Florida is home to 18 species of black flies, small biting insects that breed in the state’s rivers and streams. The most common species, Simulium slossonae, is found from the southern tip of the state all the way north to Jacksonville and west to Pensacola. Black flies are most prevalent in early to late spring, and while they’re not as talked about as mosquitoes or no-see-ums, their bites can be surprisingly painful and sometimes trigger a systemic reaction known as “black fly fever.”
What Black Flies Look and Act Like
Black flies are small, measuring just 1 to 5 mm long, with a stout, hump-backed body shape that earns them the nickname “buffalo gnats.” Their mid-section has a shiny appearance and ranges from black to shades of gray or yellow depending on the species. They’re also sometimes called turkey gnats, a nod to their impact on poultry. Floridians have coined other, less printable names for them.
Only the females bite. They feed during daylight hours, with activity peaking between about 9:00 and 11:00 in the morning and again from 4:00 to 7:00 in the late afternoon. They tend to hover around the eyes, ears, and nostrils of people and animals before landing and puncturing the skin. Black flies are strong fliers, capable of traveling 7 to 10 miles from their breeding sites, which means you don’t need to live right next to a river to encounter them.
When and Where They Breed
Black flies breed exclusively in running water. You won’t find their larvae in lakes, ponds, or swamps. The immature stages need flowing water to deliver food and oxygen, so streams, rivers, and even the outlets of ponds and lakes are prime habitat. Females lay eggs on vegetation in streams or scatter them across the water surface, and the larvae attach to rocks, leaves, grass, or other submerged objects.
In Florida, the most common species can be present from August through July, essentially year-round, though populations are most prevalent in early to late spring. Florida’s warmer water temperatures allow larvae to develop earlier in the year compared to northern states, where emergence doesn’t begin until late April or May.
How Black Fly Bites Differ From Other Insects
Black flies don’t pierce the skin with a needle-like probe the way mosquitoes do. Instead, they lacerate the skin with their mouthparts and then suck blood from the wound. Their saliva contains a strong anticoagulant that keeps blood flowing freely while they feed. This cutting action is why black fly bites tend to hurt more than mosquito bites and often leave a visible lesion rather than just a small bump.
The bitten area typically itches and can swell noticeably. In some people, multiple bites trigger a collection of symptoms called black fly fever: headache, nausea, actual fever, and swollen lymph nodes in the neck. This isn’t an infection but rather your body’s reaction to the compounds in the fly’s saliva.
Black Flies vs. No-See-Ums and Other Biters
Florida has no shortage of biting flies, and it’s easy to confuse them. Here’s how to tell the main culprits apart:
- Black flies are 1 to 5 mm, hump-backed, and bite during the day. They breed in flowing water and are most active in spring.
- No-see-ums are even smaller (under 1.6 mm), dark gray to black with spotted wings, and most active at sunrise and sunset. A light breeze stops them from feeding. They’re most abundant in summer but can bite year-round in Florida.
- Horse flies and deer flies are much larger, also daytime feeders, and use large piercing mouthparts to lacerate skin. Like black flies, only the females bite, and they inject an anticoagulant while feeding.
If you’re getting bitten during the middle of the day near a river or stream in spring, black flies are the most likely suspect. If the bites happen around dawn or dusk and a fan seems to help, no-see-ums are probably the problem.
Health Risks for People and Animals
Black flies in the United States don’t transmit the parasitic worm that causes river blindness, a disease associated with black flies in tropical Africa and Central America. But they aren’t harmless vectors either. In Florida, S. slossonae transmits a protozoan parasite that causes leucocytozoonosis, a malaria-like disease, in turkeys. Other species carry parasites that affect ducks, geese, and cattle. Several North American black flies also show evidence of transmitting vesicular stomatitis virus to livestock, cattle, horses, and pigs.
For people, the primary health concern is the bite reaction itself. Most bites resolve on their own, though the itching and swelling can last several days. Black fly fever, while uncomfortable, is self-limiting. The real danger tends to fall on poultry and livestock, where large swarms can cause significant blood loss or disease transmission.
Repellents That Work on Black Flies
DEET remains the most broadly effective repellent against black flies and is specifically labeled to work against them along with mosquitoes and other biting insects. Products containing up to 30% DEET are considered the upper recommended limit. Higher concentrations don’t offer significantly better protection and aren’t recommended.
Picaridin-based repellents are labeled for mosquitoes, biting flies, and chiggers, though products with lower concentrations may offer shorter protection times. Oil of lemon eucalyptus (the refined version, PMD) is another option, with products at 8 to 10% concentration listing black flies on their labels. IR3535, the active ingredient in some Avon products, is also registered for black flies, gnats, mosquitoes, no-see-ums, and sand flies.
For an added layer of defense, permethrin can be applied to clothing (never directly to skin). You spray it on your shirt, pants, and hat, let it dry completely, and it repels ticks and biting flies on contact. Combining a skin-applied repellent like DEET or picaridin with permethrin-treated clothing gives the strongest protection when you’re spending extended time outdoors near waterways during peak black fly season.
Controlling Black Fly Populations
Because black fly larvae live in flowing water, traditional mosquito control methods like treating standing water or fogging don’t work. The most effective large-scale tool is Bti (Bacillus thuringiensis israelensis), a naturally occurring soil bacterium applied to streams and rivers via helicopter or boat. Larvae must actively ingest Bti for it to work. Once consumed, the bacterial proteins activate in the larvae’s alkaline gut, breaking down the gut lining and causing paralysis. Bti is nontoxic to humans, mammals, birds, fish, and most other invertebrates because their digestive systems are acidic rather than alkaline.
At the individual level, there’s not much you can do about breeding sites since black flies use natural waterways rather than the kind of small water sources you can dump out in your yard. Your best strategy is personal protection: repellents, treated clothing, and timing outdoor activities to avoid peak morning and late afternoon hours during spring. Light-colored clothing also helps, as black flies are generally more attracted to dark colors.