Susceptibility of salmon louse strains to pyrethroids.

Bioassays involved 30 min of exposure of parasites to deltamethrin or cypermethrin, followed by 24 h of recovery and rating of animals as normal or affected. Standard bioassays included at least five drug concentrations with duplicate batches of 10 animals (5 adult males and 5 preadult II females) per drug or control treatment. Median effective concentrations (EC50) were derived by probit analysis. Single-dose bioassays were conducted where availability of parasites was restricted and involved one deltamethrin concentration (2 μg L-1) and untreated controls.