Carbon dioxide (CO2) present in exhaled air is the most important sensory cue for female blood-feeding mosquitoes, causing activation of long-distance host-seeking flight, navigation towards the vertebrate host1 and, in the case of Aedes aegypti, increased sensitivity to skin odours2. The CO2 detection machinery is therefore an ideal target to disrupt host seeking. Here we use electrophysiological assays to identify a volatile odorant that causes an unusual, ultra-prolonged activation of CO2-detecting neurons in three major disease-transmitting mosquitoes: Anopheles gambiae, Culex quinquefasciatus and A. aegypti. Importantly, ultra-prolonged activation of these neurons severely compromises their ability subsequently to detect CO2 for several minutes. We also identify odours that strongly inhibit CO2-sensitive neurons as candidates for use in disruption of host-seeking behaviour, as well as an odour that evokes CO2-like activity and thus has potential use as a lure in trapping devices. Analysis of responses to panels of structurally related odours across the three mosquitoes and Drosophila, which have related CO2-receptor proteins, reveals a pattern of inhibition that is often conserved. We use video tracking in wind-tunnel experiments to demonstrate that the novel ultra-prolonged activators can completely disrupt CO2-mediated activation as well as source-finding behaviour in Aedes mosquitoes, even after the odour is no longer present. Lastly, semi-field studies demonstrate that use of ultra-prolonged activators disrupts CO2-mediated hut entry behaviour of Culex mosquitoes. The three classes of CO2-response-modifying odours offer powerful instruments for developing new generations of insect repellents and lures, which even in small quantities can interfere with the ability of mosquitoes to seek humans.
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We thank E. Lacey and S. McInally for help setting up behaviour experiments; the Malaria Group team members in ICIPE Mbita Point Research Station, Y. Afrane and G. Yan for logistical support; S. M. Boyle for help with Pharmacophores; K. Klingler for help with statistics; J. Perecko for building traps and excitorepellency chambers; A. Dahanukar and G. Tauxe for comments on the manuscript; W. Walton, P. Atkinson, P. Wirth and A. Khalon for mosquitoes. Part of this work was funded by a grant to A. Ray from the Bill & Melinda Gates Foundation through the Grand Challenges Exploration Initiative, and part supported by a grant to A. Ray, Award Number R01AI087785, from the NIAID (NIH). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIAID or NIH.
The file movie contains a representative video of a mock-treated female A. aegypti mosquito navigating upwind and through a CO2 turbulent-plume releasing ring in real time.
This movie file contains a representative video of a pretreated (ultra-prolonged odour blend, 10-2) female A. aegypti mosquito, unable to find upwind CO2 source in real time. For the remaining duration of the 300 sec assay the mosquito does not move after coming to rest on the glass wall.
This file movie contains a representative video of a pretreated (ultra-prolonged odour blend, 10-1) female A. aegypti mosquito, unable to activate upwind flight from the cage and find upwind CO2 source during the entire duration of the 300 sec assay (video speeded up).