HARRISBURG — The Pennsylvania Department of Environmental Protection has begun aerial spraying on more than 1,600 stream miles in 37 Pennsylvania counties — including the Clarion River in Elk County — to control black flies, also referred to as gnats.

The first aerial spraying took place on April 20 on the Juniata River and Susquehanna River and several of their smaller tributaries. Other waterways will be treated as stream flows and weather conditions allow.

“This annual initiative reduces the nuisance of black flies so Pennsylvanians can enjoy our state’s waterways,” Acting DEP Secretary Patrick McDonnell said.

Pennsylvania rivers and streams will be monitored and treated as needed. Spraying activities will be performed both by helicopter and ground crews; their frequency will depend upon weather and biological conditions. Treatments cannot occur during periods of heavy rain or when water levels are high as high water conditions lower the effectiveness of the treatment.

DEP uses Bti, a naturally occurring bacterium, to treat the larval stage of four specific human pest black fly species. This bacterium degrades quickly in the environment and does not harm the aquatic ecosystem, birds, or other insects.

DEP notifies county and local emergency management officials prior to any spraying activity. Helicopters seen flying over waterways and ground crews using backpack sprayers off bridges are conducting control operations on behalf of DEP’s black fly program. Anyone concerned at the sight of a helicopter or ground personnel is encouraged to call their county emergency management office to verify that a black fly treatment is taking place in their area on that day.