Changes in diazinon concentrations during shower and plunge dipping of Merino sheep

Abstract

The rate of removal of diazinon from solution (stripping) during dipping of adult short wool Merino sheep was similar in a standard and modified Buzacott 60R shower dip. During 12 min showering, dip concentration fell from about 100 mg/L to about 20 mg/L. Addition of insecticide concentrate (reinforcement) and sufficient fresh solution to re-establish original sump volume (topping up or replenishment) at this point restored dip concentration but was contrary to product label instructions. Adherence to the label directions to reinforce and top up after the sump was depleted by 25% (after 2 pens of sheep in the case of the standard dip) caused dip concentration to drop below the limit of quantification (10 mg/L). Similarly, current directions for continuous replenishment (commonly referred to as constant replenishment) shower dipping failed to offset diazinon stripping. Continuous replenishment plunge dipping with diazinon for the control of lice on sheep is an off-label use of the product; however, we demonstrated that a working concentration of about 100 mg diazinon /L (the nominal rate for lice control) was maintained in a 5000 L non-draining dip if charged and replenished at 200 mg diazinon/L. By maintaining 100 mg/L, this regimen was more likely to eradicate lice from sheep. This work also identified, by calculation, large and potentially significant variations in the dose of diazinon applied per sheep due to the application method, reinforcement practice and whether the sheep were treated early or late in the dipping sequence.