Novel insecticides and application strategies for malaria vector control

Abstract

Targeting malaria vector mosquitoes outdoors has become a research priority to address residual malaria transmission. Mosquito larval source management provides an excellent and well established tool. However, there is a need to reduce the cost and effort of larviciding programmes by testing persistent larvicides that reduce the frequency of application and by exploring novel strategies of application. This thesis aimed to evaluate two larval control agents, with unique mode of actions: the self-spreading silicone-based film Aquatain Mosquito Formulation (AMF) and the pyriproxyfen-based insect growth regulator Sumilarv®. Dose-response tests and standardized field tests were conducted to assess the susceptibility of Anopheles gambiae sensu stricto and An. arabiensis to the two insecticides and determine their residual activity. Adults that survived exposure to larvicide-treated water at the larval stages were assessed for possible effects on fecundity and fertility. Both species were highly susceptible to both control agents at very low doses. Both control agents provided residual control of up to six weeks. Adults that emerged from larvicide-treated water laid fewer eggs and had low egg hatching rates. Consequently, the impact of three-weekly operational application of pyriproxyfen to habitats in the western Kenya highlands was assessed by comparing adult vector emergences from aquatic habitats in intervention and non-intervention sites. Pyriproxyfen application led to over 80% adult vector emergence inhibition from treated aquatic habitats. To assess if larvicide-treated water could serve as ‘reproductive sinks’ for gravid mosquitoes, the oviposition response of gravid An. gambiae s.s. to water treated with pyriproxyfen or surface film was tested under semi-field conditions using squares of electrocuting nets. Larvicide-treated water did not affect the pre-oviposition behaviour of gravid females. This study however did not demonstrate that ‘attract and kill’ strategies could be used for control of malaria vectors as the addition of an oviposition attractant to ponds containing larvicide-treated water did not increase the proportion of gravid females orienting towards this pond. To explore the effect of pyriproxyfen exposure on adults, individual An. gambiae s.s. and Cx. quinquefasciatus females were exposed to pyriproxyfen at seven time points around blood feeding. Fecundity and ability to transfer pyriproxyfen to an oviposition substrate 4 were studied in the laboratory. The impact of pyriproxyfen was dependent on the time of exposure. Females were nearly completely sterilized when exposure occurred around the blood meal while pyriproxyfen was only transferred by females that were exposed while gravid and close to egg-laying time. Consequently, a baiting station for gravid females was developed and semi-field experiments implemented to explore the transfer of pyriproxyfen by gravid An. gambiae s.s. from the baiting station to aquatic habitats. Horizontal transfer was observed but the extent of emergence inhibition was dependent on the distance of the habitat from the baiting station. Only the closest habitats received sufficient pyriproxyfen to control significant numbers of offspring. In conclusion, this study demonstrated great potential of the two control agents for the control of vector immature stages and adults caused by sterilizing effects of pyriproxyfen. Results suggest that they are suitable for inclusion into integrated vector management programmes for malaria control. Auto-dissemination of pyriproxyfen however, appears not to be a feasible strategy for malaria vector control.