Single-dose field bioassay for sensitivity testing in sea lice, Lepeophtheirus salmonis: development of a rapid diagnostic tool.

Sea lice on farmed salmonids are often treated with chemicals. Sensitivity testing of sea lice can reduce the number of treatments by identifying substances the sea lice are susceptible to. This study describes a simpler protocol for field sensitivity testing than today's six-dose bioassay. The protocol, which uses a single dose of the delousing agents deltamethrin, azamethiphos and emamectin benzoate, was developed on four different strains of sea lice and their subsequent generations. A sensitive strain and a strain showing reduced sensitivity were identified for each chemical after performing traditional bioassays and small-scale treatments. The single doses for each chemical were established by modelling dose-response curves from 24-h bioassays on strains with differences in sensitivity. The largest difference between the lower 80% prediction interval for the sensitive strain and the upper 80% prediction interval for the strain showing reduced sensitivity was identified for each delousing agent. The concentration of the chemical and the % mortality corresponding to each of the 80% prediction intervals were subsequently established. To validate the protocol for field use, further studies on both sensitive and resistant strains of sea lice under field conditions are required.