Bioprospecting For Anti-Mosquito Phytochemicals Associated With Olfaction And Larvicidal Activities From Selected Kenyan Plants

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%)/βpinene (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 αpinene increased repellency, while neral, geraniol, perill aldehyde and cinnamaldehyde-αpentyl 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.