Female-biased tolerance to delousing treatments in the sea louse Caligus rogercresseyi increases with compound lipophilicity

Sex-biased pesticide tolerance is increasingly recognized in arthropods, yet its implications for parasite management in aquaculture remain poorly understood. This study evaluated sex-specific antiparasitic tolerance in the sea louse Caligus rogercresseyi , a marine ectoparasite and a major challenge for salmon aquaculture. Here we analyzed 340 bioassays conducted between 2018 and 2026 across southern Chile, exposing male and female sea lice to four chemical actives plus one botanical formulation with differing modes of action and lipophilicity. Using generalized linear mixed models and dose-response analysis, we found that females consistently exhibited lower mortality than males, indicating marked sex-dependent differences in susceptibility. Significant sex effects were detected for azamethiphos, deltamethrin, and cypermethrin, whereas significant sex-by-concentration interactions were observed for both pyrethroids. Sex-specific log-logistic models further showed that females displayed higher EC 50 and LD 50 values across all treatments. Although EC 50 differences were compound-dependent, female LD 50 values were significantly higher for all compounds, ranging from 1.6 to 3.7-fold those of males. Interestingly, the strongest sex-dependent responses were observed for lipophilic compounds, whereas water-soluble antiparasitics showed smaller differences between sexes. Our findings highlight the importance of incorporating sex as a factor in resistance monitoring and treatment evaluation, which may improve management strategies against sea lice in salmon aquaculture.