Nearly everyone has experienced the irritating bite of a fly. While mosquitoes are often the prime suspects in the biting fly world, numerous other fly species are also culprits. This article delves into the question of “Why Do Flies Bite,” focusing on these often-overlooked biting flies, excluding mosquitoes (which are covered extensively elsewhere). We aim to provide a comprehensive understanding of these pests, their biting habits, and how to manage and prevent encounters.
What exactly defines a fly? Distinguished by their two wings – unlike most winged insects that have four – flies possess specialized mouthparts. These mouthparts are designed for a liquid diet, functioning like a sponge to soak up fluids. However, in biting flies, these mouthparts are adapted for a more intrusive purpose: piercing skin to access blood.
Like their mosquito cousins, biting flies are adept at locating their next meal. They utilize sophisticated senses to detect hosts, homing in on cues such as the carbon dioxide and moisture we exhale, dark colors, movement, body heat, and perspiration. Once a suitable host is identified, a biting fly lands and initiates its bite. It employs its piercing mouthparts to lacerate the skin. To ensure a steady flow of blood, the fly injects saliva containing an anticoagulant. This saliva, while facilitating their blood meal, is also the source of irritation and allergic reactions in sensitive individuals, sometimes even triggering severe, life-threatening responses.
Beyond the immediate discomfort of their bites, biting flies pose a significant health concern globally. They are vectors for debilitating diseases affecting millions. Sand flies, for instance, are responsible for transmitting sand fly fever, bartonellosis, and leishmaniasis across many regions of the globe. In the United States, the deer fly Chrysops discalis is known to transmit tularemia. Biting midges are vectors of various diseases, notably blue tongue virus in livestock within the U.S. Furthermore, the bites from black flies, horse flies, and stable flies can induce severe allergic reactions in some people.
IDENTIFYING COMMON BITING FLIES
To effectively manage and prevent bites, it’s crucial to identify the types of biting flies you might encounter. Here are some of the most common culprits:
DEER FLIES and HORSE FLIES (Tabanidae)
Deer flies hold the unfortunate distinction of being among the few types of flies in the United States that transmit diseases to humans. They are vectors of tularemia, a bacterial disease also known as “rabbit fever.” Tularemia can be contracted through various means, including contact with infected animals or contaminated objects, tick bites, and, occasionally, deer fly bites from Chrysops discalis.
Deer flies are most prevalent during the spring months. They are medium-sized flies, around ¼-inch in length, similar in size to a common house fly. Their coloration typically ranges from yellow-brown to black, often featuring distinctive dark bands across their wings. A notable characteristic of some deer flies and horse flies is their iridescent green eyes.
The larval stage of deer flies is aquatic, residing in maggot-like form in water. Adult deer flies are frequently encountered near bodies of water such as streams, lakes, ponds, marshes, and swamps, especially along trails. They are notorious for their bothersome behavior, buzzing persistently around people’s heads, particularly when present in large numbers. Both deer flies and horse flies are equipped with scissor-like mouthparts that slice into the skin to procure blood, which they then lap up. This relatively crude feeding method often results in bites that are notably painful.
Horse flies can be significantly larger than deer flies, reaching an inch or more in length. Their coloration varies; some are entirely black, while others, known as “greenheads,” exhibit a light brown body with striking shiny green eyes. Horse flies are robust and rapid fliers that primarily feed on the blood of livestock and other animals. Similar to deer fly larvae, horse fly larvae typically inhabit aquatic or moist environments, preying on other insects as they grow and eventually migrate to drier soil to pupate. Their life cycle can extend up to two years to complete.
STABLE FLY (Stomoxys calcitrans)
The stable fly, approximately ¼-inch long and gray, is identifiable by the four dark stripes on its thorax, the area behind its head. It bears a striking resemblance to a house fly, but can be distinguished by its pointed proboscis located beneath its head, which it uses for blood-sucking. Stable flies are most abundant in late summer and fall. They are known to travel several miles in search of a blood meal, targeting livestock (hence their name), pets, and humans alike. They typically bite during the early morning or late afternoon hours, often attacking ankles and inflicting a sharp, stabbing pain.
Stable flies lay their eggs in decaying vegetable matter, such as haystacks, grass clippings, manure, and shoreline vegetation. Both adult and larval stable flies are nearly indistinguishable from house flies in appearance.
BLACK FLIES (Simuliidae)
Adult black flies are small, typically no larger than 1/8-inch in length, with broad wings and a characteristic humpbacked appearance. Like many other biting flies, black flies thrive in moist environments. Also known as “buffalo gnats,” they are commonly found near creeks and rivers, where their larvae attach themselves to submerged stones.
Black flies are capable of flying considerable distances, up to 10 miles, in search of blood. While they do not transmit diseases to humans in the United States, their bites can pose a serious threat to livestock and even humans when present in large numbers, especially during late spring and early summer. Fatalities have been reported due to allergic reactions, blood loss from numerous bites, and even from inhalation of the flies themselves. Black fly bites often result in significant swelling and bleeding, and can be intensely itchy and slow to heal. They exhibit a preference for biting the head and areas where clothing fits tightly.
BITING MIDGES (Ceratopogonidae)
It’s important not to confuse biting midges with other non-biting midges (Chironomidae), which are larger and resemble mosquitoes but do not bite. Biting midges are significantly smaller, measuring no more than 1/32 of an inch long. They are also commonly referred to as “punkies,” “no-see-ums,” or simply “gnats.” Their diminutive size allows them to penetrate standard window and door screens. Biting midges can be particularly troublesome pests along coastlines, lakes, ponds, and rivers, biting both during the day and night.
The larvae of biting midges are minute and worm-like, inhabiting moist sand or soil, decaying vegetation, tree holes, and other damp locations, often near the edges of bodies of water like ponds, marshes, and lagoons. While several species feed on human blood, others target insects, including mosquitoes, for their blood meals.
SAND FLIES (Psychodidae)
Sand flies are related to non-biting drain flies. Their larvae are tiny and worm-like, thriving in moist, decaying plant matter, moss, mud, or aquatic environments. Adult sand flies are long-legged, no more than 1/8-inch long, hairy, and range in color from brown to gray. They characteristically hold their wings in a “V” shape when at rest.
Most sand fly species are nocturnal feeders, preying on the blood of mammals, reptiles, and amphibians. In various parts of the world, including southern Texas in the United States, certain sand fly species (Lutzomyia) are suspected vectors of cutaneous leischmaniasis, a disfiguring protozoan disease affecting humans.
MANAGING BITING FLIES AROUND YOUR PROPERTY
Controlling biting flies on a large scale can be challenging due to the often-hidden larval habitats and the considerable distances some adult flies can travel from these sites. However, effective management strategies can significantly reduce their presence and impact around your home and property.
Sanitation is a cornerstone of biting fly control, particularly for stable flies. Their larvae thrive in decaying organic materials like hay, straw, and manure mixed with plant matter. Eliminating these potential breeding sites is crucial. For other types, such as biting midges and sand flies, removing decaying vegetation that harbors their larvae can be effective.
Exclusion methods are also valuable. Stable flies are known to enter buildings in search of blood meals, making screens on windows and doors essential. For smaller biting flies like midges, standard screens may not suffice; finer mesh screens might be necessary to keep them out.
Fly paper has limited effectiveness against biting flies, as they are more attracted to warm-blooded hosts than to fly paper. Fans can be a more useful tool, particularly for smaller flies whose flight is disrupted by air currents. Similarly, smoke and air currents produced by burning candles or torches can help deter smaller species in localized areas.
Pesticide application offers limited control for biting flies. Ultra-low volume (ULV) treatments, like mosquito fogging, and space sprays using non-residual pesticides are most effective when flies are concentrated in a small area. These methods only kill flies on contact and degrade quickly, providing only temporary relief. Residual pesticides can be applied to surfaces where flies may rest, such as vegetation or building walls, but their effectiveness depends on flies actually landing on these treated surfaces.
Larviciding, the application of pesticides to kill fly larvae, is another control strategy. Formulations containing Bacillus thuringiensis (BTI) or growth regulators like methoprene have been successfully used against mosquito larvae in standing water. BTI is also effective against black fly larvae in streams.
PREVENTING FLY BITES ON A PERSONAL LEVEL
Repellents are your primary personal defense against biting flies. Those containing DEET (N,N-diethyl-meta-toluamide) or picaridin are considered most effective. While generally effective against mosquitoes, repellents may be less effective against certain biting fly species.
Using repellents should be complemented with other preventive measures. Avoid areas known to be heavily infested with flies, especially during peak biting times. Wear protective clothing: heavy-duty, light-colored garments, including long-sleeve shirts, long pants, and hats. When black flies are abundant, head nets, similar to “bee bonnets” used by beekeepers, can offer crucial protection. For very small biting flies like midges, applying thick lotions or oils to the skin can sometimes trap them.
Despite diligent use of control and preventive measures, complete elimination of biting flies is often unrealistic. However, by combining fly management strategies with personal protective measures, you can significantly minimize bites from these irritating pests and reduce their impact on your outdoor experiences.
Note: Always read and follow pesticide product label directions carefully. It is a violation of federal law to use pesticides in a manner inconsistent with their labeling.
Disclaimer: This information is for educational purposes only and does not constitute professional pest control advice. For specific concerns or severe infestations, consult with a qualified pest control professional or your local health department.
