Improving mass rearing of mosquito for pest management: impact of Asaia on host growth and vector competence

Abstract

Several vector control strategies have been implemented to contain and mitigate the transmission of medically significant arboviruses, including dengue, yellow fever, West Nile and more. Alternative strategies for the use of insecticides and larvicides, which can have reduced efficacies over time due to resistance evolution, involve inundative release of sterile or modified males. A critical aspect for mass-release programs is ensuring an efficient production of high-quality males. A potential solution to improving mosquito mass-rearing efficiency, and fitness of the adults, is the appropriate utilisation of bacterial symbionts, such as bacteria of the genus Asaia. The research questions in this thesis are therefore motivated by an interest in improving mass rearing for vector control. Contrary to our expectation, different species of the genus Asaia were proved to be poor mutualists of important arbovirus vectors, such as Aedes aegypti (KPP Thai and Rockefeller strain), Culex pipiens and Aedes albopictus, in the sense of not being able to persist as microbiome components in presence of an intact vertically transmitted microbiome. However, despite its transient nature, Asaia spp. had lasing impacts on the larval and pupal microbiome by altering the relative abundance of other species, for example by increasing the abundance of the putative beneficial genus Klebsiella in Aedes aegypti KPP Thai. With our studies we showed that the inoculation of newly hatched larvae of several mosquito species with different Asaia species can be beneficial in terms of reducing development time: this result was repeated in independent laboratories with different insect strains. Additionally, inoculation with Asaia can lead to the production of adults of larger sizes when compared to the uninoculated counterparts. Because both sexes can potentially benefit from the exposure to Asaia spp., and the sex separation systems currently used for mass-rearing can be prone to rare erroneous release of females, we studied the effect of Asaia exposure on female vector competence. We showed that exposure of Culex pipiens larvae to Asaia bogorensis and Asaia krungthepensis leads to a West Nile virus did not increase percentage virus infection, dissemination or transmission rate but did result in a small increase in viral titer in adults’ saliva and legs, suggesting a possible enhanced vector competence of Culex pipiens females. Our work together with future additional research, may position Asaia spp. as a choice for improving the efficiency of mass-rearing mosquitoes for vector control programs, in addition it highlights the importance of carefully evaluating specific host-symbionts interactions prior to the potential release of mosquitoes with enhanced vector competence.