FIELD TRIALS ON THE EFFICACY OF THE ORGANOPHOSPHORUS COMPOUND AZAMETHIPHOS FOR THE CONTROL OF SEA LICE (COPEPODA, CALIGIDAE) INFESTATIONS OF FARMED ATLANTIC SALMON (SALMO-SALAR)

A study of the efficacy of azamethiphos dihydro-2-oxo-1,3-oxazolo[4,5- b]pyridin-3-ylmethyl 0,0-dimethyl phosphorothioate) is described when used to treat Atlantic salmon (Salmo salar) infected with sea lice (Le peophtheirus salmonis and Caligus elongatus) at several salmon farms o n the west coast of Scotland. Prior to commencing the cage trials, rel ative lice sensitivity was evaluated through a series of in vitro expo sures which demonstrated differences in the sensitivities of different lice populations to both azamethiphos and dichlorvos. Treatments were carried out by completely enclosing pens within tarpaulins, or by sur rounding pens with a tarpaulin skirt, and were 1 h in duration. A rang e of treatment concentrations were studied which included 0.05, 0.1 an d 0.2 mg(-1) L azamethiphos, in addition to reference treatments with dichlorvos (0,0-dimethyl-2,2-dichlorovinyl phosphate) at a concentrati on of 1.0 mg(-1) L. Lice response was measured before treatment, 1 h, 24 h, and 1 week after treatment. Azamethiphos was highly efficacious (85% or better) (Site 3) against adult and pre-adult lice, from a sens itive population, at a concentration of 0.05 mg(-1) L where complete t arpaulins were used. Where resistant populations were tested, a concen tration of 0.2 mg(-1) L (complete tarpaulins) resulted in efficacies r anging from 7.1 to 82.1% (mean 57.1%) (Site 1) and 41 to 96.6% (mean 6 9.4%) (Site 2) against adult and pre-adult lice, and were generally mo re variable depending on local treatment conditions. Skirt treatments, tested only on resistant populations of lice, were found to be ineffe ctive. A general trend was observed indicating that pre-adult I male l ice were the most sensitive followed by, in order of sensitivity, pre- adult I females, pre-adult II females, pre-adult II males, adult males and adult females. Larval lice numbers, from both sensitive and resis tant populations, appeared unaffected by treatment with azamethiphos f ollowing treatments with either complete tarpaulins or skirts. When us ed at the concentrations discussed above, fish tolerated treatments an d did not display signs of acute mortality. However, hyperactivity was displayed by several of the treatment groups during treatments.