Catches in the Gambia, West Africa, of Anopheles gambiae Giles and A. gambiae var. melas Theobald in Entrance Traps of a baited portable wooden Hut, with special Reference to the Effect of Wind Direction

Abstract

A portable wooden hut with two windows, on opposite sides, fitted with traps to catch mosquitos as they entered the hut was tried, with human bait, as a sampling device for mixed populations of Anopheles gambiae Giles and A. gambiae var. melas Theo. in and near Keneba village in the West Kiang district of the Gambia.In eighteen night catches, between 24th August and 22nd September 1954, 5,901 females of this species and its variety were taken in the traps, five high catches (642 to 1,240 females) being made before 3rd September and thirteen low catches (27 to 306 females) between 6th and 22nd September. Possible causes for the different size of catches in the two periods are discussed with the conclusion that the differences in catch are not simply attributable to erratic functioning of the hut as a trap.Four night catches of 186, 741, 797 and 1,240 females of A. gambiae and its variety are reported in detail. They reveal that mosquitos were taken virtually exclusively in the leeward trap if the wind persisted from one direction. But if the wind was variable, or conditions were calm, they were taken in both traps, entering the traps alternately as the wind shifted in the first instance and, perhaps simultaneously in calm. The results show that the use of only one trap could give misleading information about the numbers of mosquitos available outside the hut.There is no doubt that, with wind, the mosquitos approached the host in the hut from down-wind. Since much of the catching occurred in darkness it would seem that the mosquitos were attracted by the emanations of the host and that their approach to the hut was not directed by a visual mechanism controlling up-wind flight.Heavy rain for about seven hours did not prevent flight.From hourly collections, made from dusk to dawn, a curve of activity for A. gambiae with its variety melas was obtained resembling the biting cycle reported elsewhere by other authors, the greatest activity occurring between midnight and 0600 hr. All the females (except two instances of incomplete fresh blood-meals) were unfed and, in a small sample, ovarian development was stage II or earlier. First entries occurred just before complete darkness in the evening and the last in good light about 30 minutes after sunrise.No males of A. gambiae or var. melas were taken in the traps.The numbers, all females, of other species taken were: 1 of A. funestus Giles, 1 of A. rufipes var. ingrami Edw., 59 of Taeniorhynchus (Mansonioides) spp., 30 of Culex nebulosus Theo., 19 of C. thalassius Theo., and 15 of unidentified species. Some Culicoides occurred in the traps and in the hut itself.The hut, if modified to include a window trap in each of the four walls and a cowl over each window to keep out rain, appears to be a simple and convenient sampling device for certain types of investigations on the biology and behaviour of A. gambiae and its variety melas.