Emamectin benzoate resistance and fitness in laboratory reared salmon lice (Lepeophtheirus salmonis)

• Sea lice sensitive or tolerant to emamectin benzoate were bred and interbred.
• No fitness costs were found in tolerant and hybrid lines.
• This is the first study on drug tolerance and fitness in sea lice.

Reduced sensitivity to the avermectin emamectin benzoate (EB) was suspected in salmon lice, collected from five Norwegian salmon farms located in Middle and Western Norway in the period January 2008–February 2009. After being transferred to the laboratory, lice from each farm were compared to an EB sensitive laboratory strain (LsGulen, collected from the field in June 2006) using bioassays. All five samples were thereafter confirmed as displaying reduced sensitivity to EB. A strain (LsAustevoll) based upon lice from one of these farms was subsequently established in the laboratory. LsAustevoll was compared to the EB sensitive strain, LsGulen, for 4 generations. A hybrid strain (LsHybrid), established by crossing LsGulen and LsAustevoll, was included in the comparisons for 3 generations. Sensitivity to EB was assessed for each generation, and the fitness parameters: egg production, hatching success and copepodid survival were assessed for selected generations. LsAustevoll retained reduced sensitivity to EB throughout all four generations, while LsHybrid displayed intermediate EB sensitivity to its parental strains. No fitness costs associated with reduced EB sensitivity were detected for any of the parameters studied.