78 - Field Cage Bioassays Identify Key Factors Affecting the Success of ULV Spraying Against Aedes aegypti

Abstract: Operational insecticide evaluations often rely on laboratory bioassays that do not capture real-world environmental variability. To bridge this gap, we used mosquito bioassay cages to measure how multiple factors—including insecticide class, mosquito strain, house location, and operational cycle—influence mortality during ultra-low volume (ULV) applications in Aedes aegypti control programs.

A year-long field study was conducted in Tapachula, Mexico, using eight clusters treated with either bifenthrin (pyrethroid) or malathion (organophosphate). During four operational cycles, cages containing local Ae. aegypti and the susceptible New Orleans strain were placed at the front, indoor, and patio areas of selected houses. Mortality was assessed 24 h post-exposure, while larval collections before and after the intervention were used for bottle bioassays and kdr genotyping. Bayesian and generalized linear model (GLM) analyses quantified the effect of biological and environmental factors on mortality. Mortality varied significantly by insecticide, strain, location, and cycle (p < 0.001). Front-located cages consistently showed higher mortality than indoor or patio placements. Malathion initially achieved >90% mortality but declined to ~65% after one year. Bifenthrin remained effective only against the susceptible strain, consistent with >30-fold resistance ratios in local mosquitoes. kdr mutation frequencies (V1016I, F1534C) remained high and unchanged.

The cage-bioassay approach enabled the simultaneous measurement of insecticidal, genetic, and environmental determinants of mosquito mortality under operational conditions. This method provides a valuable framework for linking resistance data with field efficacy, identifying performance-limiting factors, and guiding adaptive insecticide resistance management in vector control programs.