'A mate or a meal' – Pre-gravid behaviour of female Anopheles gambiae from the islands of São Tomé and Príncipe, West Africa

Background: Malaria prevalence differs between the two islands that comprise the archipelago of Sao Tome and Principe. This may be due to differences in the biology of local Anopheles gambiae, the only vector on the islands. Survival rate and feeding frequency are two factors influencing vectorial capacity. Anophelines generally feed just once per gonotrophic (oviposition) cycle. Newly emerged insects, however, may feed two or more times during their first oviposition cycle thus increasing the likelihood of becoming infected. The reasons for multiple feeding are not clearly understood and it is still uncertain whether the behaviour is facultative or obligatory. We, therefore, determined survival and sporozoite rates, and examined the behaviour of An. gambiae from the two islands during their first gonotrophic cycle. Methods: The wing size of 1,410, abdominal condition of 687, gonotrophic age and mated status of 7,264 female M form An. gambiae collected by light-trap, landing catch, resting outdoors or in copula, was determined from four sites in the archipelago. Sporozoite rates assessed by ELISA in 15,533 females from Sao Tome and 2,111 from Principe were determined. Results: Estimated survival rates ranged between 0.834–0.849 per day in Sao Tome and 0.801– 0.818 per day in Principe. Sporozoite rates of 0.63% in Sao Tome were significantly higher than the 0.24% from Principe. Overall 49% of females mated on the second night after emergence before feeding, and 51% on the third night and thus fed before mating. The likelihood of mating before feeding increased with wing size. None of the 3,776 parous insects collected showed evidence of recent mating. All but two of the 198 females collected in copula had undeveloped ovaries. Mean wing sizes and the number of insects collected in a sentinel light-trap varied but the proportion of newly emerged insects in the collection did not. The estimated survival rate of the smallest insects was lower than other size groups, but the overall size distribution of each age group was normal. Parous insects were gonotrophically concordant.