Reduced sensitivity of the salmon louse, Lepeophtheirus salmonis, to the organophosphate dichlorvos


Dr R. Wootten, Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland.


Abstract. Anecdotal reports from the Scottish fish farming industry suggested a reduction in the sensitivity of salmon lice, Lepeophtheirus salmonis, to the organophosphate dichlorvos. In this study, pro-adult and adult lice were collected from marine salmon sites showing a variety of ‘sensitivities’ to dichlorvos and subjected to dichlorvos exposure at a range of doses for a 48-h period in vitro. From the lice mortalities, 24 and 48 h LC50 values were calculated, and using a ‘sensitive’ source of lice as a reference, resistance ratios calculated. The data generated suggests that L. salmonis from different geographic areas shows differences in sensitivity to dichlorvos. The implications for the salmon farming industry are discussed.