additional information.

If you have any questions or would like more information regarding the PA Black Fly Suppression Program and its operations in Pennsylvania, you can contact the Black Fly Coordinator listed under your county. Any other fly related issues or questions unrelated to black flies (gnats) should be directed to the Penn State Extension Office in your County .


Joining the Program

  1. Citizens report adult black fly (gnat) problems to county personnel or DEP.
  2. County commissioners must submit a written request to join the program to DEP.
  3. Black Fly Suppression Program personnel conduct an extensive one-year countywide survey of black fly larval, pupal and adult populations to determine the need for a control program.
  4. DEP provides results of the survey, data analysis, recommendations, and treatment cost estimates for the county and state, if a control program is recommended in the county.
  5. If county commissioners choose to participate, and adequate funding is available, a county agreement is signed and executed.
  6. DEP water pollution biologists begin monitoring and treating black fly populations the following season under a new or existing aerial application contract.

Program Funding

  1. State/county cost-sharing of treatment operations, as set forth in county agreement.
  2. All administrative and monitoring costs are paid entirely by DEP, at no cost to the county.
  3. Commonwealth funding for the program is achieved through an annual state budget appropriation.
  4. Counties are invoiced for the local share of the project at the end of each treatment season.

History of the Black Fly Program

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.

NAGFrom 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.