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Order: Diptera There are around 40 species of small biting black flies, <\/span>Simulium spp. <\/span><\/i>(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 <\/span>S. griseum<\/span><\/i>, <\/span>S. bivittatum<\/span><\/i>, and <\/span>S. apricorium<\/span><\/i>. 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 <\/span>Simulium spp.<\/span><\/i>\u00a0<\/span><\/p>\n 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 <\/span>Simulium spp. <\/span><\/i>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 <\/span>Simulium spp. <\/span><\/i>are 2-3 mm (~1\/10 inch) long.<\/span>\u00a0<\/span><\/p>\n Adults are most active during the day and can migrate miles from breeding sites. An individual <\/span>Simulium spp. <\/span><\/i>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.\u00a0<\/span>\u00a0<\/span><\/p>\n 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.<\/span>\u00a0<\/span><\/p>\n Reactions to bites inflicted by <\/span>Simulium spp. <\/span><\/i>vary. On some individuals, bites produce a small red spot that bleeds initially. More severe reactions to bites of <\/span>Simulium spp.<\/span><\/i> are characterized by<\/span> 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 <\/span>Simulium spp. <\/span><\/i>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.\u00a0<\/span>\u00a0<\/span><\/p>\n Biting<\/span> 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.<\/span>\u00a0<\/span><\/p>\n Species of <\/span>Leptoconocops <\/span><\/i>tend to be most problematic in the West Slope, most notably in areas around Colorado National Monument and within canyon country. In many <\/span>Leptoconcops spp.<\/span><\/i>, 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 <\/span>L. kertszi<\/span><\/i>, 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.<\/span>\u00a0<\/span><\/p>\n 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 <\/span>Culicoides spp. <\/span><\/i>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. <\/span>Culicoides <\/span><\/i>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.<\/span>\u00a0<\/span><\/p>\n Most <\/span>Culicoides spp.<\/span><\/i> 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.<\/span>\u00a0<\/span><\/p>\n In Colorado, biting midges are not considered a concern to public health. However, bites produced by <\/span>Leptoconcops spp. <\/span><\/i>and <\/span>Culicoides spp.<\/span><\/i> 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.<\/span>\u00a0<\/span><\/p>\n [\/et_pb_text][\/et_pb_column][et_pb_column type=”1_2″ _builder_version=”4.16″ custom_padding=”|||” global_colors_info=”{}” custom_padding__hover=”|||”][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Culicoides-adult-female-1.jpg” alt=”adult brown dog tick” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Culicoides adult female” sticky_enabled=”0″]<\/p>\n Adult<\/span> female<\/span> of <\/span><\/span>Culicoides<\/em> spp.<\/span><\/span> After <\/span>acquiring<\/span> a blood meal, the underside of the abdomen often appears red. The females of many <\/span>biting fly species<\/span> must feed on blood to complete their reproductive development. Image credit: <\/em><\/span>Whitney Cranshaw, Colorado State University, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Simuliidae-adult.jpg” alt=”winter tick” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Simuliidae adult” sticky_enabled=”0″]<\/p>\n Adult fly <\/span>of<\/span> the family <\/span>Simuliidae<\/span>.<\/span> Note the pronounced hump on the thorax. Image credit: <\/em><\/span>Darren Blackford, USDA Forest Service, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Simulium-decorum-larvae-and-pupae.jpg” alt=”Adult Otobius lagophilus” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Simulium decorum larvae and pupae” sticky_enabled=”0″]<\/p>\n Larvae<\/span> and pupae of <\/span><\/span>Simulium<\/span> decorum<\/span><\/span><\/em>. Image credit: <\/em><\/span>Whitney Cranshaw, Colorado State University, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Culicoides-bites.jpg” alt=”adult ear tick” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Culicoides bites” sticky_enabled=”0″]<\/p>\n Bites inflicted by <\/span><\/span>Culicoides<\/em> spp.<\/span><\/span> can cause severe itchiness around the affected area for days. Image credit: <\/em><\/span>Whitney Cranshaw, Colorado State University, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Chrysops-adult.jpg” alt=”rash outbreak rocky mtn spotted fever” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Chrysops adult” sticky_enabled=”0″]<\/p>\n Adult<\/span> of <\/span><\/span>Chrysops<\/em> <\/span>spp<\/span><\/span>, which <\/span>are <\/span>classified as a group of flying insects commonly called deer flies<\/span><\/span>. <\/span><\/span>Note that the antennae are longer than the length of the head. Image credit: <\/em><\/span>Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Chrysops-egg-mass-and-larvae.jpg” alt=”rash outbreak rocky mtn spotted fever” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Chrysops egg mass and larvae” sticky_enabled=”0″]<\/p>\n Egg mass <\/span>and young larvae <\/span>of <\/span><\/span>Chrysops<\/em> spp. <\/span><\/span>Image credit: Clemson University – USDA Cooperative Extension Slide <\/span>Series ,<\/span> Bugwood.org<\/span><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Chrysops-allergic-reaction.jpg” alt=”rash outbreak rocky mtn spotted fever” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Chrysops allergic reaction” sticky_enabled=”0″]<\/p>\n Allergic reaction to bites<\/span> inflicted by <\/span><\/span>Chrysops<\/em> spp. <\/span><\/span>Image credit: <\/span>Whitney Cranshaw, Colorado State University, Bugwood.org<\/span><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Symphoromyia-fulvipes-adult-feeding-on-human.jpg” alt=”rash outbreak rocky mtn spotted fever” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Symphoromyia fulvipes adult feeding on human” sticky_enabled=”0″]<\/p>\n Adult of <\/span><\/span>Symphoromyia<\/span> <\/span>fulvipes<\/span><\/span><\/em> feeding on human blood. Image credit: <\/em><\/span>Whitney Cranshaw, Colorado State University, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][et_pb_blurb image=”https:\/\/agsci.colostate.edu\/agbio\/wp-content\/uploads\/sites\/127\/2025\/04\/Adult-stable-fly-grooming.jpg” alt=”rash outbreak rocky mtn spotted fever” _builder_version=”4.27.4″ _module_preset=”default” body_line_height=”1.4em” image_icon_custom_margin=”||-22px||false|false” hover_enabled=”0″ global_colors_info=”{}” title_text=”Adult stable fly grooming” sticky_enabled=”0″]<\/p>\n Adult stable fly<\/span> grooming<\/span>.<\/span> Image credit: <\/span>Whitney Cranshaw, Colorado State University, Bugwood.org<\/span><\/em><\/span>\u00a0<\/span><\/em><\/p>\n [\/et_pb_blurb][\/et_pb_column][\/et_pb_row][et_pb_row admin_label=”row” _builder_version=”4.16″ background_size=”initial” background_position=”top_left” background_repeat=”repeat” hover_enabled=”0″ global_colors_info=”{}” sticky_enabled=”0″][et_pb_column type=”4_4″ _builder_version=”4.16″ custom_padding=”|||” global_colors_info=”{}” custom_padding__hover=”|||”][et_pb_text _builder_version=”4.27.4″ _module_preset=”default” header_font_size=”36px” hover_enabled=”0″ global_colors_info=”{}” sticky_enabled=”0″]<\/p>\n 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 <\/span>Hybomitra spp.<\/span><\/i>,<\/span> which occur at higher elevations. Evidence suggests that adults of <\/span>Chrysops spp<\/span><\/i>. and <\/span>Silvius spp.<\/span><\/i> can transmit a bacteria (<\/span>Francisella tularensis<\/span><\/i>) 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.<\/span>\u00a0<\/span><\/p>\n Chrysops spp. <\/span><\/i>and <\/span>Silvius spp.<\/span><\/i> 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 <\/span>
Family: Simuliidae<\/span>, <\/span>Ceratopogonidae<\/span>, Tabanidae<\/span>, <\/span>Rhagionidae<\/span>, Muscidae<\/span><\/p>\nDescription, life history, and bites<\/h2>\n
Simulium <\/i>spp.<\/h3>\n
Biting midges<\/h3>\n
Leptoconcops <\/i>spp.<\/i><\/h3>\n
Culicoides <\/i>spp.<\/i><\/h3>\n
Chrysops <\/i>spp. <\/i>and Silvius <\/i>spp.<\/i><\/h3>\n