Determination of predator ultrasound parameters and acoustic startle response in the African female Anopheles gambiae s.s.

Abstract

The female Anopheles gambiae, a malaria vector, detects ultrasound by its antenna, which can initiate an attractive or repulsive response. Modern electronic mosquito repellent devices exploit this concept in attempt to control malaria. However, they have shown only 20 % effectiveness in repellency. This work determines the transmission parameters of Amolops tormotus, Coleura afra sounds and their combination and hence their startling effects on female Anopheles gambiae. The sound of C. afra and A. tormotus, which have not been investigated, were recorded with the 112 Avisoft and 702 digital recorders respectively, from Kit-Mikayi caves in Kenya and Huangshan Hot Springs in China, respectively. A bioassay was set up with 3-4 day old female A. gambiae exposed to the three sounds at varied frequencies and the number of activities and behavioural responses noted. The fundamental frequency of the sound of A. tormotus, C. afra and` their combination was 5.371 kHz, 6.836 kHz and 4.883 kHz, respectively. The spectrograms of the sound of A. tormotus, C. afra and` their combination showed FM and CF components. The frequency range for significant startle of mosquitoes was 35-60 kHz for individual predator sounds whereas 10-34 kHz for the combined sound. Similarly, the amplified mean peak amplitude for the sound of A. tormotus, C. afra and` their combination was 134.08 dB SPL, 134.28 dB SPL and 133.60 dB SPL, respectively. The mean bandwidth of the combined sound was 16.4 kHz, narrowed by 3.4 kHz and 3.0 kHz from that of the sound of C. afra and A. tormotus respectively. The mean bandwidth of the combined sound was significantly narrowed from 77.24 kHz, the reported mean bandwidth of EMR sound. The sound of A. tormotus and C. afra recorded a mean peak frequency of 47.60 kHz and 45.9 kHz respectively with the combined sound recording 29.4 kHz in the optimum frequency range. The maximum acoustic energy in the optimum frequency range for the sound of A. tormotus, C. afra and` their combination were 10.84 Pa2s, 12.32 Pa2s and 6.08 Pa2s, respectively. The combined predator sound and sound of A. tormotus evoked evasive behavioural responses in 30 % and 46 % of the mosquitoes, higher than the reported 20 % effective repulsion by EMR sound. The evasive response was characterized by 58.5o antenna erection, physical injury, unusual rest and movement, fatigue and falls; attributed to stress on nervous system and fear of predation. This work established that ultrasound affects the female A. gambiae by repelling it in addition to determining the optimum acoustic transmission parameters needed for the design of an effective electronic mosquito repellent. Further investigation into the repellency of sonar sound of C. afra and the effective repulsive frequency band of the combined sounds is recommended.