Synergism between nonane and emanations from soil as cues in oviposition-site selection of natural populations of Anopheles gambiae and Culex quinquefasciatus

Abstract

Background: Olfactory cues have been shown to have an important role in guiding gravid mosquito females to selected sites for egg laying. The objective of this study was to determine the influence of emanations from soil from a breeding site and the putative oviposition pheromone nonane on oviposition-site selection of natural populations of Anopheles gambiae s.l. and Culex quinquefasciatus.Methods: This field-based study was conducted in Mvomero District in East-central Tanzania. In a dual-choice experimental set up, clay bowls were dug into the ground and filled with one of the following treatments: (i) distilled water + autoclaved soil (control), (ii) distilled water + soil from a natural mosquito breeding site, (iii) distilled water + nonane and (iv) distilled water + nonane + soil from a natural breeding site. Soil was either left untreated or was autoclaved before use. The number of larvae present in the bowls after 10 d was used as outcome measure.Results: Autoclaved soil had a significant effect on oviposition behaviour of Cx. quinquefasciatus (P<0.005) but no effect on An. gambiae (P= 0.18). Nonane and emanations from untreated soil significantly influenced the selection of oviposition sites by both An. gambiae (P< 0.0001) and Cx. quinquefasciatus (P<0.0001). A mixture of nonane and untreated soil caused a synergistic effect on oviposition behaviour compared to either nonane or untreated soil alone, in both An. gambiae (Oviposition Activity Index (OAI) = 0.56; P< 0.001) and Cx. quinquefasciatus (OAI =0.59; P<0.0001).Conclusion: The larval pheromone nonane and emanations from breeding-site soil both induced oviposition in wild An. gambiae s.l. and Cx. quinquefasciatus, with a synergistic effect when both stimuli were present simultaneously. This is the first study in which nonane is shown to cause oviposition under natural conditions, suggesting that this compound can potentially be exploited for the management of mosquito vectors.