Black flies, known also as "buffalo gnats" and "turkey gnats," are very small, robust flies that are annoying biting pests of wildlife, livestock, poultry, and humans. Their blood-sucking habits also raise concerns about possible transmission of disease agents. You are encouraged to learn more about the biology of black flies so that you can be better informed about avoiding being bitten and about their public health risk.
Are Black Flies a Public Health Risk?
Black flies can be annoying biting pests, but none are known to transmit disease agents to humans in
the U. S. However, they transmit one parasitic nematode worm that infects humans in other regions
of the world.
Onchocerca volvulus causes a significant human disease known as onchocerciasis or "river blindness"
in equatorial Africa and mountainous regions of northern South America and Central America.
The bites of black flies cause different reactions in humans, ranging from a small puncture wound where
the original blood meal was taken to a swelling that can be the size of a golf ball. Reactions
to black fly bites that collectively are known as "black fly fever" include headache, nausea,
fever, and swollen lymph nodes in the neck.
In eastern North America, only about six black fly species are known to feed on humans. Several
other species are attracted to humans, but they typically do not bite. However, the non-biting
species fly around the head and may crawl into the ears, eyes, nose, or mouth, causing extreme annoyance
to anyone engaged in outdoor activities.
Black flies can be found throughout most of the U. S., but their impact on outdoor activities varies
depending on the specific region and time of year. For example, in parts of the upper Midwest and
the Northeast, black fly biting can be so extreme, especially in late spring into early summer, it may
disrupt or prevent outdoor activities such as hiking, fishing, and kayaking.
Besides being a nuisance to humans, black flies can pose a threat to livestock. They are capable
of transmitting a number of different disease agents to livestock, including protozoa and nematode worms,
none of which cause disease in humans. In addition to being vectors of disease agents, black flies
pose other threats to livestock. For example, when numerous enough, black flies have caused suffocation
by crawling into the nose and throat of pastured animals. On rare occasions, black flies have been
known to cause exsanguination (death due to blood loss) from extreme rates of biting. Saliva injected
by biting black flies can cause a condition known as "toxic shock" in livestock and poultry,
which may result in death.
How Many Types of Black Flies Are There?
Black flies are true flies (Order Diptera) in the family Simuliidae, which includes more than 1,700
species worldwide. In North America, 255 species in 11 genera have been identified, but additional
species remain to be discovered and named. Very little is known about black flies in Indiana, and there
are no estimates of the number of species in the state. For perspective, 12 species have been documented
in Illinois, while over 30 species have been documented in both Minnesota and Wisconsin, where black
fly habitats are more abundant.
How Can I Recognize an Adult Black Fly?
Black flies range in size from 5 to 15 mm, and they are relatively robust, with an arched thoracic region
(Figure 1). They have large compound eyes, short antennae, and a pair of large, fan-shaped wings.
Most species have a black body, but yellow and even orange species exist.
What Is the Life Cycle of Black Flies?
Black flies undergo a type of development known as "complete metamorphosis" (Figure 2). This
means the last larval stage molts into a non-feeding pupal stage that eventually transforms into a winged
adult. After taking a blood meal, females develop a single batch of 200-500 eggs. Most species
lay their eggs
in or on flowing water, but some attach them to wet surfaces such as blades of aquatic grasses.
The length of time it takes an egg to hatch varies greatly from species to species. Eggs of most species
hatch in 4-30 days, but those of certain species may not hatch for a period of several months or longer.
The number of larval stages ranges from 4-9, with 7 being the usual number. The duration of larval
development ranges from 1-6 months, depending in part on water temperature and food supply. The life
cycle stage that passes though winter is the last stage larva attached underwater to rocks, driftwood,
and concrete surfaces such as dams and sides of man-made channels.