Missouri’s warm, humid climate and abundant water sources create ideal conditions for biting flies. Several generations of these pests emerge through a Missouri summer, making outdoor activities challenging from spring through fall.
Understanding which species you’re dealing with and when they’re most active can help you protect yourself and enjoy Missouri’s outdoors more comfortably. You’ll encounter seven main types of biting flies across the state, each with distinct behaviors and peak activity periods.
Mosquitoes
Mosquitoes (Culicidae) are the most widespread biting flies you’ll encounter in Missouri. A mosquito bite results in a wheal, a small swelling mound, that itches, and the discomfort persists until your immune system breaks down proteins in the mosquito’s saliva.
These tiny insects breed in standing water, making areas near ponds, marshes, and even backyard containers prime mosquito habitat. Most mosquitoes only live a few weeks as adults, so you’ll experience several generations through a Missouri summer. Some species survive winter as adults, eggs, or larvae, emerging again when temperatures rise.
Only female mosquitoes bite, seeking blood meals to develop their eggs. Males feed on plant nectar and pose no threat to humans. Worldwide, mosquitoes are a major transmitter of diseases, including malaria, yellow and dengue fever and encephalitis. In Missouri, West Nile virus remains the primary concern.
Despite their nuisance factor, mosquitoes do pollinate flowers and provide food for bats, turtles, fish and birds. This ecological role doesn’t make their bites any less irritating, but it explains their persistence in Missouri’s natural ecosystems.
Peak Activity Times
Mosquitoes in Missouri are most active from late April through October, with population peaks occurring after periods of rain when breeding sites fill with water. Activity increases dramatically during warm, humid evenings when temperatures remain above 60°F.
Deer Flies
Deer flies (Chrysops species) are persistent pests that make Missouri summers particularly challenging. Deer flies are most prevalent in the spring. They are medium-size flies, approximately ¼-inch long, about the size of a house fly. Their distinctive dark-banded wings and brightly colored eyes make them easy to identify.
Unlike mosquitoes, tabanids do not use scent to find their hosts. They use their keen vision and key in on anything that moves. This visual hunting strategy explains why deer flies are so difficult to evade—they track movement with remarkable precision.
Both deer flies and horse flies bite with scissor-like mouthparts that cut into skin, causing blood flow which the flies lap up. Because of this relatively crude means of obtaining blood, the bites can be painful. The cutting action creates wounds that bleed more freely than mosquito bites.
You’ll most commonly encounter deer flies near streams, lakes, ponds, marshes and swamps, where their aquatic larvae develop. People apparently make a good target for deer flies, especially on or near streams or at the beach.
Peak Activity Times
These insects thrive in moist environments and are most active during June and July within their geographic range. Deer flies prefer warm, sunny days and are particularly aggressive during midday hours when temperatures peak.
Horse Flies
Horse flies (Tabanus species) are among the largest and most intimidating biting flies in Missouri. Horse flies can be more than 1 inch long, so they can be larger than deer flies. Their substantial size and powerful bite make encounters memorable for all the wrong reasons.
Horse fly wings are clear, while deer fly wings have dark markings or patterns. This distinction helps you identify which species is pursuing you. Many horse flies have iridescent green eyes, earning them the nickname “green-headed flies” in Missouri history.
Missouri history has many examples of places that were deemed unfit for settlement due to the presence of so-called green-head flies. For example, in the 1800s, the upland prairies of Missouri’s Audrain County were late to be settled in large part due to the presence of these insects.
The bite mechanics are brutal. The bite of these flies is very surgical. The fly’s mouthparts act like scissors and a sponge. After cutting the skin, the female fly soaks up blood, which flows more freely because of anti-clotting proteins in their saliva.
The airborne attack of horse flies occurs around Missouri’s rivers, lakes and streams, which are prime breeding habitat. Larvae develop in muddy soil near water, sometimes remaining in larval stages for up to three years before emerging as adults.
Peak Activity Times
Horse flies tend to be active in warmer temperatures. In Missouri, peak activity occurs from June through August during the hottest parts of summer days. They prefer to fly in sunlight, avoiding dark and shady areas.
Stable Flies
Stable flies (Stomoxys calcitrans) look remarkably similar to common house flies but pack a painful bite. Stable flies have light grey bodies, broad abdomens adorned with a “checkerboard” pattern of several dark spots, and notably long, protruding, piercing mouthparts at the front of their heads.
Unlike house flies that simply annoy, stable flies actively seek blood meals. These annoying insects feed on blood. While large hoofed mammals like cattle and horses are the primary food sources for stable flies in the U.S., these insects are also known to feed on a variety of other creatures, including goats, pigs, sheep, cats, donkeys, dogs, and even humans.
They often bite through clothing, especially on the ankles. Peak biting times are in the early morning and late afternoon. This ankle-biting behavior makes them particularly frustrating when you’re trying to enjoy outdoor activities.
Breeding sites tell an important story about stable fly populations. Adults lay eggs in moist decaying vegetation such as compost piles, hay bales, straw animal bedding, decaying fruits and vegetables, and grass clippings. Managing these materials around your property can significantly reduce local populations.
The bite itself creates distinct symptoms. Being adept fliers that feed during the daytime, their bites result in itchy, painful spots and lumps, often with a central crust. These wounds can become irritated if scratched excessively.
Peak Activity Times
Mainly found in late summer and early fall, these biting flies primarily feed on livestock. In Missouri, stable fly populations peak from July through September, with highest activity during early morning and late afternoon hours.
Black Flies
Black flies (Simuliidae) are small but formidable biters that emerge in Missouri during spring and early summer. These compact flies measure just a few millimeters long but can create serious problems when present in large numbers.
Black flies will fly up to 10 miles in search of blood, making them capable of affecting areas far from their breeding sites. This extensive flight range means you might encounter black flies even in locations without nearby flowing water.
They do not transmit disease to humans in the United States. However, injury from black fly bites can threaten the lives of livestock and even people when present in very large numbers, typically in late spring and early summer. Deaths have been reported from allergic reactions and blood loss from the bites, and even from inhaling the flies.
The bite characteristics are distinctive. Black fly bites often cause considerable swelling and bleeding, may be itchy and slow to heal. They prefer to attack the head and where clothing fits tightly. This targeting behavior makes head and neck protection particularly important.
Larval black flies require specific habitat conditions—they develop in flowing water, attaching to rocks and vegetation in streams and rivers. This habitat requirement limits their distribution but concentrates populations near suitable waterways.
Peak Activity Times
Black flies are most active in Missouri during late spring and early summer, typically from May through June. They prefer daytime feeding and are most aggressive during warm, calm days when winds don’t interfere with their flight.
Biting Midges (No-See-Ums)
Biting midges (Ceratopogonidae), commonly called no-see-ums or punkies, are Missouri’s smallest biting flies but deliver disproportionately painful bites. Biting midges are tiny (about 1.5 mm long) flies. They have piercing-sucking mouthparts and attack almost any warm-blooded animal in search of a blood meal. The bites of most species cause an immediate sharp pain.
Their minute size creates unique challenges. Since they rarely grow larger than 1/32 of an inch long, biting midges easily get into houses through window and door screens. This ability makes them one of the most pervasive flies that bite.
These flies breed in coastal salt marshes, mud-caked flatlands, freshwater areas, and damp holes in trees. Breeding places are often in densely shaded areas at the edge of grass marshes. In Missouri, you’ll find them near ponds, streams, and wetland edges.
The bite aftermath can be more severe than the initial pain suggests. This is followed by the development of red areas. For most people these lesions last up to a week or more and are more painful than mosquito bites.
The larval stage develops in standing water, wet soil, or wet organic matter. It takes about a month to complete development from egg to adult. Because of the aquatic or semiaquatic habitat of the larvae, adults are more likely to be a biting problem in areas near wet environments.
Peak Activity Times
Most biting activity takes place at twilight. In Missouri, biting midges are most active from May through September, with peak populations occurring during warm, humid evenings. Midges fly only during the warm months of the year and are most active before and during dusk.
Sand Flies
Sand flies (Psychodidae: Phlebotominae) represent a relatively minor threat in Missouri compared to other biting flies. The common name “sand fly” usually is reserved for blood-sucking species in the subfamily Phlebotominae. There are an estimated 600 species of phlebotomine sand flies in the world, including 14 species in the U.S. Only one of these species bites humans, and it is not known to be involved in the transmission of disease agents.
The disease transmission potential varies dramatically by location. Sand flies transmit sand fly fever, bartonellosis and leishmaniasis in many parts of the world. However, these diseases are not established in Missouri, making local sand flies primarily a nuisance rather than a health threat.
Sand flies share habitat preferences with other biting flies. They breed in moist organic matter and are typically found near wetland areas. Their small size—around 2-3 millimeters—makes them difficult to see and swat.
Bite reactions from sand flies can be persistent. The initial bite may go unnoticed, but affected areas often develop into itchy, raised welts that can last several days. Some individuals experience more severe allergic reactions with significant swelling and discomfort.
Peak Activity Times
Sand flies in Missouri are most active during warm summer months from June through August. They prefer humid conditions and are typically most active during evening and nighttime hours. Unlike some other biting flies, sand flies are weak fliers and avoid windy conditions.
Protection Strategies for All Biting Flies
Defending yourself against Missouri’s biting flies requires a multi-faceted approach. Defend yourself by wearing long pants and long-sleeved shirts when outdoors. You can keep deer flies from their favorite perch—the top of your head—with a hat.
Repellent selection matters, but effectiveness varies by species. Most insect repellents contain diethyl-toulamide (DEET) in concentrations that range from less than 5 percent to 100 percent. The stronger the concentration, the longer it will keep insects from biting you. However, remember that visual hunters like deer flies and horse flies respond less to chemical repellents.
Timing your outdoor activities strategically can significantly reduce exposure. Most biting flies have predictable activity patterns—mosquitoes and midges peak at dawn and dusk, while horse flies and deer flies are most active during warm, sunny midday hours. Planning around these patterns helps minimize encounters.
Environmental management reduces breeding sites. Sanitation can be an important method of controlling some biting flies. The larvae of stable flies develop in piles of decaying hay, straw and other vegetation, including manure containing plant matter. These potential sites for larval development should be eliminated where practical.
For areas with persistent problems, screens should be installed and maintained on windows and doors. However, the mesh of standard household screens is not fine enough to keep out the tiniest biting flies and should be replaced with finer mesh where these flies are a problem.
When to Seek Medical Attention
Most biting fly encounters result in temporary discomfort, but certain symptoms warrant medical evaluation. Seek professional care if you develop:
- Spreading redness or warmth around bite sites indicating possible infection
- Fever, chills, or flu-like symptoms following bites
- Severe swelling affecting breathing or circulation
- Signs of allergic reaction including hives, difficulty breathing, or dizziness
- Symptoms of tularemia after deer fly bites, including sudden fever, swollen lymph nodes, or skin ulcers
Understanding Missouri’s seven types of biting flies and their activity patterns empowers you to enjoy outdoor activities while minimizing uncomfortable encounters. Each species has distinct behaviors, peak seasons, and vulnerabilities that you can exploit for better protection.
By combining protective clothing, strategic timing, environmental management, and appropriate repellents, you can significantly reduce your exposure to these persistent pests throughout Missouri’s warm months.
