Age effects on the insemination rate ofAnopheles gambiae s.l. in the laboratory

Age effects on the insemination rate of the Galisua strain ofAnopheles gambiae s.s. (Diptera: Culicidae) and of the Nyanza strain ofAn. arabiensis were investigated in the laboratory. Batches of 7-day old males and females were kept together for 24 h with batches of mosquitoes of the opposite sex of ages ranging from 1–7 days. Males and females were also kept together continuously from emergence. The effect of increasing male/female ratios on the insemination rate was investigated as well. The insemination rate ofAn. arabiensis in the laboratory (96%) was similar to that found in nature, whereas that of our strain ofAn. gambiae s.s. was consistently low (maximum 72%), particularly after females had been together with males for a 24 h period only. The optimum age for insemination was 7 days for males and females of both species. The insemination rate of our strain ofAn. arabiensis was significantly higher at all ages than that of our strain ofAn. gambiae s.s.. The latter strain became inseminated at the earliest when 4 days old. An increase in the male/female ratio significantly enhanced the insemination rate in both strains. It was found that motility of spermatozoa inside the spermatheca, once in contact with saline, could be used as a marker for the approximate time of insemination. It is concluded that the relatively low insemination rate of the Galisua strain ofAn. gambiae s.s. is unlikely to be caused by the rearing conditions. Other factors that may be responsible are discussed.