Monitoring of the sensitivity of sea lice (Lepeophtheirus salmonis) to pyrethroids in Norway, Ireland and Scotland using bioassays and probit modelling

Treatment failures against sea lice with the pyrethroids deltamethrin (AlphaMax™), high-cis-cypermethrin (Betamax™) and cypermethrin (Excis™) have been anecdotally reported from Norway, Scotland and Ireland. In this study, a total of 19 Norwegian, 9 Irish and 4 Scottish sea lice strains collected in the period 2001-2003 were investigated for sensitivity to pyrethroids using bioassays and probit modelling. The sensitivity was expressed as EC50-the concentration (ppb) immobilizing 50% of sea lice in bioassays. Treatment failures with cypermethrin were reported in connection with 10 of the samples, 9 Norwegian and 1 Scottish. Treatment failures with deltamethrin were reported in connection with six of the Norwegian samples. A total of 15 strains had significant (P<0.05) higher EC50 to cypermethrin compared to a control strain, while 6 strains had significant higher EC50 to deltamethrin, indicating reduced sensitivity. Of the samples associated with reported treatment failure to cypermethrin, 7 out of 10 showed significant reduced sensitivity, while only 2 out of 6 showed reduced sensitivity to deltamethrin. The resistance ratio (resistance ratio=recorded EC50/control EC50) to cypermethrin ranged from 0.6 to 7.0 among the investigated strains, while the resistance ratio for deltamethrin ranged from 0.7 to 11.4. The study demonstrated that reduced sensitivity to pyrethroids occurs occasionally, but currently it does not seem to be an increasing problem.