Determination of reduced sensitivity in sea lice (Lepeophtheirus salmonis Kroyer) against the pyrethroid deltamethrin using bioassays and probit modelling

Four Norwegian populations of the ectoparasitic copepod Lepeophtheirus salmonis K. were used to develop a bioassay for sensitivity determination to the pyrethroid deltamethrin (AlphaMax, Alpharma, Oslo, Norway, 10 mg deltamethrin ml(-1)). Bioassays using preadult II stage lice were performed with the sea lice exposed on salmon (Salmo salar L.) and with the sea lice exposed in polystyrene boxes. The purpose was to investigate: (1) the difference in response in sea lice attached to salmon versus sea lice in polystyrene boxes, (2) the optimal time point for evaluation of the response after the end of the 30-min exposure period and (3) the difference between the immobilizing and the lethal concentrations of deltamethrin. The EC50 (EC50: the concentration immobilizing 50% of the exposed sea lice) was calculated with probit modelling; The EC50 value for sea lice in polystyrene boxes, evaluated at 24 h after termination of the exposure period, was considered a good indicator of sensitivity. The results were used to establish criteria for identification of strains with reduced sensitivity to deltamethrin. Bioassays with preadult II lice in polystyrene boxes demonstrated a significant (P < 0.05) reduction in sensitivity as the cause of a treatment failure with deltamethrin against sea lice in one salmon farm (Rogaland; EC50=0.25 ppb). The other populations tested (Nord Trondelag, EC50 = 0.04 ppb; Sor Trondelag, EC50 = 0.06 ppb; Aust Agder, EC50=0.10 ppb) were not significantly (P > 0.05) different with respect to sensitivity towards deltamethrin. (C) 2003 Published by Elsevier Science B.V.