ANNAPOLIS, MD (June 30, 2025) – 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, as part of a pilot program to control black fly populations in the area. Treatment is scheduled to occur on Monday, June 30.
MDA will apply a Bacillus thuringiensis israelensis (Bti)-based larvicide via helicopter (Bell 206 Long Ranger, green and white paint scheme. Tail number is N641HA). Bti is a naturally occurring bacteria found in soils. It is a bacterial insecticide, not a chemical, and is only harmful to a very limited variety of organisms, including midge, black fly, and mosquito larvae and a few other aquatic Dipteran (flies) insects.
The treatment may cause temporary discoloration of the water, but it is completely nontoxic and is not harmful to humans, fish, crabs or other aquatic invertebrates. A fact sheet on Bti can be found here.
The Black Fly Suppression Pilot Program was established in 2016. For more information on the program, please visit MDA’s website.
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