The late biting habit of parous Anopheles mosquitoes and pre-bedtime exposure of humans to infective female mosquitoes.

Using the all-night landing catch method (18:00-06:00) we showed, for Anopheles gambiae in Sierra Leone and A. punctulatus in Papua New Guinea, that parous females have a tendency to bite later than nulliparous ones. The biting habit of sporozoite-infected A. punctulatus was also investigated. The sporozoite rates for Plasmodium falciparum and P. vivax were 1.8 and 1.4% respectively, but only one (1.3%) of 76 females infected with P. falciparum was caught between 18:00 and 21:00. A significantly higher proportion (11.6%) of mosquitoes infected with P. vivax was caught in the same period. The late biting habit of mosquitoes infected with P. falciparum is discussed in relation to the differential biting habits of parous and nulliparous females. We conclude with a hypothesis that, in areas where Anopheles mosquitoes have a late-biting cycle and low parous rate, exposure to mosquitoes infected with P. falciparum during the pre-bedtime period (18:00-22:00) is very low. This hypothesis could explain why insecticide-treated bed nets protect children better in areas of seasonal transmission, where nulliparous females tend to predominate, than in areas of perennial transmission, where parous females are usually more numerous. The same hypothesis is compatible with the finding in Papua New Guinea that insecticide-impregnated bed nets are more protective against P. falciparum than against P. vivax malaria.