The Effect of the Acoustic Swarm Environment on the Behaviour and Hearing of Male Culex quinquefasciatus Mosquitoes

Abstract

Background The mating behaviour of many mosquito species is mediated by sound: males pursue and mate with a female mid-flight by using their antennae to detect and track the whine of the female flight-tone. Upon detection of the female's flight-tones (or artificial female-like tones), males track the sound source and display stereotypical rapid frequency modulation (RFM) acoustic behaviour when very close to the female. Under natural conditions, flying males of these mosquito species form relatively dense agglomerations – swarms – while waiting for sexually receptive females to pass nearby. Individual male mosquitoes thus face intense competition to detect, track, and access a passing female. Aims We aim to understand the significance of the swarm acoustics on the individual mating behaviour of male mosquitoes. Specifically, we tested the influence of simulated swarm sound on the RFM behaviour and on the hearing physiology of male mosquitoes. Methods We derived the behavioural threshold responses of individual male mosquitoes exposed to female-like tones with and without the presence of a banded 700-900 Hz synthesised swarm sound. Two electrophysiological experiments were also performed: 1) measurement of the threshold of the mosquito auditory organ, the Johnston's organ (JO), to female-like tones with and without swarm sound; 2) measurement from the JO of electrical difference-tone distortion products generated by tones at the frequencies of the male and female flight tones. Results Behavioural experiments with free-flying mosquitoes and electrophysiological preparations indicate that swarm sound has no apparent affect on the behavioural and hearing thresholds of males for female-like sounds. Conclusion This indicates that from an individual male mosquito perspective, swarms are not a source of acoustic noise and males are neither facilitated nor impaired by synthetic swarm noise in detecting females. Furthermore, the swarm noise does not appear to impair or facilitate the ability of males to detect their own flight tones and those of nearby male mosquitoes.