Investigation into the 35 Khz - 60 Khz Frequency Range of the Naturally Generated Ultrasound of the African Bat, C. Afra, Eliciting Optimal Evasive Response in the African A. Gambiae S. S.
1Department of Physics and Mathematics, Moi University. P. O Box 3900, Eldoret, Kenya
2Physics Department, SEKU. P. O Box 170-90200, Kitui. Kenya
3Physics Department, Egerton University. P. O Box 536, Egerton, Njoro. Kenya
4Entomology Department, Kenya Medical Research Institute Centre for Global Health Research. P. O Box 1578, Kisumu. Kenya
Abstract
This research investigated the 35 kHz - 60 kHz frequency band of the naturally generated ultrasound of the
African sheath tailed bat, Colure afra, which elicited optimal evasive response in the African Anopheles
gambiae. Recent study findings with the natural sounds of C. afra had shown ultrasonic components (35 kHz - 60
kHz) with capability to evoke avoidance response in the female A. gambiae s. s. Malaria whose vector are mated
female A. gambiae is a health challenge in Africa and responsible for many deaths. Efforts to reverse the trend
have shown low impact as manifested in the 2006 and 2008 World Health Organization statistics on Malaria.
Currently, the effective vector control measures include indoor residual spraying, and the long-lasting insecticide treated nets. Therefore, there was need to critically investigate the 35 kHz - 60 kHz sound of C. afra with a view
of exploiting it as an additional vector control measure. The study therefore aimed at filtering the 35 kHz - 60
kHz frequency band, determine and analyze the acoustic transmission parameters of the sound of C. afra in the 35
kHz - 60 kHz frequency range; determine the activity and the behavioral response of the female A. gambiae to
the ultrasound in the 35 kHz - 60 kHz frequency range. A set of ten 3–5-day old female A. gambiae bred and
reared at the Kenya Medical Research Institute, Kenya were used in the bioassay study. The temperature and
humidity was maintained at 25±2 oC and (60-80 %) respectively. The sound samples of C. afra were recorded
using the Avi soft recorder from Kit-Mikaya caves, Kenya. The 35-60 kHz frequency band was filtered and
analyzed using the Aviso ft SAS Lab Pro version 5.1 and Raven Pro. version 1.4 software. The mosquitoes'
behavioral response to the 35-60 kHz sound of C. afra and associated activities were observed and noted. It was
established that the 35-60 kHz sound of C. afra consisted of 5046 calls of FM and CF modulated harmonics. The
calls were generated through tongue clicks at the rate of 493.016 calls/minute. The calls were dominated by the
short duration high frequency signals with an average acoustic energy of 9.2433 x 10-4
Pa2
s which was lowest.
The non-pulsate sound had a minimum and maximum amplitude of 71.21 Pa and 104.82 Pa respectively, with
2,519 calls between 90.00 - 99.00 Pa peak amplitude range. The signal power steadily declined with the increase
in signal frequency. Statistically, there was a highly significant relationship between the acoustic energy with the
amplitude, frequency and bandwidth. The female A. gambiae assumed a normal posture with the body inclined at
45o
accompanied by occasional rubbed wings and legs under the bioassay control experiment. There was no
remarkable change in physical behavioral activities in 60 % of the sample mosquitoes on exposure to the 35 - 60
kHz sound. Only 40 % exhibited immobility and excitation tendencies. At 95 % confidence level, a paired T-test
showed that the acoustic energy significantly affected the mean activities of the female A. gambiae (p = 5.6477 x
10-5) with a low positive correlation. It was established that the mean mosquito activities under the influence of
35 - 60 kHz differed significantly from the mean activities under the control (p = 0.008). Only 30 % of the
mosquito samples showed significant difference in the individual total mosquito activities under the influence of
the 35 - 60 kHz sound of C. afra and the individual total activities under the control. The mosquitoes recorded a
mean rate of activities of 1.5598/minute when exposed to the 35 - 60 kHz sound of C. afra, 2.5195 times above
the rate of activities at the control experiment. The rate of mosquito activities was significantly affected by thepeak amplitude, peak frequency and the bandwidth. The low significance in evasive response was attributed the
non-pulsate nature of the sound of C. afra, declining signal power with increase in frequency, mixed sonar and
social calls, and short duration high frequency calls. These results of this study give an insight into the reasons for
low evasive response in female A. gambiae on exposure to the 35 - 60 kHz sound of C. afra. The acoustic
transmission parameters of the sound required modifications in order to yield improved results. The improved
results would provide Ultrasound as an addition malaria vector control measure which is locally available in
Africa and hence cut down on mortality and economic burden resulting from Malaria.
