DEP Launches 2026 Black Fly Suppression Program Across 1,800 Miles of Waterways

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Department of Environmental Protection Launches 2026 Black Fly Suppression Program Across 1,800 Miles of Pennsylvania Waterways

April 13, 2026
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Harrisburg, PA – The Pennsylvania Department of Environmental Protection (DEP) has begun its annual Black Fly Suppression Program, treating 48 rivers and streams across more than 1,800 miles of waterways to reduce black fly populations ahead of peak outdoor recreation season.

“As the weather turns warmer, Pennsylvanians will be spending more time outdoors, and black flies can make that difficult,” said DEP Secretary Jessica Shirley. “This program helps reduce those nuisance populations so families can fully enjoy Pennsylvanian’s rivers, streams, and outdoor spaces.”

Now in its 41st year, the program uses targeted applications of Bacillus thuringiensis israelensis (Bti), a naturally occurring soil bacterium that specifically controls black fly larvae in flowing water. The treatment is applied by helicopter and ground crews in river and stream sections where black fly larvae are present.

Bti breaks down quickly in the environment and is not harmful to fish, birds, people, or other insects. Treatments focus exclusively on the larval stage of the black fly life cycle and are not conducted on land. DEP conducts aerial and ground-based applications throughout the spring and early summer, with timing and frequency dependent on weather and stream conditions.

Prior to any treatment, DEP notifies county and local emergency management officials. Residents may see helicopters flying low over waterways or crews applying treatments from bridges or in streams. Spray notifications can also be found on DEP’s Vector Management Program website. The published schedule is subject to change depending on weather and water conditions.

Visit the DEP website for more information about the DEP Black Fly Suppression program. Residents with concerns about black fly activity can report them directly to DEP through the Black Fly Suppression Website.

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