Egg hatching, larval movement and larval survival of the malaria vector Anopheles gambiae in desiccating habitats

Abstract

Background: Although the effects of rainfall on the population dynamics of the malaria vector Anopheles gambiae have been studied in great detail, the effects of dry periods on its survival remain less clear.

Methods: The effects of drying conditions were simulated by creating desiccated habitats, which consisted of trays filled with damp soil. Experiments were performed in these trays to (i) test the ability of An. gambiae sensu stricto eggs to hatch on damp soil and for larvae to reach an artificial breeding site at different distances of the site of hatching and (ii) to record survival of the four larval stages of An. gambiae s.s. when placed on damp soil.

Results: Eggs of An. gambiae s.s. hatched on damp soil and emerging larvae were capable of covering a distance of up to 10 cm to reach surface water enabling further development. However, proportions of larvae reaching the site decreased rapidly with increasing distance. First, second and third-instar larvae survived on damp soil for an estimated period of 64, 65 and 69 hrs, respectively. Fourth-instar larvae survived significantly longer and we estimated that the maximum survival time was 113 hrs.

Conclusion: Short-term survival of aquatic stages of An. gambiae on wet soil may be important and adaptive when considering the transient nature of breeding sites of this species in sub-Saharan Africa. In addition, the results suggest that, for larval vector control methods to be effective, habitats should remain drained for at least 5 days to kill all larvae (e.g. in rice fields) and habitats that recently dried up should be treated as well, if larvicidal agents are applied.

Figure 1

Experimental set-up of the egg hatching experiment showing the positions of the filter paper relative to the simulated breeding site.

Figure 2

Observed and expected proportions of larvae in the simulated breeding sites, placed during the egg stage at different distances from the site. Different letters indicate significant differences (P <0.05, Tukey HSD) between the observed proportions at the different distances. Significant differences (P <0.05, paired t-test) between observed and expected proportion of a distance are indicated by '*'; n.s. = not significant

Figure 3

Average proportion (± s.e.) of larvae of An. gambiae s.s. surviving on damp soil after exposure to different periods of drought. The lines show the models after back-transformation of the linear model (arcsine(√ p) = a * time + b) obtained after arcsine square root transformation of the original data. Model descriptions are given in Table 2. A: first-instar larvae, B: second-instar larvae.

Figure 4

Average proportion (± s.e.) of larvae of An. gambiae s.s surviving on damp soil after exposure to different periods of drought. The lines show the models after back-transformation of the linear model (arcsine(√ p) = a * time + b ) obtained after arcsine square root transformation of the original data. Model descriptions are given in Table 2. C: third-instar larvae (observed for 48 hours only), D: fourth-instar larvae