Bioprospecting for Anti-Mosquito Phytochemicals Associated With Olfaction and Larvicidal Activities From Selected Kenyan Plants
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Date
2020
Authors
Ochola, John Bwire
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
Mosquitoes are of high public health concern since they are significant disease vectors of
different tropical and subtropical life-threatening diseases like malaria, dengue, yellow fever,
chikungunya, zika virus and encephalitis. Use of synthetic insecticides for control of
mosquitoes causes development of resistance in vector species and have negative effects to the
environment and human. This study aimed to find alternative, effective tools against these
vectors from plant essential oils. Plant-based products are promising alternatives of lowtoxicity, eco-friendly and low-cost. Oils from air-dried aerial parts of Satureja biflora, Satureja
abbysinica, Conyza newii and Plectranthus marrubioides from Aberdare and Ocimum
kilimandscharicum from Kakamega in Kenya were extracted using Clevenger apparatus. Oil
yields were determined and analyzed by Gas Chromatography/Mass Spectrometry. Repellency
of the oils was assessed on adult mosquitoes in ‘WHO arm in the cage’ method, while 3rd instar
larvae were used to assess larvicidal activities based on WHO protocol. Oil with better
larvicidal activity was formulated into a water-miscible solution for laboratory and semi-field
testing. GC linked to EAG detector was used to determine the constituents that elicited
chemosensory responses from the antenna of An. gambiae. Individual constituent contribution
to mosquito repellence was established through a subtractive-combination bioassay. From the
results, oils from different habitats and seasons showed qualitative and quantitative differences.
Thirty three (33) compounds were identified in S. biflora oil with 3 chemotypes based on
location: geranial(31%)/neral (24%)/linalool (12%) of Kinale, linalool (28%)/neridol (21%)/β-
pinene (7.4%) of Nyahururu, and geranial (27%) neridol (Z) (21%)/linalool (16%) of KieniGakoe. S. abyssinica had also 33 compounds comprising majorly menthone (44.1%) and
pulegone (33.3%). C. newii had 19 components forming 2 chemotypes p-mentha-1,8-dien-7-yl
acetate (24%)/limonene (23%)/5-methyl-2-phenyl-2-hexenal (21%) associated with Nyahururu,
and p-mentha-1,8-dien-7-yl acetate (27%)/ limonene (38%). P. marruboides and O.
kilimandscharicum had 35 and 41 components respectively. The major compounds in P.
marrubioides were carene-2-δ (18.7%) and camphor (17.9%), whereas for O.
kilimandscharicum was camphor (36.6%) and limonene (18.6%). C. newii (Nayahururu) S.
biflora (Kieni) and S. biflora (Nyahururu) gave a high repellence of ED50 at < 1.95 ppm,
whereas S. biflora (Kinale) had ED50 at < 2.35 ppm against Ae. aegyptei and An. gambiae.
Besides, C. newii had strong repellence with ED50= 0.5 ppm against An. gambiae.
Comparatively DEET gave the highest protection time of 389 min against An. gambiae,
followed by C. newii (cream) against the 3 mosquito species; 241, 206 and 60 minutes for An.
gambiae, Cx. quinquefasciatus and Ae. aegyptei respectively. S. biflora cream gave 208, 180
and 56 minutes against An. gambiae, Cx. quinquefasciatus and Ae. aegypti respectively.
Addition of vanillin to the cream formulation resulted in a longer protection time of 382 min
against An. gambiae. Nine EAG-active constituents were identified. Perillyl alcohol and α-
pinene increased repellency, while neral, geraniol, perill aldehyde and cinnamaldehyde-α-
pentyl reduced the repellency of the respective blends. Limonene and linalool interchangeably
increased or decreased the repellence based on the resulting blend. O. kilimandscharicum oil
showed the highest larvicidal effect against 4 mosquito species, with LD50 of 0.292 and
0.41ppm at 24 and 48 hrs respectively against An. gambiae larvae. The O. kilimandscharicum
water miscible formulation recorded a LC50 of 0.13, 0.14, 0.16 and 0.13ppm against larvae of
An. gambiae, Ae. aegypti, Cx. quinquefasciatus and An. arabiensis respectively. In a semi-field
setup, the larvicidal formulation at 0.5ppm on day 8 attained 100% larval reduction when B.ti
had 96.5%. The larvicidal and repellent results of the plant formulations generated in this study
have demonstrated high potential for practical application in control of mosquito vector borne
diseases and need to be deployed for large scale field trials and registered with the relevant
bodies for adoption for control of mosquitoes.
Description
A Thesis Submitted in Fulfilment of the Requirement
for the Award of the Degree of Doctor of
Philosophy (Chemistry) in the School of Pure And
Applied Science of Kenyatta University
Keywords
Anti-Mosquito Phytochemicals, Larvicidal Activities, Olfaction Activities, Kenya, Mosquito