Simulium species — black flies or buffalo gnats

Flies of the genus Simulium (blackflies or buffalo gnats) occur in almost all parts of the world, including Canada.


Flies of the genus Simulium (blackflies or buffalo gnats) occur in almost all parts of the world, including Canada.  The life cycle involves egg laying in fast-flowing ("white") water, and development of the hatched larvae through several stages before pupation, still in the water,  and emergence of the adult flies.  The major problem caused by Simulium for domestic animals, particularly cattle, is blood feeding by the adult females, which often move around and attack in swarms.  This behaviour can be very distressing for affected animals and can interfere with grazing and production.  The nasty bite wounds themselves are often troublesome, and frequently take some time to heal.  Adult Simulium can travel considerable distances to find hosts or water suitable for egg laying.  Simulium species will also attack people, causing considerable irritation.

Adult females Simulium are also vectors for several pathogens, most notably Onchocerca volvulus, a nematode parasite of people in west and central Africa, Yemen, and areas of south and central America, that causes river blindness - so-called because of the rivers required for the life cycle of the fly, and because microfilariae of the parasite can invade the cornea, causing opacity.  Simulium can also transmit other species of Onchocerca of domestic animals, and the protozoan Leukocytozoon among birds.


In horses

Black flies, or buffalo gnats, of the genus Simulium are dipterans that have an essentially free-living life cycle during which the adult males and females blood feed on a variety of mammals, including people, and birds.  Black flies breed in white water in rivers and streams, are strong fliers and tend to attack in swarms.  They are vicious biters, preferring areas of the body with little hair, for example the face and the ventral thorax and abdomen.  Black flies secrete a toxin that exacerbates the effects of the bites.  They produce papules and wheals, which may become necrotic and haemorrhagic.  Simulium can also be associated with hypersensitivity reactions in horses, which may be systemic if the animals are attacked by swarms of flies.



Phylum: Arthropoda
Subphylum: Uniramia
Class: Hexapoda
Order: Diptera
Suborder: Nematocera
Family: Simuliidae

The Family Simuliidae contains approximately twenty genera and fifteen hundred species. The genus Simulium contains approximately 1700 species, among which S. arcticum and, more recently, S. luggeri are the most important livestock pests in the prairies.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.


Adult Simulium species are small, dark and characteristically hump-backed, measuring up to approximately 5 mm in length. The wings are short.

Host range and geographic distribution

Depending on the species, black flies blood-feed primarily on either mammals or birds. They are found in almost all parts of the world.

Life cycle - direct

Adult Simulium are free-living and most species mate on the wing close to sites for egg laying. Female black flies blood-feed prior to egg laying. Male black flies do not blood-feed. The eggs are deposited in moving water, either directly into the water – these eggs drop to the bottom - or on objects in the water. The eggs are attached to the substrate by a silky thread. Under ideal conditions, the larva in each egg develops over several days and hatches. The larvae spin a long silk thread so that they remain attached to the substrate and drift downstream until they find a new substrate, to which they attach. The larva then spins a cocoon and pupates, and subsequently the adult fly emerges within an air bubble, which floats to the surface. Almost all black fly species in Canada have an annual cycle, with one generation of adults each year; an exception is S. luggeri that has several. Some species survive over winter as eggs, others as larvae.



Black flies are a major pest for domestic animals, including poultry and for wildlife and people, in many areas of Canada. Essential for their success is access to moving water, especially rapidly moving water, which is abundant in Canada.

It is interesting that man-made changes to hydrology can influence the ecology of black flies. For example, construction of the Gardiner Dam on the South Saskatchewan River removed rapids and made the river much less suitable for breeding by S. arcticum, a major pest species. The slower river is, however, ideal for S. luggeri, which is fortunately a less serious pest.

Another important aspect of the epidemiology of black flies is the ability of many species to travel great distances – up to 150 km – from the breeding sites, and to form huge swarms that attack animals and, very rarely, people. The greatest risk of bites is often in areas closest to the breeding sites.

Black flies in the prairies bite during the day, and when it is sunny biting is more common in the early morning and in the evening. On cloudy days, however, biting activity can persist all day.

Pathology and clinical signs

In Canada, the major problem with black flies is the blood feeding by the adult females. In other areas of the world they are vectors for a range of pathogens, notably Onchocerca volvulus, the nematode that causes river blindness in people in areas of Africa and Central and South America, and Leucocytozoon species, important tissue protozoans of birds in Canada and elsewhere.

Female black flies find their hosts initially by odor and rising carbon dioxide concentration and, when within a meter or two, by sight. Biting involves probing and attachment by the proboscis, slashing by other mouthparts and the formation of a small subcutaneous haematoma, which is then ingested. The feeding process takes approximately five minutes.

Cattle and other animals attacked by black flies during the spring and summer may be adversely affected by the irritation and/or blood loss associated with the bites, especially if there are large numbers of flies attacking simultaneously, for example from swarms, by a hypersensitivity to the flies’ saliva, or by toxins introduced from the flies during feeding. The irritation can cause a range of non-specific clinical signs, and reduce the productivity of cattle. Massive attacks can be fatal for cattle, and occasionally other hosts, probably as a result of the introduction of large amounts of toxins. Cattle in areas endemic for black flies may be less affected than newly introduced, naïve animals.

Treatment and control

There are a few topical products approved in Canada for control of black flies on cattle.

 Drug(s)  Product(s)
 Pyrethrins  VARIOUS

The use of these products is controlled by the Pest Control Product (PCP) Act. 

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

The best control measures are directed against the larvae in rivers and streams and are often based on biological control. If only small numbers of animals are involved and a swarm is anticipated, housing the animals will be helpful.

Public health significance

Female black flies will blood feed on people, usually singly or in small numbers, but sometimes in swarms. The affected person may not realize they are being bitten until a typical lesion develops: round, pink and itchy, often with blood at the centre. Undisturbed by scratching, these bites heal in a few days. Rarely, people may develop a systemic reaction to the bites – "black fly fever". In Canada there is minimal pathogen transmission to people by black flies.
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