Deltamethrin resistance in the sea louse C aligus rogercresseyi ( B oxhall and B ravo) in C hile: bioassay results and usage data for antiparasitic agents with references to N orwegian conditions

The sea louse C aligus rogercresseyi is a major threat to C hilean salmonid farming. Pyrethroids have been used for anticaligus treatments since 2007, but have shown reduced effect, most likely due to resistance development. Pyrethroid resistance is also a known problem in L epeophtheirus salmonis in the N orthern H emisphere. This study describes the development of deltamethrin resistance in C . rogercresseyi based on bioassays and usage data for pyrethroids in C hilean aquaculture. These results were compared to bioassays from L . salmonis from Norway and to N orwegian usage data. Available deltamethrin bioassay results from 2007 and 2008, as well as bioassays from N orway, were collected and remodelled. Bioassays were performed on field‐collected sea lice in region X in C hile in 2012 and 2013. Bioassays from 2007 were performed prior to the introduction of pyrethroids to the C hilean market. Both the results from 2008 and 2012 showed an increased resistance. Increased pyrethroid resistance was also indicated by the increased use of pyrethroids in C hilean aquaculture compared with the production of salmonids. A similar trend was seen in the N orwegian usage data. The bioassay results from C hile from 2012 and 2013 also indicated a difference in the susceptibility to deltamethrin between male and female caligus.