Biting flies
Order: Diptera
Family: Simuliidae, Ceratopogonidae, Tabanidae, Rhagionidae, Muscidae
Description, life history, and bites
Simulium spp.
There are around 40 species of small biting black flies, Simulium spp. (family: Simuliidae), inhabiting Colorado. These small biting flies are nuisance pests that feed on people, birds, and livestock animals. The most problematic species in Colorado include S. griseum, S. bivittatum, and S. apricorium. Outbreaks of these flies tend to occur in areas with continuous running water. For example, in 1995 large populations of these flies were observed in parts of eastern Colorado due to unusually heavy spring runoff, which resulted in a substantial increase in biting problems in these areas. These flies are typically considered nuisance pests that do not enter buildings to feed, though several complications can result on livestock due to heavy feeding of Simulium spp.
These flies range in size from 5-15 mm (1/5-3/5 inches) long, depending on the species, and have a pronounced arch on the thoracic region. Most species are black, although some are yellow or orange. The eggs of Simulium spp. are very small, measuring under 0.5 mm, and oval or triangular with rounded edges. The eggs are initially white but darken as the embryo develops. Mature larvae are about the same size as adults and have elongated bodies that can be brown, green, grey, or black. The larvae have a large head with labral fans used for feeding by filtering out microorganisms from running water. The pupae of many species are orange and produce a pupation chamber with silk. The pupae of Simulium spp. are 2-3 mm (~1/10 inch) long.
Adults are most active during the day and can migrate miles from breeding sites. An individual Simulium spp. can live for about three or four weeks. Male and female flies feed on nectar or plant sap, while only female flies feed on animal blood to complete their reproductive development. Favored feeding sites include body regions not covered with dense hair such as around the eyes, in the ears, or on the belly.
A single female can lay hundreds of eggs in layers or strings on the surface of moist objects directly in water or closely associated with moving water, including large rivers, icy mountain streams, trickling creeks, and waterfalls. The larvae adhere to substrates in water and feed by filtering organic matter. When mature, larvae enter a nonfeeding pupal stage that remains attached to substrates submerged in water. Newly emerged adults rise to the water surface in an air bubble before finding a suitable location to rest and allow their exoskeleton to harden.
Reactions to bites inflicted by Simulium spp. vary. On some individuals, bites produce a small red spot that bleeds initially. More severe reactions to bites of Simulium spp. are characterized by enlarged swellings that can impede vision or walking, depending on the location of the bite, and such swellings can produce lesions that may persist for months. While Simulium spp. are not known to transmit any human diseases in North America, intense feeding can lead to scabbing and puts livestock at risk of vesicular stomatitis and bovine ochocerciasis. Massive attacks can kill livestock through exsanguination (bleeding out), suffocation, or infections of the respiratory tract when large quantities of flies are inhaled.
Biting midges
Biting midges (family: Ceratopogonidae) are very small flies, about 1-3 mm (~1/10 inche) long, with gray bodies. Both male and female flies feed on nectar and plant sap, and female reproductive development often requires the blood of vertebrates, which serves as a source of protein. Some species are host-specific, while others are generalists. Mammals often serve as the primary host for biting midges, but these flies can also attack birds, reptiles, and amphibians. While their bites may be extremely annoying, these flies are not known to transmit any human diseases in the United States. However, there are several important livestock diseases transmitted by biting midges, such as the Blue Tongue virus which can infect sheep, goats, and cattle. Biting midges tend to overwinter as larvae and pupate the following spring or early summer. Most species of biting midge produce multiple generations each year with differing seasonal patterns of population abundance.
Leptoconcops spp.
Species of Leptoconocops tend to be most problematic in the West Slope, most notably in areas around Colorado National Monument and within canyon country. In many Leptoconcops spp., adult females must acquire a blood meal to complete egg development. The larvae are small and resemble white worms or miniature caterpillars. Adults typically emerge after rain in late spring or early summer, and emergence can be delayed during periods of drought. Eggs are laid in masses on moist surfaces in streams, ponds, or organically rich semi-aquatic habitats such as marshes, bogs, tree holes, and saturated rotting wood. Species associated with livestock tend to reproduce in saturated soils of wastewater ponds and seepage from watering troughs. Some species, such as L. kertszi, inhabit semiarid regions of the southwest and can disperse up to 10 miles from the larval development site. Bites are most likely to occur during the day, especially during periods of midmorning and dusk, which can be a nuisance to campers, fishermen, hikers, and others who spend time outdoors during these periods.
Culicoides spp.
Midge species in this genus are widely distributed throughout Colorado. Adults are under 3.2 mm (1/8 inch) long and have grey bodies with dense hairs on the wings, which can be used for species identifications. These flies have large compound eyes and well-developed mouthparts adapted for blood sucking in females. The eggs of Culicoides spp. measure about 0.25 mm long, are cigar-, banana-, or sausage-shaped, and are initially white but darken as the embryo develops. The worm-like larvae are white and roughly 2-5mm (<1/5 inch) long. Culicoides pupae are pale yellow or brown and roughly the same size as larvae. As with the patterns of wing hairs on adults, the arrangement of spines on the pupal integument can be used for species identifications.
Most Culicoides spp. can only disperse 0.5-1 mile from the development site of larvae in search of vertebrate blood and tend to be most abundant near productive breeding sites. Adults tend to be most active at dusk and dawn. A single female can lay anywhere from 25 to over 100 eggs after each blood meal, depending on the species. Favored egg-laying sites include muddy or sandy surfaces in and around swamps, the shores of streams or ponds, and in saturated substrates. Larvae feed on decaying organic matter in mud, wet soil, intertidal sand, water holes, or seepage areas, and require high moisture for their development. Larvae develop through four instars before pupating, and adults emerge from the puparium after two to three days.
In Colorado, biting midges are not considered a concern to public health. However, bites produced by Leptoconcops spp. and Culicoides spp. can be irritating, painful, and cause long-lasting painful lesions. Individuals bitten by these small flies often report experiencing a sensation of being bitten, but an inability to see the biting insect, which often leads to them being mistaken for mosquito bites. Some individuals have an allergic reaction to insect bites, which results in significant itching around the feeding site.
Adult female of Culicoides spp. After acquiring a blood meal, the underside of the abdomen often appears red. The females of many biting fly species must feed on blood to complete their reproductive development. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult fly of the family Simuliidae. Note the pronounced hump on the thorax. Image credit: Darren Blackford, USDA Forest Service, Bugwood.org
Larvae and pupae of Simulium decorum. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Bites inflicted by Culicoides spp. can cause severe itchiness around the affected area for days. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult of Chrysops spp, which are classified as a group of flying insects commonly called deer flies. Note that the antennae are longer than the length of the head. Image credit: Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Egg mass and young larvae of Chrysops spp. Image credit: Clemson University – USDA Cooperative Extension Slide Series , Bugwood.org
Allergic reaction to bites inflicted by Chrysops spp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult of Symphoromyia fulvipes feeding on human blood. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Adult stable fly grooming. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Chrysops spp. and Silvius spp.
The relatively large flies (family: Tabanidae) are commonly called deer flies and are closely related to another group of flies commonly referred to as horseflies, which are occasional nuisance pests of livestock in Colorado and do not usually bite people, except for Hybomitra spp., which occur at higher elevations. Evidence suggests that adults of Chrysops spp. and Silvius spp. can transmit a bacteria (Francisella tularensis) that cause tularemia in the western United States, also known as deer fly fever and rabbit fever. However, they play a minor role in the disease transmission.
Chrysops spp. and Silvius spp. are medium-sized flies that measure about 7-10 mm (~1/3-2/5 inch) long and are often gray, yellow to brown, or dark brown with large eyes. Some species also have dark markings on the wings and body in the form of dark patches or stripes, respectively. The antennae of Chrysops spp. and Silvius spp. are longer than the head and slender. The mouthparts of adults include two blade-like cutting structures used for piercing skin and a sponge-like mouthpart for sucking blood. The larvae measure 1.9-3.8 cm (3/4 – 1½ inches) long and are tan, white, or brown spindle-shaped maggots that taper at both ends. The cylindrical eggs measure 1-2.5 mm (<1/10 inch) long and are white initially but become darker as the embryo develops. The pupae are brown with a rounded front end and a tapered rear.
Females require a blood meal to complete egg development and rely on visual cues that indicate the shape, color, and movement of potential hosts. Shades of blue, black, or red appear to be attractive to these flies, and they tend to bite during the day. Once a host is identified, the initial probing stages involve repeated and persistent attacks until blood flow begins. The fly then begins feeding, which introduces chemicals in saliva that prevent coagulation and maintain blood flow. When not mating or feeding, adults can be observed resting on shrubs or tall grasses.
These flies overwinter as larvae. Eggs are laid on grasses or other aquatic vegetation along the banks of small ponds or other permanent bodies of standing water. One or more generations can be produced each year, depending on species. After hatching, the young larvae begin developing in substrates around the water’s edge such as mud and plant matter. The larvae feed on decaying organic matter and small invertebrates.
Bites inflicted by Chrysops spp. and Silvius spp. tend to be painful and itchy. Some individuals can experience allergic reactions and develop welts or red bumps. Although uncommon, these biting flies can transmit tularemia, which is characterized by skin ulcers, fever, and headaches. Antibiotics can help individuals recover from tularemia, which can be fatal if left untreated.
Symphoromyia spp.
Symphoromyia spp. (family: Rhagionidae), commonly referred to as “snipe flies”, are closely related to tabanid flies and a very similar feeding mechanism. Symphoromyia spp. tend to be distributed near forested areas at higher elevations in Colorado and can sometimes enter homes. Species in this genus feed on the blood of mammals, and these flies commonly attack bison in the summer after they have shed their winter coats. Other hosts include deer, cattle, horses, dogs, and humans.
Adults are medium to large flies that measure 8-15 mm (3/10-3/5 inch) long and have round heads with long legs. The thorax of Symphoromyia spp. is brown, grey, or black with yellow or orange markings, and the abdomen is grey, black, or black and yellow and tapers at the rear. The biology and habits of Symphoromyia spp. are not well-understood, but larvae develop in moist soils of grassy woodlands, rotting wood, or decaying vegetation.
Bites tend to occur on the face, neck, and hands. The bite is described as painful and can result in moderate to severe inflammation. Droplets of blood can pool at the feeding site. There is no definitive connection between bites of Symphoromyia spp. and the transmission of diseases.
Stable fly (Stomoxys calcitrans)
The stable fly (family: Muscidae) is a close relative of the common house fly (Musca domestica). Unlike the house fly, this blood-feeding insect is known to attack any warm-blooded and is considered a major pest of livestock throughout the world. The stable fly is gray, measures about 4-7 mm (~3/20-3/10 inch) long and can be distinguished from the house fly based on the presence of a dark red or brown proboscis, two broad pairs of dark thoracic stripes, black abdominal markings, and wing venation patterns. When at rest, stable flies partially spread their wings and hold their head higher than the abdomen. The worm-like larvae have black, rounded triangular spiracles widely spaced at the rear. Newly emerged larvae are about 1.25 mm (<1/20 inch) long and translucent, while mature larvae are pale yellow or white and reach lengths of 11-12 mm (<1/2 inch). The pupal stage is characterized by a red-brown body and measures about 4.5-6mm (1/5-1/4 inch) long. The eggs of stable flies are sausage-shaped, white, and roughly 1 mm (<1/20 inch) long.
In addition to feeding on nectar and other sugar sources, both male and female stable flies feed on blood and persistently bite warm-blooded animals, which is necessary for sexual maturation. The adults are strong fliers, and stable flies will readily bite humans when livestock are not available. When fly densities are low, this pest tends to attack the ears of dogs and the lower legs of cattle, horses, and humans, but will feed on other body regions at high fly densities. Stable flies orient their heads upwards when feeding on livestock, unlike some other economically important blood-feeding flies such as the horn fly (Haematobia irritans), who orient their heads toward the ground when feeding. This pest feeds during the day and is most active during the early morning and late afternoon. Adults are very persistent and often ignore swatting, stamping, or other behaviors meant to disturb the flies.
In temperate regions, stable fly populations often peak in early and late summer. Female stable flies only mate with one partner, while male flies can inseminate multiple females. After mating, females lay eggs in small batches on suitable substrates. As many as 800 eggs have been produced by a single female in laboratory conditions, but this number is likely much less in field conditions. Larvae develop through three instars as they feed on fermenting vegetive matter or animal waste. Development time for immature stable flies depends on temperature and humidity, with optimal conditions facilitating development in as little as 13 days. Overwintering is accomplished as immatures decrease their development rate in heat-producing substrates that remain above freezing temperatures. In protected environments that regulate climatic conditions such as dairy barns, development can be continuous. Short- or long-term dispersal of adults can lead to recolonization in areas where stable flies cannot survive winter.
Stable flies inflict painful bites that have been described as a sharp needle prick on the feet, ankles, behind the knees, and other parts of the leg. Bites on humans are often concentrated around the ankle region and cause raised, red bumps and red rashes to develop at the feeding site. When flies feed on the ears of confined animals, bloody sores develop and are slow to heal which can result in considerable scarring. The bites inflicted by stable flies can also become infected with secondary pathogens. When densities are high, significant disease transmission can occur in the livestock population. One disease of livestock, African Swine Fever, can be transmitted through bites or when an infective fly is ingested. Other diseases transmitted from stable flies to livestock include habronematidosis, equine infectious anemia, anthrax, and anaplasmosis.
Quick Facts
- Certain species of fly must feed on the blood of vertebrates to complete their reproductive development. The exact appearance, life history, and pest impact varies among these species.
- Many of these biting flies are primarily considered a nuisance pest. However, some, such as the stable fly, can cause significant economic loss by reducing cattle production. In the United States, approximately $2 billion are lost each year due to stable fly infestations, which interfere with livestock feeding, reproduction, and resting behaviors.
- Attacks on humans can be prevented by wearing long-sleeved clothing in problematic areas. Treating clothing with chemicals such as DEET can help repel some biting flies.
- Various sanitary practices focus on removing or reducing the availability of reproductive sites.
- Chemical management of biting flies focuses on eliminating different larvae or adults, depending on the pest. Treating breeding sites with insecticides helps kill developing larvae, and treating resting sites or host animals kills adults.
Management
Simulium spp.
When entering areas where Simulium spp. are problematic, wearing light-colored clothing with minimal openings such as buttonholes is recommended. The use of fine-mesh head nets can protect exposed feeding sites on the face such as around the eyes and ears. The use of repellents containing DEET or other active ingredients can help prevent biting of Simulium spp. However, these chemical repellents should be used sparingly by children and pregnant women. Since these flies do not usually enter indoor areas, providing livestock with refuge shelters can help alleviate feeding pressure during periods of heavy black fly activity. The entrances of animal shelters can include self-application devices for repellents, which provide an added measure of protection. Sealing holes with adhesives can help exclude flies.
Chemical management of Simulium spp. focuses on the larval stage since adults are highly mobile and often migrate miles from their natal water body in search of a blood meal. For this reason, spraying insecticides around individual yards is unlikely to provide effective control. Management typically involves the use of a toxin produced by the bacterium Bacillus thuringiensis, which often involves weekly treatments of water bodies during periods of peak fly activity. The toxin produced by B. thuringiensis can be applied by hand or aerial spraying. However, such treatments can be expensive and require specialized training for safe and effective application. Large-scale treatments are often conducted by public operators hired by local government bodies.
Biting midges
Reducing the availability of breeding sites is recommended by filling low-lying areas, diking, and adopting practices that help regulate water levels such as maintaining an appropriate irrigation schedule. Removing bodies of stagnant water rich in organic matter and eliminating seepage areas associated with water troughs in or around livestock facilities can help limit the spread of certain livestock diseases transmitted via biting midges. Properly maintaining farm ponds and controlling water level fluctuations in waste lagoons can help suppress the biting midge population. However, little information is available on the magnitude by which habitats must be modified to significantly reduce populations of biting midges and limit the spread of livestock diseases. Animals should be housed at least two miles away from confirmed breeding sites.
To prevent bites, outdoor activity can be minimized during peak biting periods (i.e., dawn and dusk), and treating clothing with DEET can help repel biting midges. Providing shelter for livestock can help alleviate feeding pressures against species of biting midge that tend to avoid indoor areas. Other species will readily enter shelters, in which case a residual insecticide can be applied to surfaces such as the walls and roof. Area sprays of insecticides must be applied daily and have a very short residual activity, making them costly and less efficient than other control measures.
Chrysops spp. and Silvius spp.
Removing vegetation around ponds will reduce the availability of egg-laying sites. Wearing long-sleeved clothing treated with an insect repellent can be an effective approach to prevent bites. Trapping can be an effective approach to reducing populations of these biting flies in confined and manageable areas. One trap, called the “Manitoba trap” uses a glossy black or red heat-absorbing sphere with a diameter of 0.6 m (2 feet) suspended about 0.91 m (3 feet) above the ground in a cone-shaped foundation. On arrival, flies enter a space around the trap and move upward into a chamber that captures and prevents them from escaping. Another trap called the “trolling deer fly trap” uses a nursery pot or blue plastic cup covered with a sticky substance and placed upside-down on a rod. The trap is then affixed to a slow-moving truck or lawnmower traveling at slow speeds (<7 miles per hour), or on a hat worn atop a person’s head. The goal is to move the trap through space, which provides the visual cues necessary to attract flies to the object. Flies are then captured upon contact with the sticky exterior surface.
Insecticides can be used in a variety of ways. Resting sites, such as shrubbery or tall grasses, can be treated with insecticides to control adults. Treating livestock ear tags and head collars with insecticides can also be effective. The inner walls of the Manitoba trap can also be treated with a residual insecticide, which renders the capture chamber unnecessary.
Symphoromyia spp.
Installing door and window screens or keeping these entrances shut is recommended to prevent these flies from entering homes. The use of sticky fly paper ribbons can help eliminate small numbers of flies indoors. Lastly, all development sites should be removed from the vicinity, such as garbage, rotting fruits, animal waste, and animal carcasses. Chemical management is not usually necessary for controlling Symphoromyia spp.
Stable flies
Seemingly small, ephemeral development sites can produce large numbers of stable flies. Therefore, removing and sanitizing potential immature development sites for stable flies is an important component of stable fly management. Practices that promote easy removal of bedding and waste or drying of manure and waste will help reduce the availability of reproductive sites for stable flies. Promptly removing rotting feed and waste-contaminated bedding can be helpful, and in situations where bedding cannot be easily removed, treating with an insect growth regulator can help limit stable fly development. During removal, it is important that the materials be disposed of properly to ensure the breeding site is not simply being moved from one location to another. Separating animal waste and vegetation is recommended, in addition to moving bale hay placement sites, keeping dry silage above ground level, covering silage, and preventing the accumulation of undisturbed silage. Cleaning any spillage of feed and choosing the right bedding can help reduce the availability of development sites for stable flies. Sand, gravel, and sawdust are poor substrates for stable fly development, while straw, corn, and soybean stalks are more favorable for stable fly development.
In addition to the sanitation practices described above, monitoring is necessary for effective management. Counting the number of flies on the front legs of at least 15 animals in the early morning and early afternoon can help determine whether treatments are necessary. On cattle, if greater than 15 flies per animal are observed, this suggests that a productive site for stable fly development may be present in the vicinity. The use of certain sticky traps may help when monitoring stable fly populations near horses since horses tend to be more sensitive to bites and will readily disturb flies, making it more difficult to obtain accurate counts. Traps also exist for monitoring adults as they emerge from development sites or for monitoring mature larvae when they leave the development site in search of a suitable location to pupate. The use of traps can help reduce nearby fly populations as well, but requires effective placement, trap density, and regular maintenance. In addition to sampling for adults, core or trowel samples allow for direct assessments of the immature life stages. The sample does not need to be very deep as most larvae will be occupying the top 5 cm (~2 inches) of the surface to be sampled. It is important to note that house fly larvae often share development sites with stable flies, and distinguishing between immature stages of these two species is critical for this monitoring approach to be effective.
General use pesticides registered for managing other biting flies, lice, or ticks are usually effective against stable flies. Such products often contain pyrethroids as the active ingredient and are applied to animals via spraying or pouring or sprayed directly on the resting sites of adult stable flies. On-animal applications should be directed and thoroughly applied on the lower legs where stable flies prefer to feed. However, such applications are often short-lived since the lower legs often become wet or dirty as the animals move. Furthermore, these flies only visit animal hosts when seeking a blood meal, which means the application window for insecticide contact is very limited. Resistance to pyrethroids has been documented in several countries, including the United States.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
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Connelly, R., 2019. Biting midges, no-see-ums. University of Florida – Extension. Available here.
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EPA. 2023. Biting Midges and Schools. United States Environmental Protection Agency. Available here.
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Mizell, R. 1998. The Trolling Deer Fly Trap. North Florida Research and Education Center. Available here.
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