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Black Fly - Biology

Black Fly Biology and Life History

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Adult black flies (Diptera: Simuliidae) are a severe worldwide pest of humans and animals. Female adults, commonly known as gnats or buffalo gnats, swarm around the head and body of selected hosts in order to obtain blood required for egg production. Through blood feeding, black flies can be important vectors of diseases. They are responsible for transmitting onchocerciasis (river blindness) to millions of people in tropical areas of the world. Black flies can vector other serious diseases such as bovine onchocerciasis, mansonellosis, and leucocytozoonosis in wild and domestic animals. Eastern equine encephalitis (EEE) virus has been detected in two bird-feeding black fly species in Wisconsin, but these species have not been shown to transmit EEE to humans. The first evidence that black flies may play a role in the transmission of arthropod borne viruses was discovered in Arizona in 1992 with vesicular stomatitis virus-New Jersey serotype (VSNJ). Epidemiological research demonstrated that a common black fly species was a competent biological vector of VSNJ and that black flies may be responsible for periodic outbreaks of this disease in livestock, wildlife, and humans in the western United States.

Fortunately, most Pennsylvania black fly species do not feed on human blood and, therefore, do not transmit parasites, pathogens, and diseases to people. However, some species of black flies do cause serious annoyance and discomfort by their persistent biting, swarming, and crawling behavior. This can result in significant economic losses for areas that depend on recreation and tourism. These small (2-5 mm in length) dark flies reduce our enjoyment of outdoor activities and deter our desire to visit areas with annoying adults. Adult females enter the eyes, ears, nose, and mouth of humans and animals as they locate hosts in their search for blood. Severe allergic reactions to black fly bites are known to occur in some cases. Large black fly populations have been shown to cause enormous agricultural losses through livestock mortality and by decreasing milk, beef, and egg production in livestock and poultry industries.

Fifty-three (53) black fly species in 7 genera have been found in Pennsylvania. A total of 254 species in 12 genera have been described from North America and approximately 2,000 species have been discovered worldwide. The aquatic stages are often abundant and important organisms in river ecosystems, where the larvae filter and eat fine food particles from the water column. Black flies play in important role in aquatic and terrestrial food chains where they are preyed upon by many insect predators, fish, amphibians and birds.

The immature stages of black flies are aquatic and exclusively inhabit flowing streams and rivers. They can be found in almost every stream in Pennsylvania, from our tiny, cold spring flows to our very large, warm rivers. Only streams that are severely impaired by acid mine drainage or pollution will not support black fly populations. However, black flies are not found in lakes, ponds, swamps, and other standing water habitats. Stream flow is essential for transporting food and oxygen to the immature stages.


Figure 1. Typical Black Fly Life Cycle Diagram including egg, larval, pupal, and adult stages.


The life cycle includes four stages: egg, larva, pupa, and adult (Figure 1). All are aquatic except the adults, which leave the water to search for food and mates. Black fly immatures have three general life history strategies. One group of species produces 1 generation per year (univoltine) that matures in late winter or early spring. A second group is also univoltine, but these species develop during late spring or summer. DEP does not treat for these early-season black flies, as treatments would be ineffective due to their widespread distribution and short life cycles. The third and final group of species produces 2 or more generations per year (bivoltine or multivoltine) that typically develop from early summer through fall.

Adult females deposit from 150 to 500 eggs in flowing water. Egg laying occurs near dusk for many species. The eggs are dropped singly from the air or deposited in masses on trailing vegetation, rocks, debris and other substrates. Eggs hatch in 2 days to 8 months, depending on black fly species and water temperature. Incubation time in some species is delayed by a prolonged diapause, or resting period. Eggs of many species can successfully withstand temperature extremes, fluctuating water levels, and desiccation associated with alternating flood and drought conditions during seasonal changes. Many species overwinter in the egg stage, but a few black flies spend the winter months as larvae and pupae, or rarely, as adults.

Larvae anchor themselves to clean vegetation, rocks, or debris by spinning a small silken pad with their mouthparts and inserting a row of hooks at the end of their enlarged abdomen into the silk pad. This technique allows the larvae to secure themselves in areas of very fast water velocity and orient their body with the abdomen pointed upstream, and head positioneddownstream to feed. Larvae can easily relocate to other areas by drifting downstream on a silken thread, spinning a new silk pad, and reattaching themselves in areas with more acceptable substrates or food supplies. Feeding is accomplished by expanding a pair of fan-like structures on their hardened head capsule to efficiently filter microscopic food particles from the water column. The larvae filter or scrape very fine organic matter, filamentous algae, bacteria and tiny aquatic animals from the current or substrates. Larvae are often infected with various parasites and pathogens, including nematode worms, bacteria, fungi, protozoa and viruses. Larval instars vary from 4 to 9, depending on species, with many species passing through an average of 7 instars. Larval development time varies from 1 week to 6 months depending on species, water temperature, stream turbidity and food availability. Larval growth is very temperature dependent, with relatively slow growth during the cold winter months and very rapid growth during warm summer water temperatures. Some summer-developing, multivoltine species are capable of completing their entire life cycle in justa few weeks. Mature larvae, with fully developed respiratory filaments visible as a dark area on each side of the thorax, stop feeding, and construct a silken pupal cocoon where metamorphosis takes place.

Pupae secure themselves inside their cocoons with rows of spine-like hooks on their abdomen. The tightly woven or loose cocoons, characteristically shaped for each species, are attached to substrates with the closed end facing upstream to protect pupae from current and sediments. Some species have a lateral aperture, or window, on each side of the cocoon to increase water circulation around the pupa. The branched respiratory organs that project from the pupal thorax are designed to function in or out of water. This adaptation allows pupae to obtain oxygen at all times, and survive normal fluctuations in water levels. Number of respiratory filaments in Pennsylvania black flies varies from 4 to 77 per side, depending on species. The pupal stage may last from 2 days to several weeks depending on the species and water temperature.

Adults emerge from the pupal skin through an elongate slit at the top of the thorax and ride a bubble of air that propels them to the water surface. Freshly emerged adults fly to streamside vegetation where their wings and bodies quickly dry and harden. Mature adults immediately seek food sources and mates. Both sexes feed on nectar, sap, or honeydew to obtain the sugar used for flight and energy. Only females feed on blood. In most species, mating takes place in flight, with females flying into male swarms that form over landmarks such as waterfalls, vegetation or host species. Males utilize their large eyes to detect and seize females entering the swarm. Male and female pairs exit the swarm, and mating takes place in flight in just a few seconds. Females then seek a host to obtain the blood meal required to nourish their eggs. Adults are strong fliers, capable of dispersing many miles from their larvalhabitats.

Black fly females are attracted to their specific hosts by size, shape, color, carbon dioxide, body odor, body movement, skin texture, temperature and humidity. Females use their mouthparts to cut, or lacerate the host skin, and then drink from the resulting pool of blood. Anticoagulants in the saliva are injected into the bite to facilitate bleeding. Many domestic and wild animals have been killed by outbreaks of adult black flies. Deaths have been attributed to acute toxemia from large numbers of bites, anaphylactic shock, and weakness due to blood loss. In humans, lesions can develop at the bite, accompanied by reddening, itching, and swelling. In severe cases, allergic reactions may occur, resulting in nausea, dizziness, and fever.

Host specificity in black flies varies from highly specific species that will feed on blood from only 1 host, to much more generalized species that will draw blood from a number of different hosts. Although host preferences for many North American black flies are poorly understood, it is estimated that 67% feed on mammals and 33% feed on birds. Approximately 10% of North American species will feed on the blood of humans. In Pennsylvania, 4 closely related black fly species in the Simulium jenningsi complex are responsible for most of the human pest problems during the summer recreational season. The immature stages of this group prefer large, clean, warm streams and rivers. Unfortunately, these species can produce many generations of annoying adults during a single season. The Pennsylvania Black Fly Suppression Program focuses control efforts on these 4 pest species from April to September each year, to reduceblack fly populations in the Commonwealth to acceptable levels.


Further Reading

Adler, P.H. and K.C. Kim. 1986. The Black Flies of Pennsylvania (Simuliidae, Diptera): bionomics, taxonomy, and distribution. Penn. State Univ. Agric. Exp. Stn. Bull. 856:1-88.

Adler, P.H., Currie, D.C. and D.M. Wood. 2004. The Black Flies (Simuliidae) of North America. Comstock Pub. Assoc. 960 pp.

Crosskey, R.W. 1990. The Natural History of Black Flies. John Wiley, Chichester, U. K.Davies, J.B. Black flies (Diptera:Simuliidae); available from; Internet.

Green, S. Black Flies and Their Control; available from ; Internet.

Kim, K.C. and R.W. Merritt (eds.). 1987. Black flies: Ecology, population management, and annotated world list. Pennsylvania State University Press, University Park, PA. 528 pp.