Efficacy of deltamethrin in the control of Caligus rogercresseyi (Boxshall and Bravo) using bath treatment

•We study the efficacy of deltamethrin in the control of C. rogercresseyi under field conditions.•We did not record significant differences between enclosed tarpauling and skirt method in the control of adult lice.•We recorded significant differences between enclosed tarpauling and skirt method in the control of chalimus stages.

The pyrethroid deltamethrin (AlphaMax™) was authorized to control sea lice in Chile in September 2007. In the same year the first studies to assess its efficacy in the control of Caligus rogercresseyi were carried out in four salmon farms located near Puerto Montt. At two of the farms with fish weighing less than 1 kg the cages were enclosed in a tarpaulin for bath treatments, while a skirt around the cage sides was used in the other two farms with larger fish. No significant differences in AlphaMax™ efficacy in adult lice were recorded between both methods of treatment, neither between females and males at 24 h post-treatment, despite the differences in size of fish and lice abundance pre-treatment. However, significant differences in efficacy were recorded for the chalimus stages, with lower efficacy in the cages using the skirt method.