Identification Of Candidate Volatiles That Affect The Behavioural Response Of The Malaria Mosquito Anopheles Gambiae Sensu Stricto To An Active Kairomone Blend: Laboratory And Semi-field Assays

Abstract

Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) is the most important vector of human malaria in sub-Saharan Africa, affecting the lives of millions of people. Existing tools such as insecticide-treated nets and indoor-residual sprays are not only effective, but also have limitations as a result of the development of resistance to insecticides and behavioural adaptations in biting time. Therefore, novel mosquito-control tools are needed. Odour-releasing traps have a potential for both trapping and surveillance purposes. Based on the outcome of ex vivo gene expression assays and in vivo electrophysiological assays on hundreds of volatile organic compounds, 29 ‘candidate behaviourally-disruptive organic compounds' (cBDOCs) are selected, belonging to 10 chemical categories, to be assayed in the laboratory using dual-choice olfactometers for the ability to modify the ‘attractiveness’ (i.e. significantly more insects being caught in the associated trap) of a basic volatile blend consisting of ammonia, lactic acid and tetradecanoic acid but without additional carbon dioxide. Compounds that either reduce or increase trap catches by the basic blend in the olfactometer experiments are tested under African conditions in choice experiments in a semi-field facility in Kenya. The release rates of cBDOCs are determined gravimetrically to allow the calculation of aerial concentrations at the trap outlet. Aerial concentrations in the sub-p.p.m. range are reported for the first time to influence mosquito behaviour. The results of the olfactometer and semi-field assays generally correspond. Under semi-field conditions, three compounds, 3-heptanol (0.025 p.p.m.), 2-methylpropanal (0.05 p.p.m.) and 4,5-dimethylthiazole (0.73 p.p.m.), significantly increase trap catches relative to the basic blend consisting of ammonia, lactic acid, tetradecanoic acid and carbon dioxide. 2-Acetylthiophene, 2-nonanone and 2-phenylethanol decrease the number of mosquitoes caught at all concentrations tested under semi-field conditions. These compounds hold promise as attractants and spatial repellents to be applied in mosquito control programmes.