Phytochemical composition and repellency of volatiles emitted by live potted mentha piperita (peppermint) plants against Anopheles gambiae sensu stricto
Obegi, Jackson Matundura
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The malaria vector Anopheles gambiae sensu stricto remains one of the principal malaria transmission vectors in developing countries, particularly within the tropics and sub-Saharan Africa. Current methods of control are based on synthetic chemical insecticides, but these are costly and inaccessible by poor communities, and large-scale use leads to vector resistance and toxicity to beneficial non-target organisms. Plant-based products have been used for generations in traditional practices, either for space protection to repel the arthropods from distance or for personal protection to deter them from blood feeding on contact. These are eco-friendly, cost effective and readily accessible to low income earners with minimal external input in resource-poor endemic and non-endemic malarial regions of the world. However, there have been limited research and documentation studies undertaken to screen the repellency of different plants and to optimize their deployment to effectively protect human subjects from mosquito bites in households. The aim of the present study was to compare the performance of different numbers of potted M. piperita plant(s) against An. gambiae s.s. in a choice set-up in a screen-house. In addition, the compositions of volatile emissions of the plant during the day and night (trapped on Super Q adsorbent) were compared and the major constituents identified by Gas Chromatography–linked Mass Selective Detection (GC-MSD). The repellency of the major constituents and blends were evaluated by human landing bait technique in aluminium cages. There was significant incremental rise in repellency from 44.00 % to 70.09 % when the number of potted plants was doubled from 2 to 4 (p˂0.05); however, there was no significant further increase with 8 and 16 plants (p˃0.05). Thus, the level of emission of volatiles from each plant appears to be negatively affected by the presence of other con-specific plants. There was large quantitative difference between volatile blends emitted during the day and night. Monoterpenes were the major compounds emitted during the night (60.5%) followed by sesquiterpenes (25%). 1,8-Cineole was the major constituent for both blends (27.68% at night and 23.93% during the day). Of the individual constituents tested, β-Pinene (56.40±6.40) was most repellent. Subtractive assays of the five constituted blends, showed that the blend of 1,8-Cineole, β-Pinene, α-Pinene, and β-Myrcene (143:17:71:19) was most repellent (100.00±0.00) at a dose of 0.1 mg/mL which was comparable to that of the posive control (DEET). This study shows that deployment of a small number of M. piperita plants has additive space protection effects. However, further research is needed to achieve significant incremental effects and complete protection from malaria vectors.