Odour-mediated oviposition site selection in Aedes aegypti depends on aquatic stage and density

Abstract

Background Olfaction plays an important role in the selection and assessment of oviposition sites by mosquitoes. Volatile organic compounds (VOCs) associated with potential breeding sites affect the behaviour of gravid mosquitoes, with VOCs from aquatic stages of conspecific mosquitoes influencing and regulating oviposition. The purpose of this study was to conduct a systematic analysis of the behavioural response of gravid Aedes aegypti to conspecific aquatic stage-conditioned water, to identify the associated bioactive VOCs and to determine how blends of these VOCs regulate oviposition site selection and stimulate egg-laying. Methods Using a multi-choice olfactory oviposition assay, controlling for other sensory modalities, the responses of individual females to water conditioned with different densities of conspecific aquatic stages were assessed. The conditioned water samples from the most preferred density of each aquatic stage were subsequently compared to each other using the same oviposition assay and analysed using an analysis of variance (ANOVA) followed by a Tukey post-hoc test. Using combined gas chromatography and electroantennographic detection or mass spectrometry, bioactive VOCs from the preferred density of each aquatic stage were identified. Synthetic blends were prepared based on the identified ratios of bioactive VOCs in the aquatic stages, and then tested to determine the oviposition choice of Ae. aegypti in a dose-dependent manner, against a solvent control, using a dual-choice assay. This dataset was analysed using nominal logistic regression followed by an odds ratio comparison. Results Gravid Ae. aegypti responded stage- and density-dependently to water conditioned with eggs, second- and fourth-instar larvae, and pupal exuviae, but not to water conditioned with pupae alone. Multi-choice assays demonstrated that gravid mosquitoes preferred to oviposit in water conditioned with fourth-instar larvae, over the other aquatic stage-conditioned water. Gravid Ae. aegypti were attracted, and generally stimulated, to oviposit in a dose-dependent manner to the individual identified synthetic odour blends for the different aquatic stages. Conclusions Intraspecific VOCs regulate oviposition site selection in Ae. aegypti in a stage- and density-dependent manner. We discuss the need for further studies to evaluate the identified synthetic blends to modulate the odour-mediated oviposition of Ae. aegypti under field conditions. Graphical Abstract

Female Aedes aegypti mosquitoes use communal cues to manage population density at breeding sites

“…On the contrary, the avoidance response in a higher density can minimize progeny competition. Similar relationships involving the density of eggs and immatures influencing female choice have been demonstrated in laboratory and field studies 15 , 16 , 41 – 47 . Our study established this density-dependent behavior relationship among adult females of Ae.…”

“…aegypti females. The quality and quantity of these intraspecific VOCs blends are chemical codes influencing site selection in a density-dependent manner 16 . Our findings demonstrate that intraspecific combined cues from Ae.…”

“…5b ) suggest that the orco pathway is likely involved in an attraction response linked to aggregation. Identifying the volatile cues that rely on this pathway, likely intraspecific adult VOCs 16 would be helpful in determining how the OR pathway allows mosquitoes to navigate toward each other.…”

“…The ultimate decision of depositing eggs by a female mosquito demands a complex integration of sensory inputs, from finding a breeding site to assessing its quality, behaviors that have been superficially explored. Recent studies on how volatiles can modulate oviposition behavior 11 , 13 , 16 , 59 , 60 , and on how mosquito females evaluate salt concentration in the breeding site before making an oviposition decision 10 have highlighted the role of chemosensation during this critical but vulnerable step in the mosquito life cycle. Our study shows that olfactory conspecific communication can assist female mosquitoes in regulating population density at a breeding site.…”

“…Recently, geosmin has been shown to be an odor cue guiding females to breeding sites in the laboratory and the field 11 . Mosquito eggs and larvae have also been shown to influence egg-laying decisions by providing chemosensory cues to females seeking a breeding site 12 – 16 , but communication between adult female mosquitoes during the breeding site-seeking behavior has not been previously characterized. Given the importance of Ae.…”