ANNAPOLIS, MD (May 11, 2026) – The Maryland Department of Agriculture, in partnership with the Department of Natural Resources, will treat a targeted area of the Potomac River near Harpers Ferry, West Virginia to control black fly populations in the area. Treatment is scheduled to occur on Tuesday, May 12, 2026 between 9:00 a.m. – 10:00 a.m. ET.
A green-and-gold Bell 206 Long Ranger helicopter will apply a Bacillus thuringiensis israelensis (Bti)-based larvicide throughout the targeted area. The helicopter’s tail number is N637HA. Bti is a bacterial insecticide, not a chemical, that is naturally-occurring in soils. It is harmful to a limited variety of organisms, including midges, black flies, mosquito larvae and a few other species of aquatic insects.
The treatment may cause a temporary discoloration of the water but is harmless and will dissipate naturally. The treatment will not impact drinking water supplies, fish, crabs or other aquatic organisms outside the targeted insect groups.
The Black Fly Program was established in 2016 in response to resident concerns in Washington County. Additional information about the program, including further details about Bti, is available on the Maryland Department of Agriculture’s Black Fly Program website.
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