HARRISBURG, PA — The Pennsylvania Department of Environmental Protection (DEP) has commenced its annual Black Fly Suppression Program to control nuisance black fly populations across the state. The initiative, now in its 40th year, will cover 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 are a pest that can put a damper on an otherwise beautiful day,” said Acting Secretary Jessica Shirley. “This annual treatment program reduces this nuisance so that Pennsylvanians and visitors can fully enjoy outdoor activities.”
The control efforts involve targeted aerial and backpack spraying to address the larval stage of the black flies’ lifecycle in flowing sections of rivers. Helicopters and DEP ground crews will apply treatments directly over waterways using a brown liquid containing Bacillus thuringiensis israelensis (Bti), a naturally occurring soil bacterium. Bti specifically targets black fly species while posing no harm to humans, fish, birds, or other insects. The treatment degrades quickly in the environment, ensuring safety for recreational activities.
Treatment frequency will be adjusted based on weather conditions and pest activity. DEP coordinates with local and county emergency officials to notify communities of spray operations. Helicopters flying low over waterways and ground crews applying sprays from bridges or wading in streams may be visible during the program. Residents can confirm treatment activities by contacting their county emergency management office.
Spray notifications are available on the DEP’s Vector Management Program website under 2025 Spray Notifications. The schedule may change based on weather and water conditions.
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