Evaluation of the performance of pyrethroids on different life stages of Caligus rogercresseyi in southern Chile

Control of sea lice in Chile is largely based on antiparasitic treatments, synthetic pyrethroids being the most used drugs. In recent years, farmers in Chile have reported decreased performance of pyrethroid-based treatments. The aim of this study was to assess the performance of two deltamethrin-based (Alphamax® and a generic product) and one cypermethrin-based (Betamax®) product on the different life stages of Caligus rogercresseyi, while controlling potential confounders. We found that both deltamethrin products and the cypermethrin product had a significant effect on the reduction of juvenile, mobile adult, and gravid female lice, compared with untreated pens; however, the effect on juvenile lice was less than on mobile stages. There was no evidence that pyrethroids performed better on certain mobile life stages, such as gravid females. When the three products were compared, no significant differences were observed in the numbers of juvenile, adult male, and non-gravid female lice after we controlled for potential confounders; however, cypermethrin exhibited a small, yet significantly greater effect on the gravid female group when compared with one of the deltamethrin-based products. We also confirmed that other factors besides the product choice, such as the pre-treatment sea lice abundance, water temperature and salinity, and time elapsed to the post-treatment sample, affect the post-treatment sea lice level as well, and therefore, they should be taken into consideration when assessing the effect of immersion treatments.