Standardization of an in vitro test for doramectin, fipronil, chlorpyrifos, and fluralaner against Cochliomyia hominivorax (Diptera: Calliphoridae) larvae.

Cochliomyia hominivorax (Coquerel, 1858) is the main etiological agent of myiasis in Latin America. In vitro bioassays are essential for detecting insecticide resistance development in the laboratory. This study evaluated different in vitro bioassay methodologies targeting C. hominivorax third-instar larvae. Chlorpyrifos, fipronil, fluralaner, and doramectin, were tested using either a filter paper assay or larval immersion for three different durations. In the filter paper assays, 9 concentrations of each active ingredient were applied to filter paper discs. After drying, the discs were placed in Petri dishes and ten third-instar larvae of C. hominivorax were introduced to each disc. In the immersion assays, groups were submerged in solutions containing the same concentrations used in the filter paper tests. In both methods, each concentration was evaluated in 6 replicates at 24, 48, and 72 h after exposure for larvicidal effects and for eventual adult emergence. These were then reported as median lethal concentrations (LC50) and inhibition emergence concentrations (IE50) when possible. The 72-h exposure period was most appropriate for evaluating maximal mortality across the tested methodologies. For all active ingredients assessed, the administered doses required to induce mortality were sufficient to inhibit the emergence of the challenged individuals. The filter paper assay method demonstrated greater larvicidal efficacy for fluralaner against C. hominivorax larvae, whereas the larval immersion method exhibited higher efficacy for both fipronil and doramectin. These findings support the standardization of in vitro bioassays for preliminary screening of ectoparasiticides and for resistance monitoring in field populations.