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Black Flies

Appearance & Identification

image of black fly

With more than 160 species present in Canada, black flies are especially annoying pests. To make their prevalence worse, several species are known for biting people and sucking their blood, like mosquitoes. Black flies range in size from 2 to 6 mm and typically have large wings, an arched thorax, large eyes, and short antennae. Most species have a black body, although some are yellow or even orange.


Females lay their eggs on partially submerged objects in fast flowing waters. They prefer running streams to those that are stagnant. Black fly larvae remain underwater after attaching themselves to a static object like a rock or branch and can remain there for several weeks. When development is complete and the adults finally surface, they immediately fly away but tend to stay within 16 km of their breeding source unless carried farther by air currents.


Black flies feed on flower nectar, though females also bite people for a blood meal needed for egg development. This is most common shortly before they lay eggs. Depending on the species, females will also feed on other mammals or birds. They will not venture indoors specifically searching for blood, making the black fly mainly an outdoor pest.


In other parts of the world, black flies are vectors of important diseases, however biting black flies in Canada are most assuredly annoying but fortunately are not known to transmit diseases to people. A black fly bite can cause different reactions in humans, ranging from a small wound to severe swelling. There also is so-called “black fly fever,” a collection of symptoms including headache, upset stomach, and fever. Even the non-biting black flies make pests of themselves by flying around people’s heads and crawling into their ears, eyes, nose, and mouth.


Most black fly remedies are aimed at eradicating larvae by larvaciding breeding sites, as adults can be difficult to control. To protect against being bitten or ending up with a mouthful of flies, it’s best to avoid peak periods of black fly activity during daytime hours in May and June. They are particularly fond of hot, humid days. When outside in infested areas, use appropriately labeled insect repellent, wear protective clothing, and consider fine-mesh head netting, similar to those worn by beekeepers.