State-dependent domicile leaving rates in Anopheles gambiae.

BACKGROUND: Transmission of Plasmodium greatly depends on the foraging behaviour of its mosquito vector (Anopheles spp.). The accessibility of blood hosts and availability of plant sugar (i.e., nectar) sources, together with mosquito energy state, have been shown to modulate blood feeding (and thus biting rates) of anopheline mosquitoes. In this study, the influence of mosquito starvation status and availability of nectar on the decision of female Anopheles gambiae mosquitoes to leave a bed net-protected blood host was examined. METHODS: Two small-scale mesocosm experiments were conducted using female mosquitoes starved for 0, 24 or 48 h, that were released inside a specially constructed hut with mesh-sealed exits and containing a bed net-protected human volunteer. Floral cues were positioned on one side of the hut or the other. Several biologically plausible exponential decay models were developed that characterized the emigration rates of mosquitoes from the huts. These varied from simple random loss to leaving rates dependent upon energy state and time. These model fits were evaluated by examining their fitted parameter estimates and comparing Akaike information criterion. RESULTS: Starved mosquitoes left domiciles at a higher rate than recently fed individuals however, there was no difference between 1- and 2-day-starved mosquitoes. There was also no effect of floral cue placement. The best fitting emigration model was one based on both mosquito energy state and time whereas the worst fitting model was one based on the assumption of constant leaving rates, independent of time and energy state. CONCLUSIONS: The results confirm that mosquito-leaving behaviour is energy-state dependent, and provide some of the first evidence of state-dependent domicile emigration in An. gambiae, which may play a role in malarial transmission dynamics. Employment of simple, first-principle, mechanistic models can be very useful to our understanding of why and how mosquitoes leave domiciles.

Energy-state dependent responses of Anopheles gambiae (Diptera: Culicidae) to simulated bednet-protected hosts.

In nature, Anopheles gambiae mosquitoes are found at various energy levels and such females must choose between seeking somatic energy from sugar sources and obtaining both somatic and gametic energy from blood hosts. We used a straight-tube olfactometer containing a simulated unobtainable blood host (human foot smell protected by a net) as well as a sugar source (honey odor). We assessed female probing rate and residence time at the net as a function of energy state (0, 24, 48, 72-h starved). In our trials, 0-h starved females showed low response to human odor, low probing rate, and residence time at the human odor site. By contrast, both 48 and 72-h individuals showed high response to foot odor, longer residence time, and higher probing rates. Seventy-two-h females also flew towards the honey source less often than other groups. Our findings suggest that managing sugar sources might be a viable strategy for influencing mosquito biting behavior.