The Identification of cereal volatile compounds that attract gravid malaria mosquito, Anopheles arabiensis Patton (Diptera: Culicidae)

Abstract

Malaria is a mosquito-vectored infectious disease, which causes serious human health problems in many endemic areas. The dominant malaria vectors in sub-Saharan countries are Anopheles gambiae, Anopheles coluzzii, Anopheles funestus and Anopheles arabiensis. Of these, An. arabiensis is the most widely distributed, versatile and opportunistic vector. The distribution of this vector is frequently correlated with a preference for larval habitats associated with natural and domesticated grasses (Poaceae). Over the past ten thousand years, humans have successfully cultivated grasses and altered the landscape, creating An. arabiensis favourable environments that contain excellent habitats for both larvae and adults. This may indicate that a pre-existing adaptation or preference by An. arabiensis for grass habitats may have co-evolved with human agriculture; to create highly suitable conditions for both the vector and malaria transmission. The distribution and dispersal of An. arabiensis among various larval habitats is mainly driven by the choice of the gravid mosquito for an egg-laying site. To maximise the fitness of their offspring, female mosquitoes should carefully search for habitats with high nutriment, minimal competition and reduced risk of predation. Selection of oviposition site impacts on the survival and development stages of the immature mosquitoes, since larvae are minimally mobile and mature without parental care. The larval habitats also affect adult density, distribution and vectorial capacity. Several cues from the breeding site, such as visual, tactile and olfactory signals, assist gravid mosquitoes to select a suitable oviposition site. Among these, the olfactory cues play a major role in the breeding site selection process, similar to that which has been documented for in other fitness-related activities, such as blood host seeking and sugar feeding. However, few studies have documented the odour-mediated oviposition behaviour of malaria vector mosquitoes.The present studies investigated the effect of olfactory cues emanating from breeding sites associated with the domesticated grasses, rice, maize and sugarcane, on the oviposition preference of An. arabiensis. The volatile compounds associated with the plants and pollen of these grasses were collected by headspace volatile collection, from the above-ground plant tissues of two cultivars of rice (MR1 and MR2), and the pollen of two cultivars of both maize (Bako hybrid-660 and Melkassa 2-521) and sugarcane (Coll-48 and EAK 71-402). Behavioural responses of gravid An. arabiensis to the collected headspace volatile extracts were tested under laboratory conditions, using a two-port olfactometer and a two-choice oviposition bioassay. Headspace volatiles from both cultivars of rice plants as well as maize and sugarcane pollen attracted gravid An. arabiensis and stimulated oviposition. However, the response of the mosquitoes to the volatiles of each crop differed, indicating that females prefer the volatile profiles of specific cultivars. Through combined gas chromatography and electroantennographic detection (GC-EAD) analyses of the headspace volatiles extracts of MR3 rice, BH-660 pollen and sugarcane pollen revealed eight, five and seventeen bioactive compounds, respectively. These compounds were identified using combined gas chromatography and mass spectrometries (GC-MS), revealing that only α-pinene and nonanal were shared among each cereal. Synthetic blends of MR3 rice, BH-660 pollen and sugarcane pollen odour elicited significant attraction and oviposition stimulation of gravid An. arabiensis. Subtractive blends were inferior to that of the full synthetic blends. The synthetic blend of MR3 rice was also tested under semi-field conditions showing gravid An. arabiensis are significantly attracted to the full blend when compared to a solvent control. The results show that gravid An. arabiensis prefer oviposition sites associated with domesticated grasses and that this selection is driven by olfactory cues. Synthetic blends were demonstrated to elicit the complete behavioural repertoire of gravid An. arabiensis mosquitoes. The synthetic blends identified in the present studies are novel for malaria vectors and show great potential to be integrated with other components of Integrated Vector Management (IVM).