Pyrethroid insecticides induce an excito-repellent effect that reduces contact between humans and mosquitoes. Insecticide use is expected to lower the risk of pathogen transmission, particularly when impregnated on long-lasting treated bednets. When applied at low doses, pyrethroids have a toxic effect, however the development of pyrethroid resistance in several mosquito species may jeopardize these beneficial effects. The need to find additional compounds, either to kill disease-carrying mosquitoes or to prevent mosquito contact with humans, therefore arises. In laboratory conditions, the effects (i.e., repellent, irritant and toxic) of 20 plant extracts, mainly essential oils, were assessed on adults of Anopheles gambiae, a primary vector of malaria. Their effects were compared to those of DEET and permethrin, used as positive controls. Most plant extracts had irritant, repellent and/or toxic effects on An. gambiae adults. The most promising extracts, i.e. those combining the three types of effects, were from Cymbopogon winterianus, Cinnamomum zeylanicum and Thymus vulgaris. The irritant, repellent and toxic effects occurred apparently independently of each other, and the behavioural response of adult An. gambiae was significantly influenced by the concentration of the plant extracts. Mechanisms underlying repellency might, therefore, differ from those underlying irritancy and toxicity. The utility of the efficient plant extracts for vector control as an alternative to pyrethroids may thus be envisaged.

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Essential oil samples of Cinnamosma fragrans from two regions in Madagascar, Tsaramandroso (38 samples) and Mariarano (30 samples), were analysed by GC/MS. Fifty-seven components were identified, accounting from 88.3% to 99.4% of the oils' composition. The major components were linalool (72.5 ± 23.3%) in Tsaramandroso and 1,8-cineole (47.3 ± 10.2%) in Mariarano. Samples B8 (95.8% linalool) from Tsaramandroso and B143 (71.6% 1,8-cineole) from Mariarano containing the highest proportions of the two main components identified, were selected to determine antimicrobial activities against 10 microbial strains. Bacillus subtilis and Staphylococcus aureus were the most sensitive strains to both oils. Minimum inhibitory concentration (MIC) values were lower for B143 against all tested Gram-negative strains than pure 1,8-cineole. B8 showed higher MIC values than pure linalool against Salmonella typhimurium and Vibrio alginolyticus, and similar MIC values to linalool towards the other Gram-negative strains. Both essential oils exhibited higher MIC values towards Fusarium oxysporum than their respective pure major component. These results suggested the occurrence of synergism or antagonism effects between the different oil constituents.