Black Flies
All You Wanted to Know...and More

No other biting flies inspire such apprehension as do black flies. Black flies often land and take off repeatedly without biting. Their numbers, and their tendency to bite, increase as sunset approaches. Relief comes after dark, for unlike mosquitoes and biting midges, black flies do not attack at night. Also unlike mosquitoes, black flies seldom attack indoors or ven in a vehicle; once they sense being trapped their attention seem permanetly diverted to escape.

Although they cannot bite through clothing, black flies have a predilection for crawling into their hair or under clothing, biting in inaccessible places, such as the ankles and belt line. Tucking trouser cuffs into socks will normally prevent them from getting at the ankles.

Black flies are strongly influenced by colour – they find dark hues more attractive than pale ones, and blue, purple, brown, and black more attractive than white or yellow. A light-coloured shirt, therefore, is a much better choice of clothing than a dark blue one.

In southern Canada, black flies are on the wing from early May (coincident with the bursting of buds of forest trees, especially sugar maple, before mosquitoes appear in numbers) until mid-June.

There are more species of black flies than of mosquitoes in Canada; over 100 have already been recorded, and there are more than have not even been named. Black flies are more selective in their choice of host than are mosquitoes, and comparatively few species take human blood. Most species seem to feed only on the blood of birds and a substantial percentage apparently do not take blood at all, because their mouthparts have degenerated and appear useless for bloodsucking. It is the female of the species that are blood feeders.

Females deposit eggs, 200-800 per female, on vegetation just below the water surface. Larvae emerge from eggs and attach themselves to acquatic or emergent vegetation as well as rocks. Most black fly larvae are filter feeders, with the larvae feeding on nutrients in the water as it flows by. Larvae pass through six stages before reaching the pupal stage.

Pupae are encased in a silken cocoon attached to vegetation or other objects in the stream. Adults emerge from the pupal case through a slit and float to the surface on a bubble of air. Some species matwe as soon as adults emerge. The length of the cycle from egg to adult is variable, depending on the black fly species and water temperature. Emerging adults live from two to three weeks, to a long as 85 days. Almost all species of black flies in Canada have a single annual generation.

This article first appeared in the Ottawa Rambling Club newsletter, the Rambler, Summer 2001 (Vol. 8 No. 2).