Settling of insecticide from dip wash mixed with dam water and zinc sulphate and used to control sheep lice (Bovicola ovis)

Abstract

Insecticidal dipping fluid emulsions, mixed in vitro in dam water containing suspended clay particles and 1% w/v zinc sulphate, were analysed to determine rates of settling of diazinon, cyhalothrin and cypermethrin. Fifteen minutes after mixing, the concentration of the insecticides 5 cm below the surface had declined by 72.5%, 72.8% and 89.4%, respectively. On remixing, the concentration of insecticide in suspension was close to or greater than the initial concentration. In 2 trials, lice were eradicated from sheep showered with dip wash mixed in cloudy dam water to which 1% w/v of zinc sulphate was added. In 12 flock treatments in which 1000 to 2000 sheep were dipped with added zinc sulphate, the concentration of insecticide remained above the minimum lethal for susceptible strains of lice. However, lice were still present 6 months later in 8 of these flocks. When zinc sulphate is added to dip wash, agitation is needed to maintain the insecticide in suspension.

Further evidence that zinc sulphate compromises the efficacy of dipping treatments using diazinon to control sheep lice (Bovicola ovis)

“…The reduction was significant for zinc sulphate concentrations between 0.2% and 1.5%, which covers the range usually used by farmers to prevent dermatophilosis. 6,7 The in-vitro results indicated that increasing the concentration of zinc sulphate from 0.5% to 1.5% made little difference to the wetting ability of the dip wash. In both the field and in-vitro experiments, treatments with higher concentrations of zinc sulphate were not consistently those with poorer wetting.…”

“…Downing 5 published some concerns about the effect of adding zinc sulphate to dip wash, a common practice for the purpose of minimising the spread of dermatophilosis. 6,7 He reported flocculation of the dip wash, which was made using dam water, and a subsequent decrease in the diazinon concentration in the supernatant. Similar findings were reported by Morcombe et al 7 with diazinon, cypermethrin and cyhalothrin.…”

“…The reports of Downing 5 and Morcombe et al 7 indicate that further investigation is required to determine if zinc sulphate compromises the efficacy of dipping treatments and, if so, at what concentrations. The following report includes data from in-vivo and in-vitro studies that compared the wettability and efficacy of dip wash made with and without the addition of zinc sulphate.…”

“…The samples were refrigerated until assayed by the Chemistry Centre of Western Australia using standard procedures. 7 After dipping, the degree to which the fleece was wet was assessed by measuring the length of the staple and that part wetted at five partings on the dorsal mid-line of the neck and withers of eight sheep chosen at random. The treatment groups were held on separate pasture plots at Wongan Hills Research Station secured by electric outriggers on both sides of common fences.…”

“…However, concentrations below 0.2% may be ineffective for the prevention of dermatophilosis. 6 Problems associated with adding zinc sulphate to dip wash were identified by Downing 5 and Morcombe et al 7 The report by Downing 5 on the effect of clay particles on concentration of diazinon in dip wash, highlighted a potential risk of not agitating dip wash throughout a dipping event. Similarly, Morcombe et al 7 found that the concentration of insecticide decreased in dip wash when zinc sulphate was added at small concentrations (2 kg and 4 kg per 1000 L).…”