Sensitivity assessment of Caligus rogercresseyi to anti-louse chemicals in relation to treatment efficacy in Chilean salmonid farms

Abstract Control of the sea louse Caligus rogercresseyi in the Chilean salmonid industry relies mainly on chemical treatments, with reported resistance to deltamethrin and emamectin benzoate. In this study, twenty four-hour exposure bioassays were performed to explore the sensitivity level of adult male and female C. rogercresseyi towards the three main chemicals currently in use (azamethiphos, pyrethroids and emamectin benzoate) at four farms located in the Regions X and XI (Chile). Lice abundance was monitored weekly at the farms and anti-louse treatment efficacy and re-infestation rate were calculated. The correlation for adult lice between sensitivity level found in bioassays and treatment efficacy was explored. Azamethiphos and the pyrethroids deltamethrin and cypermethrin were the most commonly applied treatments. Sensitivity towards azamethiphos was lower in the farms from Region X than in those in Region XI. Yet azamethiphos treatment efficacy for adult lice was above 90% in all farms and the re-infestation rate low. Previous use of other organophosphates in Region X was suggested as the possible cause for the difference in sensitivity between regions. Low sensitivity towards deltamethrin coupled with low deltamethrin and cypermethrin treatment efficacies, high number of treatments and high re-infestation rate was detected in mid-west of Region X, indicating a severe resistance problem. In addition, this area probably had high lice infestation pressure since high number of treatments with the still effective azamethiphos was needed to keep the lice abundance low. In other areas, those parameters indicated that the resistance level and the infestation pressure were lower, but with differences between farms. Sensitivity towards emamectin benzoate was low and similar in the four farms (no treatment efficacy data available). Differences in sensitivity between louse genders were explored. Females tended to be less sensitive than males to azamethiphos only at low louse sensitivity. Females were more resistant than males to deltamethrin, although the differences decreased when sensitivity was low. Males tended to be less susceptible than females to emamectin benzoate when sensitivity decreased. The combination of methods and data presented in this study proved to be useful for detecting resistance issues and problematic areas regarding lice infestation. They may also be used for anticipating problems in the future, which is crucial for improving the use of anti-louse chemicals and reducing the risk of resistance development. Statement of relevance Tools for Caligus rogercresseyi resistance monitoring.