Black flies or buffalo gnats (Figure 1) are 1/8-inch long hump-backed dark flies with wide, clear wings. Females use their sharp blade-like mouthparts to slice the skin and feed on blood that wells up. The bite may bleed for some time after feeding has stopped and it may itch intensely for several days. Black flies are most closely related to mosquitoes, but their feeding habits are most similar to horse flies. One of the most common species in Kentucky attacks horses and cattle, and swarms around humans and occasionally bites. These flies feed during the day and frequently attack animals in the ears and around the eyes. They also may bite along the underbelly. The bites are painful; animals under attack can become very head-shy and hard to handle. They may run in an attempt to escape the torment of these small gnats.
Generally, insect control efforts are based on source reduction, but this is not an option with an insect that develops in streams and rivers. While there is an insecticide registered to control black fly larvae, it is expensive and impractical to use over wide areas. Animal protection alternatives are limited, too. Insecticides used to control nuisance flies on livestock and horses will provide some protection but must be reapplied frequently during the black fly “season”. Feeding can be intense in horse ears; thick, repeated applications of petroleum jelly will provide a physical barrier to fly feeding and will allow injured areas to heal. In some areas, KDA Pest and Noxious Weeds personnel are spraying with Anvil, a synthetic pyrethroid insecticide, in an effort to control adult flies. The larval and pupal stages of black flies live in flowing water of streams and rivers. Hundreds may be found on stones or submerged objects. Brush-like fans on the head are used to strain small microorganisms from the water. The pupal stage is attached to underwater surfaces in a silken case. An adult emerges from the pupal stage, rises to the surface, and flies away in search of a blood meal. They are strong fliers and may move long distances from water to feed. Historically, black flies have been a problem for livestock along permanent streams and rivers. Buildup of debris, especially fallen trees, can slow stream flow and make long stretches unsuitable for breeding. Stream clean-up that increases current flow allows black fly populations to increase to nuisance levels.
By Lee Townsend, Extension Entomologist