Susceptibility of Aedes aegypti to spinosad larvicide and space spray adulticides in Brazil.

Background: Insecticides play a critical role in controlling insect vectors, particularly during epidemics. Effective chemical control relies on the robust monitoring of insecticide resistance to guide evidence-based decision-making in vector control strategies.

Objectives: This study assessed the susceptibility of Aedes aegypti, the primary vector of dengue, Zika, and Chikungunya viruses, to various larvicides and adulticides deployed during Brazil's national campaigns from 2020 to 2023.

Methods: Mosquito collection was performed in 46 Brazilian municipalities using ovitraps. Eggs were transported to FIOCRUZ to establish the F1 and F2 generations. The Rockefeller strain was employed to determine the discriminating concentrations (DC) for the larvicide Natular™ 20EC (spinosad) and the adulticides Cielo™ (imidacloprid and prallethrin) and Fludora® Fusion (clothianidin and deltamethrin) using a modified World Health Organization (WHO) bottle bioassay. These DCs were then used to estimate the resistance status of Ae. aegypti populations in the tested formulations. Resistance intensity was assessed by exposing mosquitoes to five, 10, or 20 times the DC concentrations.

Findings: All Ae. aegypti populations were fully susceptible to larvicide spinosad. However, resistance to both adulticide formulations was detected based on WHO criteria (mortality rates < 90%). Intensity assays revealed high to very high resistance to combined adulticide products.

Main conclusions: Our findings indicate the full susceptibility of Ae. aegypti populations in Brazil to spinosad, but substantial resistance to adulticides used in space spraying and residual applications, likely due to pre-existing pyrethroid resistance. However, the specific contributions of each active ingredient remain unclear, owing to the evaluation of the combined formulations. The efficacy of both traditional and alternative vector control strategies must be continuously evaluated and closely monitored to ensure the real-time assessment of their performance. For chemical control, future studies should prioritise the assessment of combination products in field trials, refining laboratory assays, and sustaining insecticide resistance surveillance to optimise control efforts in Brazil.