THE POPULATION AND ACTIVITY OF ADULT FEMALE BLACK FLIES IN THE VICINITY OF A STREAM IN ALGONQUIN PARK, ONTARIO

Abstract

The population of females of Simulium venustum Say on the wing was measured by hand-netting. The population varied annually and seasonally coincident with the number and longevity of immigrant and emergent flies. Longevity in turn was probably related to the rainfall.Activity was divided into flying, attraction, landing, and biting, the first two measured by hand-netting and the last two by counts on a unit area of human skin in the shade. Flying usually varied diurnally, a large peak in the evening and a small one in the morning. Attraction varied with none of the meteorological factors measured. Biting and flying increased and landing decreased with rapidly changing, especially falling, pressure. Other factors influenced flying and landing, but not biting, when the area was in the shade. Flying was greatest between 60–80° F., at low but not zero saturation deficiencies, in light winds, and in zero to low rates of evaporation. Landing on the host was least below 55 °F. and at 65–75 °F., at zero and intermediate saturation deficiencies, and at moderately high rates of evaporation. These factors affected the flies landing on the host directly, and indirectly by curtailing the flying that brought flies to the host in the first place. In the direct sunlight landing decreased to one-half and biting; to one-quarter of that in the shade.