Studies on the attraction of Simulium damnosum s.l. (Diptera: Simuliidae) to its hosts. I. The relative importance of sight, exhaled breath, and smell.

Four traps used in capturing hunting female S. damnosum s.l. are discribed: the slat trap, the enclosure trap, the fan trap, and the sticky trap. In forest a man hidden inside a trap, but removing his exhaled breath through a long hose, attracted 4 times as many flies as did an unbaited trap. When the man exhaled normally into the trap the number of flies taken rose again by a further factor of 4. Carbon dioxide gas, emitted from inside the trap attracted some two-thirds as many flies as a man exhaling normally inside the trap. In both forest and Sudan-savanna environments the catches of a man stationed on an open stretch of land and fully exposed to view while his exhaled breath was removed through a long hose, were not substantially lower than those of a man in a similar situation who exhaled normally. In forest a motionless man, partly hidden among vegetation, attracted fewer flies than a moving man who was fully exposed to view (a) when both men removed their exhaled breath through long hoses, and (b) when they exhaled normally. In the forest a trap baited with CO2 gas (250 cc per min) caught more flies than a similar, unbaited trap, but the preformance of CO2-baited traps, as compared with that of human collectors, was highly variable. Catches in CO2-baited traps were uniformly low in the Sudan-savanna. In forest, removal of olfactory substances from the human skin, by vigorous washing and application of petroleum jelly, or by wearing impermeable clothing, greatly reduced the numbers of flies attracted. No such reduction was observed in Sudan savanna. In the forest, cloths, worn by men for several days and then used as bait, attracted about 10 times as many flies as did similar, unworn cloths. Addition of CO2 gas produced a 4-fold increase in the attractiveness of worn cloths and an 18-fold increase in that of unworn cloths. No corresponding experiments were done in the savanna. Fan traps could become contaminated by human handling during the assembly and setting-up procedures. They were thus rendered attractive to flies. In the Sudan-savanna, the catches of men positioned in front of warm rock surfaces were lower than those of men stationed on coller, sandy surfaces. This suggests that body heat may be an additional attractant factor. The results suggest that "forest" S. damnosum s.l. (presumably S. squamosum) females rely heavily on smell as an attractant and, to a lesser extent, on sight and exhaled breath. Smell appears to be the only obligatory attractant, and it can act by itself. This raises the possibility that a trap might be developed which incorporates only smell stimuli as bait. For "savanna" S. damnosum s.l. (presumably S. damnosum s.s.) neither smell nor exhaled breath appear to be important attractants, and some other factor, perhaps sight, appears to be the dominant attractant in this zone.