Chemical ecology of the behaviour of the filariasis mosquito Culex quinquefasciatus Say

Culex quinquefasciatus is an important vector of urban bancroftian filariasis in the tropical world. Despite its public health importance, much of its olfactory mediated behaviour is poorly understood. Studies on resource-location behaviour, in particular the role of semiochemicals in its behaviour, are required to understand the relationship between the mosquito, its host and the surrounding environment to effectively control bancroftian filariasis. In this thesis the role of semiochemicals in the host-location behaviour and oviposition site-selection of Cx quinquefasciatus has been examined.The olfactory responses of the host-seeking female Cx quinquefasciatus to various odour stimuli in a dual-choice olfactometer showed that the mosquito responds more to foot skin emanations than to carbon dioxide and moisture. These results present evidence that kairomones used during the host-seeking process by Cx quinquefasciatus are present in human skin emanations. The preference of Cx quinquefasciatus between humans, and domestic animals, was investigated in the field. The response of the mosquito to the humans, cattle and goats was also compared to its response to carbon dioxide. The mosquito responded to human olfactory cues in significantly larger numbers than to the calf or goat cues. The response of the mosquito to either the calf or goat was similar.It can be concluded that with equal availability of the three vertebrates, Cx quinquefasciatus would respond significantly more to cues from human hosts than from either a calf or a goat. The major olfactory cue from goat or a calf, to which this mosquito responds is probably carbon dioxide. Furthermore, the response of Cx quinquefasciatus, Anopheles gambiae s.l. and An. funestus to human odour was compared with that to carbon dioxide in the field. Human odour attracted a larger number of host-seeking Cx quinquefasciatus, An. gambiae and An. funestus than carbon dioxide.In an indoor situation human body odour other than carbon dioxide is the principal cue to which these mosquito species respond and that the physical cues from a host and carbon dioxide, when used as a kairomone on its own, account for a minor part of the overall attractiveness of man. Skin emanations and various specific organochemical compounds known to attract other blood sucking Diptera were tested for Cx quinquefasciatus in the field. It was found for the first time that Cx quinquefasciatus can be trapped into traps baited with human skin residues. Furthermore a larger number of Cx quinquefasciatus were caught in traps baited with carbon dioxide than in traps baited with either acetone, octenol or butyric acid. The combination of carbon dioxide and skin emanations resulted in an additive effect. These results indicate that under field conditions Cx quinquefasciatus can be sampled by traps baited with skin emanations and/or carbon dioxide.As far as the odour-mediated oviposition behaviour is concerned it was found that the oviposition by Cx quinquefasciatus occurs more frequently in the presence of the mosquito oviposition pheromone and/or skatole than in plain water. The daily oviposition rhythm of Cx quinquefasciatus showed two peaks, the higher at twilight and the lower peak at dawn. The biological activities of the synthetic oviposition pheromone (acetoxyhexadecanolide) and skatole were found to last for over 9 and 7 days respectively.It was further found that the combination of synthetic oviposition pheromone and soakage pit water or grass infusions resulted in a synergistic response in the oviposition by Cx quinquefasciatus, Cx cinereus and Cx tigripes. However, a blend of synthetic oviposition pheromone and skatole resulted into an additive oviposition response by Cx quinquefasciatus . Oviposition semiochemicals can thus be used as ovitraps or gravid mosquito traps to monitor Cx quinquefasciatus populations or to attract mosquitoes to sites treated with a biopesticide for vector control.The efficiency of various odour baited sampling devices was also investigated. It was found that the efficiency of the widely used Centers for Disease Control (CDC) light trap in sampling an indoor population of Cx quinquefasciatus and An. gambiae is affected by the position of the trap in relation to the human-baited bed net. Significantly higher catches were recorded for both species when the trap was positioned at the foot-end of the bed, near the top of the bed net. Parity rates were significantly higher near the top of the net than at the level of the host. In addition, infective Cx quinquefasciatus were caught in the trap positioned above the foot-end of the bed net.Various trapping systems baited with carbon dioxide were compared in sampling outdoor populations of An. gambiae and Cx quinquefasciatus . The efficiency of the Counterflow Geometry (CFG) trap was similar to that of electric nets (ENT). Both CDC traps with light or without light were less efficient in collecting An. gambiae or Cx quinquefasciatus outdoors. It is possible, therefore, to collect the two mosquito species outdoors with CFG traps or ENT baited with carbon dioxide. Finally an assessment of traps baited with oviposition semiochemicals in sampling of gravid Cx quinquefasciatus is reported. A CFG trap baited with either a synthetic oviposition pheromone, grass infusions or the combination of the two was found to be a useful tool for collecting gravid Cx quinquefasciatus both indoors and outdoors. However, the proportion of gravid mosquitoes in the catches increased when the traps were placed away from mosquito emergence sites. In conclusion, the study shows that chemical ecology plays a significant role in the life cycle of Cx quinquefasciatus and this principle can be used to develop new control strategies