Bioassay tray for assessing susceptibility of Liriomyza sativae Blanchard (Diptera: Agromyzidae) to reduced-risk insecticides and resistance monitoring in Brazil

Liriomyza sativae infestations pose a threat to horticultural crops like melon and tomato, causing economic losses of up to 30%, typically managed through insecticide applications. Despite the inherent risk of resistance development in populations, reported cases are still relatively low that may be associated with the practicality of the existing bioassay methods. To address the resistance phenomenon in a species, a practical and reliable method to generate susceptibility data should be used. This study aimed to refine existing bioassay method(s) for Liriomyza against different modes of action and set up the susceptibility status of field populations to cyantraniliprole and spinetoram in Brazil. Using the leaf disc immersion method, individual larvae on leaf discs were transferred to both bioassay trays and acrylic Petri dishes after treating them, and mortality was assessed 48 and 72 h after exposure. Susceptibility parameters varied with arena type, exposure duration, and insecticide. Bioassay tray arenas showed consistency of response compared to acrylic dishes and improved parameters at 72 h exposure among the modes of action assessed. Thus, survey of populations against cyantraniliprole and spinetoram using Bioassay tray arenas showed to be consistent and data suggested resistance evidence in L. sativae populations. Resistance ratio varied from 1.2- (MSR4) to 4.2- (BRN1) and from 1.6- (MSR5) to 7.9-fold (MSR2) for cyantraniliprole and spinetoram, respectively. Diagnostic concentration (10 mg/L) for both insecticides showed survival up to 10% (spinetoram) and 2% (cyantraniliprole). The bioassay tray method may be an alternative to existing methods for toxicological assessments and detection/documenting resistance to insecticides.