Back River Midges Nuisance Midge Control Program

The Baltimore County Department of Environmental Protection and Sustainability (EPS) is working to address nuisance populations of swarming midges in Back River by conducting aerial midge suppression treatments on the upper section of Back River. EPS is also coordinating with Baltimore City Department of Public Works to treat midge larvae inside the Back River Wastewater Treatment Plant. The treatments use a nontoxic, naturally-occurring bacterial larvacide.

Midges are small nonbiting aquatic flies that often swarm near water or marshy areas where they breed. They are not dangerous, but can create a nuisance for homeowners, boaters, businesses and recreational activities.

The goal is not to eliminate all adult midges, but to reduce their numbers to tolerable levels. The 2022 and 2023 programs were successful at consistently reducing midges to well below nuisance level in the treated area.

SPRAYING DATES AND LOGISTICS

Trained, certified technicians conduct the spraying in accordance with EPS regulations and industry best practices, spraying a Bacillus thuringiensis israelensis (Bti)-based larvacide from a low-flying helicopter during daylight hours an estimated five times each year from spring through fall. 

The treatment area is a 1200-acre open water section of upper Back River. The flight pattern can vary slightly, but is generally from Walnut Point northwest to Powells Point and the I-695 crossing, including portions of Deep Creek, Duck Creek, and Bread and Cheese Creek, as shown in the 2024 treatment area map.

The County’s contractor, Helicopter Applicators Inc., deploys a black-colored Bell helicopter to apply the Bti treatment during favorable weather conditions and actively avoids boaters and human activity in and around the water. The helicopter flies approximately 50 miles per hour at a height of approximately 20 feet above the water.

EPS staff monitors the midge larval population in the substrate and repeats applications depending on need as indicated by biological monitoring results. Spraying dates are are dependent on wind and weather conditions and tidal movement. EPS monitors the area for midge larvae abundance prior to and after treatment with Bti. The Baltimore County Nuisance Midge Control Project is funded in part by the Maryland Department of Agriculture.

ABOUT BTI AND HOW IT WORKS TO REDUCE MIDGES

Bti is a naturally-occurring bacteria that is found in soil, and is only harmful to certain organisms, specifically midge, black fly and mosquito larvae and a few other aquatic flies. It does not harm people, fish, crabs or other aquatic invertebrates. It is a spore-forming bacterium that specifically targets midge larvae at particular times in their life cycles. 

Once ingested by the midge larvae, it produces endotoxins that cause mortality. Research demonstrates that Bti is nontoxic to other species, people, mammals, birds, fish and most invertebrates, when properly applied. The County’s contractor follows all applicable regulations and industry best practices. 

REPORT MIDGE ISSUES

Members of the public are encouraged to report hotspots of nuisance midge populations by entering their location on the County's Environmental Reporter online mapping tool. This crowdsourcing information tool provides specific data to help determine locations of nuisance midge populations that provide feedback to help inform future mitigation strategies by Baltimore County. 

Learn More

Learn more about Bti or midge suppression from the Maryland Department of Agriculture website.