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Title: The Role of Grasses in Natural Breeding Habitats of Gravid Female Anopheles Mosquitoes (Diptera: Culicidae) on Oviposition Preference as Well as Larval Development and Survival
???metadata.dc.contributor.*???: Dr. Habte Tekie
Asmare, Yelfwagash
Keywords: Anopheles Arabiensis;Anopheles Coluzzii;Gravid Mosquitoes;Attraction;Oviposition;Larval Survival;Grass Volatiles;Pollen;Oils
Issue Date: Aug-2017
Publisher: Addis Ababa University
Abstract: The malaria vectors Anopheles arabiensis and Anopheles coluzzii are widely distributed in sub- Saharan Africa, the region with the highest malaria burden in the tropics. Previous research findings show both positive and negative correlations between Anopheles larvae productivity and vegetation in and close to breeding habitats. The aim of the present studies is to determine species specific association between Anopheles mosquitoes and grasses in natural habitats. Larval sampling in natural breeding sites associated with grasses was used as a proxy to identify the link between grasses and oviposition preference in Anopheles mosquitoes. Grasses have their own impacts on the life cycle and oviposition behavior of mosquitoes and exploiting their potential in vector control is a novel approach. Oviposition behaviour of Anopheles mosquitoes is odour-mediated and behavioural response of An. arabiensis and An. gambiae/An. coluzzi to volatile extracts of grasses using oviposition and attraction bioassays were investigated. The results showed that grass volatiles affect differentially oviposition site selection by Anopheles mosquitoes. Both the larval Anopheles in natural breeding habitats and gravid female of An. arabiensis and An. coluzzi in laboratory demonstrated strong preference to Poaceae grasses than Typhaceae and Cyperaceae grasses (P < 0.05). The dietary effects of grasses pollen was studied to test why gravid Anopheles mosquitoes and their larvae show preference hierarchy to grasses. The hypothesis that dietary effect of pollen of grasses is based on the quality of pollens and this is characterized by C: N ratio and grain size of pollens. The results showed that there were significant differences between pollen from grasses by their C: N ratio and grain size (P < 0.0001), and these differences demonstrated differential effects on development rate and survival of larval An. arabiensis (P < 0.05). The pollens from all xvii the grasses enhanced pupation rate more than the nitrogen rich artificial diet TetraMin®. The small grained and carbon rich Typha latifolia and Pennisetum setaceum pollens resulted in more enhanced development rate of larval An. arabiensis than the large grainedcarbon-rich Zea mays pollen, nitrogen-rich TetraMin® fish food and Echnocloa pyramidalis pollen. Zea mays pollen, and TetraMin® fish food equivalently increased survival of larval An. arabiensis develop in to adulthood. The effects of grass pollens in larval diets were more pronounced in the females of An. arabiensis. This is similar with previous studies, indicate that male anophelines develop faster and emerge sooner than females. The effect of grasses oils on larval survival was also studied to test why both gravid Anopheles mosquitoes and their larvae showed lower preferences to Typhacae and Cyperaceae grasses and especially no larvae was found in natural breeding habitats associated with Cyperaceae. The absence of Anopheles larvae in habitats with Cyperaceae grass is due to higher production of natural oil and this could result in mortality of larval Anopheles due to toxic nature of Cyperaceae grasses in addition to the physical nature of oils that causes suffocation. The results showed the rhizome sections of C. papyrus had significantly higher oil content (2.09 ± 0.23) than that of T. latifolia (1.455 ± 0.011) (P <0.05). Oil extracts from both T. latifolia and C. papyrus significantly increased larval mortality on An. arabiensis in a dose dependent manner (P<0.0001). No significant differences were observed between oils of the two grass species (P = 0.624). In contrast, increasing the doses of the oils had significant positive correlation with larval mortality of the tested mosquito (P < 0.0001). Therefore, it is possible to conclude that presence/growing of C. papyrus in breeding habitats could reduce Anopheles mosquito population due to the larvicidal effect of higher amount of natural oil production by its rhizome. xviii Generally grasses close to and in breeding habitats are drivers of vector population dynamics and malaria transmission: grasses volatiles modulate oviposion site selection, grasses pollen has larval nutrition input and grasses oils markedly affect larval survival and development of Anopheles mosquitoes. The findings in this study indicated that Poaceae and Typhaceae grasses are indicators for the availability of suitable breeding habitats for Anopheles mosquito and could help for monitoring malaria vector populations and for planning malaria interventions. In contrast, Cyperaceae grasses can be promising component of integrated malaria vector management as the rhizome had higher amount of oil which could be released directly to the breeding water with larvicidal effects on An. arabiensis.
Description: A dissertation submitted to the Graduate Programme of Addis Ababa University in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Biology (Insect Sciences)
Appears in Collections:Thesis-Zoological Sciences

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