ABSTRACT:INVESTIGATIONS OF THE AFRICAN MALARIA MOSQUITO (ANOPHELES GAMBIAE S.L., DIPTERA: CULICIDAE): OVIPOSITIONAL BEHAVIOR AND TOXICITY OF BLOOD-BORNE INSECTICIDES ByMegan L. Fritz Anopheles gambiae sensu stricto is a major malaria vector in Sub-Saharan Africa. High malaria morbidity and mortality rates are experienced by humans, mainly children under the age of 5, living in endemic regions. Studiesof the basic biology of this mosquito, including oviposition, provide a background for assessing which attributes might be exploited for suppressing An. gambiae s.s. populations. Ovipositional periodicity of groups assessed using a modified battery-powered wall clock revealed bimodal egg deposition. Oviposition by caged An. gambiae s.s. groups is most probable in early to mid scotophase, or early photophase. Confined An. gambiae s.s. individuals oviposit in a single ca. 2-4 h continuous bout per 24 h. However, some oviposition can occur at any hour during 24 h, especially if females were previously deprived of ovipositional substrate. Many females sit on a dark, wet horizontal substrate while ovipositing. However, vertical resting sites are adequate perches from which oviposition can occur if they provide high humidity and the paramount dark and wet ovipositional cues for An. gambiae s.s. oviposition. Under such conditions, laboratory tests revealed that An. gambiae s.s. were as likely to rain eggs down from a vertical perch, as to oviposit while sitting horizontally on a substrate of moist mud or open water. These studies confirm remarkable ovipositional flexibility by An. gambiae s.s., and I conclude that oviposition is not a suitable target for An. gambiae s.s. management. Host choice appears to be an attribute that is relatively restricted for An. gambiae sensu lato. Despite increasing selection pressure imposed by ITN use in Western Kenya, most blood meals from An. gambiae s.s. were imbibed from human hosts, whereas sibling species, An. arabiensis, blood fed upon cattle. Prior to 1998 An. gambiae s.s. comprised between > 70% of the total An. gambiae s.l. population in Kisian village. In the present study, >50% of the An. gambiae s.l. collected were identified as An. arabiensis, revealing a shift in the dominant vector species. Ivermectin, a cattle dewormer, is highly lethal to both An. gambiae s.s. and An. arabiensis. When cattle are treated with ivermectin, ninety percent of An. gambiae s.s. that fed upon them within 2 wk of treatment failed to survive >10 d post blood meal. No eggs were deposited by An. gambiae s.s. fed on ivermectin-treated cattle within 10 d of treatment. According to the label, commercial formulations of ivermectin cannot be used on pregnant or lactating animals. Laboratory tests of eprinomectin, a commercially available dewormer for pregnant and lactating cattle, revealed equivalent toxicity to An. arabiensisat low concentrations (LC50 of 8.5 ppb). These results suggest that cattle treated with ivermectin or eprinomectin in the prescribed range of low dosages as parasiticides have blood toxic to zoophilic malaria vectors. Regionally-coordinated, seasonal treatment of cattle could suppress An. arabiensis populations, thereby reducing malaria transmission.
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