The black fly, sometimes called a buffalo gnat, is a small fly that only grows to 1/8 inch in length. It is common near streams and rivers because its larvae require fast-moving water to develop. The larvae have small brushes on the side of the head that gather food from the water that rushes by as they cling to debris. In the larval stage, they cannot gather food independently. Many areas of the country are using bacillus thuringiensis israelensis, or BTI, programs to control black fly larvae without adverse environmental consequences.
Remove brush and debris from the stream or creek where you have problems with black flies. The larvae require debris in the stream to hold onto in order to get food to develop. When you clear the debris you remove much of the habitat they need to grow to the adult stage.
Check with the local conservation department to make sure there is no local regulation against the use of BTI cakes, commonly called mosquito dunks, in the stream. You can find mosquito dunks in the pest control sections of many hardware and home improvement stores.
Put the mosquito dunk BTI cakes within 50-100 feet of each other in the stream, provided they are not banned in your area. Begin putting them out about three weeks before the beginning of black fly season in your area. Black fly season can vary dramatically depending on your location, so check with local authorities if you are not sure on the best time to start. Make sure to place the cakes in various locations throughout the stream so that all the water supply is treated. The cakes should take about four weeks to dissolve completely, but check every couple of weeks. When they are completely dissolved, reapply cakes until black fly season is over.
While treating streams near your property can reduce the problem significantly, a regional or county program is the only way to control a large population of black flies. Once they emerge as adults, black flies have a range of several miles. If there is no county or regional control program, suggest to local conservation authorities that they begin a regional BTI control program. If the black fly problem is widespread, coordinate with neighbours on the same stream to help control the problem locally. The female black fly is a bloodsucker, most often feeding on birds, but she will bite humans. Pyrethrin-based insect foggers biodegrade quickly and provide effective temporary relief from adult black flies. Most major insect-repellent manufacturers make Pyrethrin-based products.
Temporary damning can be used to kill larvae and reduce black fly populations, but mosquitoes develop in stagnant and still water. You can end up trading one problem for another with that strategy if you are not properly trained. The best solution is to get local authorities to adopt a coordinated strategy, which is by far the most effective means of black fly larvae control.