A year-long farm trial was performed in 2003/04 to determine the relative efficacies of commercial formulations of diflubenzuron and triflumuron lousicides and measure pesticide concentrations in different parts of the fleece throughout the year. Significant differences were found between the concentrations of pesticides present on samples of fleeces taken from the back and side initially (P<0.05), 5 months (P<0.001), and 12 months (P<0.05) after pour-on treatments. The bulk of the pesticides from pour-on treatments remained in the zone where they had been applied and little moved to the sides of the sheep. Five months after application pesticide concentrations were significantly higher (P<0.001) in staple tip compared with butt samples on both back and side fleece samples for pour-on or jetting treatments, confirming recent predictions that diffusion of pesticides in greasy wool will be limited. Concentrations in butt samples from 5 months onwards were near or less than the mean LD95 doses reported for diflubenzuron for susceptible – 0.31 mg/kg, 95% confidence limits (CL) 0.16-1.25 mg/kg – and resistant lice populations – 4.4 mg/kg, 95% CL 1.4-71 mg/kg. The results offer a possible explanation for the anecdotal reports of poor efficacy of some pour-on louse treatments and treatment failures on farm.

DA, Rankin, TC Reid, DJW Cole, RA Urquhart, and MS Syhre

Proceedings of the New Zealand Society of Animal Production, Volume 67, Wanaka, 320-325, 2007
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