Habitat discrimination by gravid Anopheles gambiae sensu lato--a push-pull system

Abstract

Background: The non-random distribution of anopheline larvae in natural habitats suggests that gravid females discriminate between habitats of different quality. Whilst physical and chemical cues used by Culex and Aedes vector mosquitoes for selecting an oviposition site have been extensively studied, those for Anopheles remain poorly explored. Here the habitat selection by Anopheles gambiae sensu lato (s.l.), the principal African malaria vector, was investigated when presented with a choice of two infusions made from rabbit food pellets, or soil.

Methods: Natural colonization and larval survival was evaluated in artificial ponds filled randomly with either infusion. Dual-choice, egg-count bioassays evaluated the responses of caged gravid females to (1) two- to six-day old infusions versus lake water; (2) autoclaved versus non-autoclaved soil infusions; and assessed (3) the olfactory memory of gravid females conditioned in pellet infusion as larvae.

Results: Wild Anopheles exclusively colonized ponds with soil infusion and avoided those with pellet infusion. When the individual infusions were tested in comparison with lake water, caged An. gambiae sensu stricto (s.s.) showed a dose response: females increasingly avoided the pellet infusion with increasing infusion age (six-day versus lake water: odds ratio (OR) 0.22; 95% confidence interval (CI) 0.1-0.5) and showed increasing preference to lay eggs as soil infusion age increased (six-day versus lake water: OR 2.1; 95% CI 1.4-3.3). Larvae survived in soil infusions equally well as in lake water but died in pellet infusions. Anopheles gambiae s.s. preferred to lay eggs in the non-autoclaved soil (OR 2.6; 95% CI 1.8-3.7) compared with autoclaved soil. There was no change in the avoidance of pellet infusion by individuals reared in the infusion compared with those reared in lake water.

Conclusion: Wild and caged An. gambiae s.l. females discriminate between potential aquatic habitats for oviposition. These choices benefit the survival of the offspring. Although the study was not designed to distinguish between stimuli that acted over a distance or on contact, it could be demonstrated that the choice of habitat is mediated by chemical cues based on both preference and avoidance. These cues, if identified, might be developed for 'push-pull' strategies to improve malaria vector monitoring and control.

Figure 1

Location of egg-count bioassays. (A) Sheds (10 m long × 5 m wide × 2.8 m high) with walls made of reed mats and a roof made of translucent corrugated polycarbonate sheets. (B) Interior of a shed. In each shed two tables hold up to 25 standard cages each, allowing 40 cm of space between adjacent cages.

Figure 2

Natural colonization of artificial habitats. Daily average of early instar larvae in (A) pellet infusions; (B) soil infusions.

Figure 3

Survival of Anopheles gambiae sensu stricto larvae to the pupal stage kept in different infusions or lake water. Error bars show 95% confidence intervals.

Figure 4

Proportion of gravid Anopheles gambiae sensu stricto laying eggs in infusions of different ages compared with control water. (A) Pellet infusion experiment; (B) soil infusion experiment.

Figure 5

Oviposition response of caged Anopheles gambiae sensu stricto to pellet (Set 1)and soil (Set 2) infusions of different incubation times and non-autoclaved and autoclaved 6 day soil infusion (Set 3). Multiple comparison of treatments: treatments denoted with the same letter are not significantly different.

Figure 6

Egg laying responses of Anopheles gambiae sensu stricto reared in lake water or in pellet infusion to lake water and pellet infusion (Set 4).