Back River Midges Nuisance Midge Control Program

The Baltimore County Department of Environmental Protection and Sustainability (DEPS) is working to address nuisance populations of swarming midges in Back River by conducting aerial midge suppression treatments on Back River. DEPS is also coordinating with Baltimore City Department of Public Works to treat midge larvae inside the Back River Wastewater Treatment Plant. The treatments use Bti, 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. 

June 2025 Update

Beginning in mid-May of 2025, excessive populations of swarming midges have affected waterfront areas outside of the original treatment area of upper Back River. Nuisance midges are disruptive, but not dangerous. These non-biting flies do not transmit disease, or cause serious harm to kids or pets. They can cause discomfort and irritation in eyes, ears, nose and mouth. 

This unprecedented infestation is caused by a combination of unusually cool temperatures this spring, followed by an influx of freshwater into tidal waters from recent heavy rains, further compounded by strong winds pushing more freshwater into these waterways. This has dropped salinity levels in normally brackish waterways to almost zero. Freshwater provides an extremely hospitable habitat for the primary species of midges that are present in our waters, allowing them to flourish to levels not previously seen in our area.

Tips to Reduce Midges on your property

  • Reduce outdoor lighting; try LED or yellow bulbs
  • Shine bright lights in unoccupied areas to draw midges away from homes
  • Avoid zap traps and insecticides, as treated areas will quickly repopulate
  • Hose down midge-covered surfaces with a mild, marine-safe detergent solution to ground midges permanently
  • Consider wearing a mask to cover your nose and mouth. 

County and State Response

In response to these unprecedented populations of swarming midges along Baltimore County’s waterfront this season, DEPS is modifying the mid-June helicopter treatment of Back River to double the treatment area to include the previous 1,200 acres of upper Back River plus 2,400 acres of lower Back River. Using this modified treatment methodology, the helicopter will fly 90-foot wide passes across the river rather than the previous 45-foot passes, using the same quantity of Bti. 

Please keep in mind that there are only two or three programs in the world that are treating this order of magnitude for midges. In close consultation with experts in industry and academia, we are learning as we go and making adjustments based on monitoring data and changing conditions. This remediation effort is a cooperative program in conjunction with the Maryland Department of Agriculture.

The current treatment area of Back River costs $1.3 million per year, funded jointly  by Maryland Department of Agriculture and Baltimore County. DEPS is expanding monitoring of midge larvae to include Middle River, and is exploring whether there may be additional funding from state or federal sources to cover costs of treatment in this and other Baltimore County tidal waterways where monitoring may indicate midge levels significantly above nuisance levels.

SPRAYING 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_ 2025 treatment area map. (Please note that the mid-June 2025 treatment area is expanded to include lower Back River.)

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(link is external). 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(link is external) or midge suppression(link is external) from the Maryland Department of Agriculture website.