Bioassays with salmon lice (Lepeophtheirus salmonis) copepodites in microtiter plates

The development of resistance in salmon lice (Lepeophtheirus salmonis) to antiparasitic agents presents a challenge for parasite management in aquaculture. Existing bioassay protocols for monitoring drug sensitivity primarily rely on preadult and adult lice, requiring significant resources and manual labor. In this study, we present a novel high-throughput bioassay protocol utilizing the copepodite life stage, established in 96-well microtiter plates. The protocol was evaluated for two of the ten substances tested, deltamethrin and imidacloprid, by direct comparisons between copepodite and preadult bioassays in a fully sensitive strain (LsGulen). In addition, comparison of bioassay performance with deltamethrin between the two life stages was carried out on a pyrethroid resistant strain (LsBjugn).Our results demonstrate that the copepodite bioassay reliably estimates compound efficacy, clearly discriminating between the deltamethrin sensitive and resistant strain. For both deltamethrin and imidacloprid, the bioassays on copepodids yielded higher EC50-values compared to bioassays on preadults.The copepodite assay enables screening of multiple compounds with minimal resource requirements. It could also serve as method for sensitivity testing in field, if good representation of the larger sea lice population is assured by sampling egg strings from multiple females. Furthermore, the copepodid bioassay allows for direct association between copepodite phenotypes and maternal genotypes, facilitating studies of inheritance and resistance mechanisms. While not a replacement for established preadult assays, the copepodite protocol offers complementary advantages in efficiency, throughput, and the ability to resolve intra-population sensitivity differences.These findings represent the first reported comparison of deltamethrin susceptibility between copepodid and preadult life stages in L. salmonis.