Lessons learned exploiting a multi-year large-scale data set derived from operational quality assessment of mosquito larval treatments in rain catch basins

BACKGROUND: Larval control is crucial for managing mosquito arbovirus vectors. Quality control (QC) data from insecticide-based interventions are rarely exploited to assess the effectiveness of biocides in the field. This study aims to: (i) evaluate the outcomes of large-scale catch basin treatments with diflubenzuron (DFB) and Bacillus thuringiensis var. israelensis and B. (Lysinibacillus) sphaericus (Bti + Bs) on immature stages of Aedes albopictus and Culex pipiens, with reference to critical variables, such as the intervals between catch basin treatments and inspections; and (ii) identify areas of repeated treatment failures. RESULTS: More than 30 000 catch basins were inspected from 2019 to 2021 for mosquito immature stages after treatment with DFB and Bti + Bs in 461 municipalities in north-east Italy. Overall, 5% of catch basins revealed the presence of live L3–L4 larvae and/or pupae. Model results showed opposite associations between percentages of positive catch basins treated with the two larvicides and the intervals between treatments and inspections, likely because of the different modes of action; i.e., a negative association following DFB treatments (day 7/day 21: Ae. Albopictus, 7%–4%; Cx. pipiens, 8%–4%), and a positive association following Bti + Bs treatments for Ae. albopictus (day 7/day 21: 2%–13%). Spatial analysis revealed repeated DFB treatment failures against Cx. pipiens in the area of the Venice lagoon, where the highest frequencies of alleles associated with DFB resistance have been reported. CONCLUSION: The results show that, despite unavoidable limitations, high-quality area-wide databases from multi-year QC activities of public mosquito control interventions may allow general conclusions to be reached (such as shortening the intervals between Bti + Bs treatments) accounting for real-world heterogeneities that cannot be achieved experimentally.