Role of Grasses in Oviposition site selection by Malaria Vectors in Western Kenya

Malaria continues to be among the top leading causes of illness and deaths in Africa. Its elimination is challenging in high transmission areas using insecticide-based intervention tools alone. Understanding the ecology and behaviour of disease vectors, including the olfactory cues, are essential for the development of novel, insecticide-free control tools. Recent work has highlighted that selected graminoid plants release volatile chemicals that attract malaria vectors. However, there is dearth of information on the type of dominant graminoid plants in habitats, nature of volatiles they emit and how volatiles affect behaviour of malaria vectors during oviposition sites selection. The objectives of this study include analysis of the association between graminoid plants and presence and abundance of early instar Anopheles larvae, evaluate response of gravid malaria vectors to the plant volatiles, characterize and identify attractive volatiles. A cross-sectional survey of aquatic habitats was done to sample mosquito larvae in Rusinga Island, western Kenya. The plant species were identified by an expert at East African Herbarium, National Museum of Kenya. Bioassays were implemented with live graminoid plants (Cyperus rotundus, Cyperus exaltatus, Panicum repens, Cynodon dactylon and Cenchrus setaceus). Olfactometers, WHO tubes, cages and BG-Sentinel traps were used in the experiments in Mbita and Ahero, western Kenya. The volatile compounds released in the headspace of these plants were identified using gas-chromatography/mass spectrometry and the synthetic chemicals were tested. The tests were conducted with Anopheles gambiae and An. funestus. No positive association was observed between graminoid plants and the presence and abundance of early instar Anopheles larvae. All the tested plants attracted gravid An. gambiae in an olfactometer and when tested with free-flying mosquitoes over a longer distance in large field cages. Limonene, β-pinene, β-elemene and β-caryophyllene were consistently detected in plant headspace. β-elemene (the first to be reported as being attractive to gravid malaria vector) and β-pinene elicited significant short-range attraction and egg-laying responses in gravid An. gambiae when tested individually. Addition of C. rotundus to the BG-Sentinel traps significantly increased the catches of gravid An. funestus than the control traps. This study confirms that gravid malaria vectors use chemical cues released from different graminoid plants to orientate. The potential utilization oviposition attraction of β-elemene and β-pinene should be further tested in the semi-field with free-flying mosquitoes for their utilization in attract-and-kill trapping strategies. Overall, there is need to invest more research into developing odour-blend formulations that can to improve surveillance and control of vectors