The detection of carbon dioxide and its role in the orientation to hosts by haematophagous insects

Abstract

Host orientation behaviours of mosquitoes and other blood-seeking insects are controlled to a large degree by signals released by the host, including heat, moisture and sound, as well as visual and olfactory cues. Clearly olfactory signals play a primary role in mediating such behaviours. Although other olfactory signals such as lactic acid, 1-octen-3-ol and ammonia have been implicated in orientation, perhaps the most important of these volatiles is carbon dioxide (CO2). Carbon dioxide is a by-product of cellular respiration that is released in large amounts by potential hosts and has been associated with mosquito behaviour since the early 1920s. In this report we focus on aspects of carbon dioxide detection including the morphological and physiological characteristics of sensory structures located on the maxillary palps which contain receptor neurons responsive to CO2. The relevant characteristics of these receptor neurons include the threshold and slope of the concentration response functions, the temporal pattern of the discharge, the effects of different background CO2 concentrations and the physiological condition of the insect. Among these conditions are age, diapause status and whether the species is autogenous or anautogenous. The physiological characteristics of CO2 detection in female mosquitoes are contrasted with similar characteristic in male mosquitoes as well as in other insects such as biting midges (Ceratopogonidae). The recent advances involving molecular research to further our understanding of the cellular processes of CO2 detection and the role of deterrents and repellants on such processes are discussed. Finally, the uses of CO2 as a tool in the design of control strategies including the development of traps for monitoring and potential control are discussed.