Calendar-based larval control against mosquitoes in rain catch basins in veneto: an assessment of the outcomes based on quality control data

Quality control (QC), an essential part of mosquito control quality management, plays a crucial role in the assessment of the real-world effectiveness of control interventions, but is rarely exploited for research purposes. This study assesses the outcomes of larvicide treatments largely performed in Italy against mosquito vectors of arboviruses, with two main goals: i) to evaluate the outcomes of large-scale catch basin treatments with Diflubenzuron (DFB) and Bacillus thuringensis var. israelensis and sphaericus (Bti+Bs) on immature stages of the two main European arbovirus vectors, with reference to critical variables, such as the intervals between catch-basin treatments and inspections; ii) to identify spatio-temporal areas of repeated treatment failures. In detail, the analysed QC dataset was based on the recording of live larvae/pupae during the inspection of >18.000 catch basins treated by Diflubenzuron (DFB) and >9.000 catch basin treated by Bacillus thuringiensis var. israelensis and sphaericus (Bti+Bs) inspected in 461 municipalities in Veneto region (North-East Italy) between 2019 and 2021. Overall, ~5% of inspected catch basins revealed the presence of live L3-L4 larvae and/or pupae. GLM results showed opposite associations between percentages of positive catch basins treated with the two larvicides and the intervals between treatments and inspections, 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%). The spatial analysis revealed repeated DFB-treatment failures against Cx pipiens in the area of Venezia lagoon, where highest frequencies of alleles associated to DFB-resistance have been reported. The results show how high-quality area-wide databases from multi-year QC activities of mosquito control interventions may allow reaching some general conclusions not achievable experimentally and accounting for real-word heterogeneities. The evidence that the effectiveness of DFB treatments increases in the 3 weeks following the treatments, while the effectiveness of Bti+Bs treatments decreases over time, suggest that shortening the intervals between Bti-Bs applications may increase control effectiveness, particularly in the case of Ae. albopictus, even though testing the product efficacy under control conditions suggests a longer persistence. Moreover, results show that the analysis of multi-year, area-wide data can help prioritize areas – such as the Venezia lagoon - for monitoring and managing insecticide resistance.