Abstract
Variation in sensitivity of Lepeophtheirus salmonis (Krøyer) to dichlorvos in static toxicity tests was recorded on salmon farms in Ireland. Five hour LC50s ranged from 60 to 202 μg l −1 at 15 °C for five farms in Autumn 1991 and 76–151 μg l−1 at 10 °C for four farms in Spring of 1992. Resistance ratios, calculated using the most sensitive sites as baseline, were low and ranged from 1.32 to 3.27 in 1991 and 1.28–1.98 in 1992. Repeat sampling on three farms in 1991 and 1992 also indicated short-term changes in sensitivity to dichlorvos within farms.