A NEW STRATEGY FOR MOSQUITO BIOCONTROL BASED ON THE IMPROVEMENT OF BACILLUS THURINGIENSIS EFFECTIVENESS

Mosquitoes are insects with a worldwide geographical distribution. Several species are known to spread animal and human illnesses, such as the Zika fever, Dengue fever, and malaria. This is due to the vectorial competence of adult females that is the ability to acquire a pathogen from an infected host during a blood meal and transmit it to an uninfected host during the following meals. Several anthropic activities have favoured the diffusion of invasive mosquito species of sanitary importance to non-endemic areas raising concern due to the increasing number of outbreaks of neglected diseases in these regions. Due to the lack of vaccines and prophylaxes strategies, mosquito management is a pivotal aspect to control the spread of these diseases. Beside adult stage management, larval management is also extremely important to decrease the density of adult populations and to prevent the spread of neglected diseases. However, biorational strategies for their control are still scarce. To date, the most effective bioinsecticides targeting mosquito larvae are based on the entomopathogens Bacillus thuringiensis var. israelensis (Bti) and Lysinibacillus sphaericus (Lsph). These products, active by ingestion, include Bti alone or in combination with Lsph and are safe for the environment while displaying a high and specific toxicity against the larvae of Culicidae. The key issues related to their use is due to Bti short residual activity after the application and to the possibility of the onset of resistance phenomena in target population. To preserve the efficacy of these formulations it is necessary to develop proper delivery systems able to protect the active ingredients maintaining their effective concentration. With these purposes we developed MosChito raft, a biodegradable floating hydrogel constituted of molecules of natural origin, namely chitosan and genipin. This product has been designed to include a Bti-based formulation to induce larval toxicity, Saccharomyces cerevisiae cells to induce phagostimulation and air bubbles to allow its flotation in water. Our results have shown the efficacy of MosChito rafts in laboratory conditions and in semi-field conditions against laboratory colonies and colonies with a genetic background comparable to that of natural populations of the Asian tiger mosquito, Aedes albopictus, and the common house mosquito, Culex pipiens. Differently from what stated for the Bti-based product how it is, its inclusion in MosChito raft prolongated its efficacy of at least 15 times (i.e., from two days to one month) solving one of the main issues related to its application. Beside what we expected, yeast cells inclusion did not improve the attractiveness of the hydrogel, anyhow, their role can be rethought as biofactory to produce and administrate immune-modulating double-stranded RNA molecules with the aim of weaken larval immune system while boosting the toxicity of Bti. Further studies must be carried out assessing the ecotoxicological effects of our product against off-target organisms and in field conditions. Looking at the results obtained so far, MosChito rafts could represent a valid alternative for the larval control of two highly anthropophilic mosquito species of sanitary importance at the same time in the urban context.