DEP Begins 2025 Black Fly Suppression Program

The Pennsylvania Department of Environmental Protection has begun its annual Black Fly Suppression Program, monitoring and treating 48 rivers and streams spanning more than 1,800 miles.

“As the weather turns warmer, Pennsylvanians will be spending more time outdoors, and black flies (biting gnats) are a pest that can put a damper on an otherwise beautiful day,” said DEP Acting Secretary Jessica Shirley. “This annual treatment program reduces this nuisance so that Pennsylvanians and visitors can fully enjoy outdoor activities.”

The program involves aerial and backpack spraying to control nuisance black fly populations. Spraying activities will be performed on flowing sections of the river containing the pest species by both helicopter and DEP ground crews. Helicopters perform treatments by flying low over waterways and dispersing a brown liquid containing Bti.

Bti is a naturally occurring soil bacteria, used to treat the specific black fly species that is a pest to people. This bacterium quickly degrades in the environment, and does not harm fish, people, birds, or other insects. The frequency of the treatments will depend on weather and presence of the target species.

The PA Black Fly Suppression Program is entering its 40^th year of spray operations on Pennsylvania waterways. The program focuses on the larval stage of a black fly’s life cycle in rivers and does not conduct spray operations on land to control black fly adult populations.

Prior to any spraying activity, DEP notifies county and local emergency management officials. Helicopters may be seen spraying material over waterways and ground crews may be seen applying material from bridges and wading in streams. Anyone concerned at the sight of a helicopter or ground personnel conducting sprays is encouraged to call their county emergency management office to verify that a black fly treatment is taking place in their area. Spray notifications can also be found on the DEP’s Vector Management Program website by clicking the link 2025 Spray Notifications. The published schedule is subject to change depending on weather and water conditions.