South Dakota’s warm weather brings outdoor adventures, but it also brings biting flies that can turn a pleasant day into an uncomfortable experience. From tiny no-see-ums that slip through window screens to large horse flies with painful bites, you’ll encounter various species throughout the state.
Understanding when these flies are active, where they breed, and how to protect yourself makes all the difference in enjoying South Dakota’s outdoors.
The state’s abundant wetlands, rivers, and standing water create ideal breeding grounds for multiple fly species. Weather patterns play a crucial role in fly populations—above-average precipitation leads to increased numbers, while dry conditions keep populations lower.
Knowing which flies you’re dealing with helps you choose the most effective protection strategies for your specific situation.
1. Mosquito
The mosquito (Culex pipiens and related species) is perhaps the most familiar biting fly in South Dakota. There are 43 species of mosquitoes in South Dakota, though not all of them bite people. These small insects have slender bodies, long legs, and needle-like mouthparts that pierce your skin to draw blood.
Female mosquitoes require blood meals to develop their eggs, while males feed on nectar and plant juices. The inland floodwater mosquito is often encountered near areas containing water such as grassy pools, shallow ponds, and even wet ditches, with only females biting humans for their blood. They detect hosts through carbon dioxide emissions, body heat, and visual cues.
Mosquitoes like wet and warm weather and the population can vary across the state. You’ll find them most active during early morning and evening hours, particularly in shaded areas near water sources. Their activity typically starts in spring and continues through fall, with peak populations following rain events.
Mosquito breeding sites include any standing water—old tires, bird baths, watering troughs, flower pots, and natural wetlands. Mosquitoes often lay their eggs on the surface of standing water, and even small amounts of standing water can attract mosquitoes, so eliminating sources or applying appropriate insecticides may help limit the population.
Protection strategies include DEET-based repellents, which work effectively against mosquitoes by creating a chemical barrier. Wearing long sleeves and pants during peak activity times reduces exposure. You can also reduce outdoor exposure by staying inside during early morning or early evening hours when mosquitoes are most active.
While mosquito bites cause itching and discomfort, mosquitoes can transmit the West Nile and Zika viruses. The Sioux Falls mosquito traps monitor West Nile Virus risk, though heavy biting mosquitoes typically aren’t effective disease carriers.
2. Deer Fly
Deer flies (Chrysops species) are distinctive biting flies with colorful eyes and dark-patterned wings. They are large flies with large brightly-coloured compound eyes and large clear wings with dark bands, larger than the common housefly and smaller than the horse-fly. These medium-sized flies measure about a third of an inch or longer.
In South Dakota, horse flies and deer flies start emerging in June or July and remain active throughout the summer. They prefer daytime activity, especially during warm, sunny conditions above 71°F. They are active under direct sunshine and hours when the temperature is above 22 °C.
Unlike mosquitoes that use stealth, deer flies take a more aggressive approach. They circle around your head repeatedly, bumping into you before landing to bite. When feeding, females use scissor-like mandibles and maxillae to make a cross-shaped incision and then lap up the blood, with bites that can be painful.
Deer flies are visual hunters attracted to movement, dark colors, body heat, and carbon dioxide. This makes them particularly persistent when you’re active outdoors. They prefer wooded areas and fields near water sources where their larvae develop.
Horse flies and deer flies are similar to mosquitoes in that they use areas of standing water, such as ponds or wetlands, for breeding sites. Females lay 100-800 eggs on vegetation near water, with larvae developing in moist soil or damp objects for several months.
Protection against deer flies requires different strategies than mosquito protection. DEET is not an effective repellent against these visual hunters. Instead, wear light-colored clothing and hats, as deer flies are attracted to dark colors. Some people attach sticky tape to hat tops to trap circling flies. Covering exposed skin with long sleeves and pants provides the most reliable protection, though this can be uncomfortable in South Dakota’s summer heat.
3. Horse Fly
Horse flies (Tabanus species) are the largest biting flies you’ll encounter in South Dakota. Horse flies and deer flies are much larger than most other flies, ranging in size up to 1.25 inches long. They have robust bodies, large compound eyes, and prominent downward-extending mouthparts.
These impressive insects vary in color—many are black, grey, or brown, though some display patterns of blue, green, or yellow. Black Horse Flies are about an inch long, making them highly visible and somewhat intriguing to watch, and they are fast fliers despite their hefty size.
Horse flies emerge during the same timeframe as deer flies, appearing in June or July and remaining active throughout summer. In South Dakota, horse flies and deer flies start emerging in June or July and remain active throughout the summer. They prefer warm temperatures and, unlike smaller flies, can remain active even in moderate wind conditions due to their size and strength.
Like deer flies, horse flies rely on visual cues to find hosts. They rely on visual cues to seek out hosts, searching for large moving dark-colored objects. This explains why they’re particularly attracted to livestock and vehicles. Female horse flies need blood meals to develop eggs, while males feed exclusively on nectar.
They can inflict very painful bites and draw large amounts of blood, which leads to stress and reduced weight gain in livestock. Their scissor-like mouthparts cut open skin, allowing blood to pool, which they then sponge up. Due to their large size and the intensity of their bite, horse fly bites are often considered one of the most painful.
Horse fly larvae develop in mud and high-moisture areas near standing water, though not necessarily in water itself. Their life cycle can take around 2 months to complete, so there is usually only one generation per year. This single generation means populations don’t multiply as rapidly as some other biting flies.
Protection strategies mirror those for deer flies. Wear heavy-duty, light-colored clothing including long sleeves and pants. Apply repellents, though they’re less effective against horse flies than mosquitoes. Avoid peak activity times during midday when temperatures are highest. For livestock owners, providing shaded areas and water for animals to submerge in helps reduce horse fly harassment.
4. Black Fly
Black flies (Simulium species), also called buffalo gnats, are small but notorious biters. The culprits of these bites are commonly referred to as black flies or buffalo gnats. These small dark flies have stout bodies, broad wings, and a characteristic humped appearance on their thorax.
Black flies are active for approximately three weeks starting sometime in May, and fortunately have only one generation per year and shouldn’t be around much past mid-June in South Dakota. Their relatively short activity period provides relief compared to species that persist all summer.
Black flies swarm around faces and exposed skin, creating significant annoyance even before biting. Although black fly bites aren’t necessary painful, they can cause itching in some cases. Many people don’t realize they’ve been bitten until welts develop later, unlike the immediate pain of horse fly bites.
These flies require running water for reproduction. Black flies lay their eggs around running water, and the larvae will only develop in running water. Eggs are laid on vegetation, rocks, or other objects in streams, rivers, and waterfalls. Larvae attach to substrates underwater and filter organic matter for food.
Cool, wet spring conditions extend black fly activity beyond their typical early June disappearance. The black fly populations are persisting in South Dakota beyond the expected date of early June, which is likely due to the cool conditions. Favorable spring moisture also increases their numbers significantly.
Managing black flies proves challenging. There isn’t a great way to manage black flies, but reducing skin exposure and using repellents can reduce their impact on activities, though repellents are sometimes not as effective against black flies as they are against mosquitos. Wearing long pants, long-sleeved shirts, and head nets may be necessary during peak activity. Avoiding outdoor activities during their three-week flight period provides the most reliable protection.
5. Biting Midge (No-See-Um)
Biting midges (Culicoides species), commonly called no-see-ums, are among the most frustrating biting flies due to their tiny size. Ceratopogonidae is a family of flies commonly known as no-see-ums, sand flies or biting midges, generally 1–3 millimetres in length. Their small size allows them to pass through standard window screens, making them difficult to avoid.
These tiny insects have grayish or yellowish bodies and resemble miniature mosquitoes. Since they rarely grow larger than 1/32 of an inch long, biting midges easily get into houses through window and door screens, making them one of the most pervasive flies that bite. You often feel a biting sensation without seeing the culprit.
Biting midges are most active during dawn and dusk on calm days. Biting midges are susceptible to air currents and emerge most commonly on still days. They prefer feeding on exposed areas like ankles, feet, arms, and neck. The bites cause intense itching, redness, and welts that can last up to two weeks.
Female midges need blood meals to reproduce, while males feed on nectar. It is believed that biting midges are drawn to mammals, including humans by detecting certain odors we emit, particularly the odor of carbon dioxide and the odor of lactic acid. This attraction to CO2 makes carbon dioxide traps effective for control.
Breeding sites include diverse locations—lakes, ponds, tree holes, moist soil, and plants that retain water. Biting midge larvae consume decaying organic matter in mud, intertidal sand or wet soil around water holes and seepage areas, with decaying seaweed along the coast presenting an ideal breeding habitat. Females can lay 400 or more eggs at one time, breeding 5-7 times during their adult life.
Protection requires specialized approaches. Most midges are small enough to pass through ordinary insect window screening, and they can be repelled with DEET, oil of Eucalyptus, or Icaridin. Install fine-mesh screens (smaller than 16-mesh) on windows and doors. Use fans to create airflow, as these weak fliers avoid wind. Apply repellents containing DEET, picaridin, or oil of lemon eucalyptus. Stay indoors during peak biting times at dawn and dusk.
6. Stable Fly
Stable flies (Stomoxys calcitrans) are often called biting house flies due to their similar size and appearance to common house flies. The stable fly is often called the biting house fly, is about the same in size and general appearance as the common house fly, and has a bayonet-like, needle-sharp mouth. However, their painful bite quickly distinguishes them from their non-biting relatives.
These flies measure about the same size as house flies but possess piercing-sucking mouthparts designed for blood feeding. Unlike house flies that feed on organic matter, stable flies require blood meals. Both male and female stable flies bite, making them unusual among biting flies where typically only females feed on blood.
Stable flies are found throughout South Dakota, particularly in areas with livestock. They’re active during daylight hours and prefer biting exposed skin on lower legs and ankles. Stable flies are smaller than deer or horse flies, about the size of a house fly, and this species is often noticed by those paddling canoes, as these insects seem to prefer to bite exposed skin on ankles and feet.
These flies breed in decaying organic matter rather than standing water. The larvae of stable flies develop in piles of decaying hay, straw and other vegetation, including manure containing plant matter, and these potential sites for larval development should be eliminated where practical. This makes them common around farms, stables, and areas with accumulated plant debris.
Stable fly larvae develop in moist, fermenting organic material. The life cycle progresses through egg, larva, pupa, and adult stages. Adults can live for several weeks, during which they take multiple blood meals. Their ability to breed in various organic materials makes population control challenging.
Protection involves both personal measures and environmental management. Wear long pants and closed shoes to protect ankles and feet—their preferred feeding sites. Apply insect repellents to exposed skin. For property owners, promptly remove piles of decaying vegetation, grass clippings, and hay. Keep areas around water troughs clean and dry. Proper manure management in livestock areas reduces breeding sites significantly.
7. Sand Fly
Sand flies in the context of South Dakota refer to certain species within the broader category of biting midges and related flies. The term “sand fly” is sometimes used interchangeably with no-see-ums or biting midges, though true sand flies (Phlebotomus and Lutzomyia species) are more common in tropical and subtropical regions.
In South Dakota, when people mention sand flies, they’re typically referring to biting midges or small flies found near sandy or muddy areas along rivers and lakes. No-See-Ums are also referred to as Biting Midges, Biting Gnats, Punkies or Sand Flies. These tiny insects share similar characteristics with no-see-ums—small size, painful bites, and attraction to moist habitats.
True sand flies in warmer climates feed primarily at night on mammals, reptiles, and amphibians. However, the small biting flies called sand flies in South Dakota may be active during day or night depending on species and conditions. They’re most abundant near water bodies with muddy or sandy shores.
These flies breed in moist sand, mud, or soil near water. Females lay eggs in these damp substrates, where larvae develop by feeding on organic matter. The life cycle duration varies by species and temperature, with warmer conditions accelerating development.
Bites from sand flies cause itching, redness, and small welts. These insects cut into the skin instead of using a proboscis like mosquitoes, which causes intense itching, redness, and welts, with bites lasting up to two weeks. The cutting action of their mouthparts creates more tissue damage than mosquito bites, leading to longer-lasting irritation.
Protection strategies mirror those for biting midges. Avoid sandy or muddy areas near water during peak activity times. Wear protective clothing covering arms and legs. Apply DEET-based repellents or alternatives like picaridin. Install fine-mesh screens on windows to prevent entry into buildings. Use fans to create airflow in outdoor sitting areas, as these weak fliers struggle against air currents.
For property management, eliminate breeding sites by addressing areas of standing water and moist organic matter. Keep grass mowed and vegetation trimmed around water features. Remove decaying plant material that washes up along shorelines. While complete elimination is difficult, these measures reduce populations around your property.
Understanding the seven main types of biting flies in South Dakota helps you prepare for outdoor activities throughout the warm months. Each species has distinct characteristics, activity patterns, and preferred habitats. By recognizing which flies you’re dealing with and when they’re most active, you can choose appropriate protection methods and enjoy South Dakota’s outdoor spaces with greater comfort and confidence.
