Efficacy of Emamectin Benzoate Figure 1. Profile of emamectin benzoate treatment episodes in years 2002 to 2006. Proportions are presented for all site-wide emamectin benzoate treatment episodes and for episodes with the necessary pre/post treatment sea lice data required for further analysis. (A) The proportion of treatment episodes occurring in each stage of production/season in the two year production cycle. Seasons are defined as spring [Feb–Apr], summer [May–Jul], autumn [Aug–Oct] and winter [Nov–Jan]. (B) The proportion of effective, loch-wide and effective loch-wide treatment episodes and the proportion of treatments applied when lice levels were below treatment-trigger guidelines. doi:10.1371/journal.pone.0001549.g001 they were followed up with at least one cypermethrin treatment within a matter of weeks. The other difference of note was the lower proportion of effective treatments present within the reduced data set. However, of the 77 treatments that were excluded from further analysis, it remains unclear how effective they were due to limited lice count data prior to and/or following treatment. Similarly, without a valid pre-treatment lice count, it could not be ascertained how many adult female lice were present and therefore whether guideline treatment-trigger levels had been reached. Thus the proportions shown for the full data set should be regarded as estimates, as the true incidence of effective episodes may have been lower and the frequency with which treatments were applied when lice levels were below trigger guidelines may have been higher. Trends in treatment efficacy The mean annual efficacy of emamectin benzoate treatments in controlling infestations of mobile L. salmonis, in the 83 day period following treatment initiation, is shown in Figure 2. It should be noted that efficacy percentages could not be calculated for episodes with a pre-treatment mobile abundance of zero. This resulted in four treatment episodes from 2004 and three from 2003 being excluded from this summary plot. With the exception of 2006, mean louse abundance in all years fell to less than 45% of pre-treatment levels within 27 days of treatment intervention. In 2002, abundance fell below 1% of pre-treatment levels within 34 days and remained lower than 12% throughout the rest of the 83 day period. Treatments applied in 2003 also appeared to be highly efficacious, with abundance falling below 5% between days 28–34, and not rising above 40% thereafter. Treatments administered in 2004 and 2005 appeared to take longer to reach maximum efficacy, however mean abundance did fall below 17% and 30% of pre-treatment abundance respectively. In contrast to all other years, lice abundance in 2006 remained above pre-treatment levels until around 5 weeks after treatment intervention. While abundance did drop to 35% of pre-treatment levels between days 56 and 62, it remained above 40% in all other weeks. The percentage changes, as presented in Figure 2, should be considered together with the absolute mean lice abundance prior to treatment intervention. As illustrated in Figure 3, mean lice abundance prior to treatment varied considerably over the five- year period. At 14.4 and 10.7 lice per fish respectively, the mean pre-treatment abundance of L. salmonis mobiles in 2003 and 2004 was approximately two to three times higher than in 2002, 2005 and 2006. However, with the exception of 2006, mean abundance in all years dropped to below four lice per fish within a month of treatment initiation. PLoS ONE | www.plosone.org 4 February 2008 | Volume 3 | Issue 2 | e1549