The protection of sheep against blowfly strike. III. The effect of different formulations of gamma benzene hexachloride (B.H.C.)

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1. Three groups each of twelve merino sheep were treated separately with Lindane in emulsion, suspension and dust form to determine the influence of the formulation on the duration of protection against blowfly strike. 2. The emulsion and suspension contained a wetting agent of anionic type. The emulsion, containing 970 p.p.m. gamma B.H.C. and the suspension with 940 p.p.m. gamma B.H.C. were applied by immersion in a small vat of 120 gallons (546 litres) capacity. Twelve sheep were used for each formulation. The removal of dip wash per animal and the depletion rate of the insecticide were determined. 3. The rate of depletion of insecticide from the dipping bath was higher per sheep treated in the emulsion than in the suspension. This is supported by the finding that the average deposition of Lindane per sheep was 30•6 grams for the emulsion, but only 16•9 grams after immersion in the suspension. 4. The percentage rate of exhaustion of insecticide continued to rise progressively as the absolute quantity available in the tank decreased. This rise in percentage depletion was far more marked in the case of the emulsion that the suspension. 5. There was no significant difference between the amount of insecticide deposited in the fleece of the first and last of twelve sheep treated in either the emulsion or the suspension formulation. This may be ascribed to the finding that in the case of the emulsion the progressively higher percentage depletion accounted for the higher rate of deposition in the wool, in other words the higher rate of selective removal. In the case of the suspension the lower absolute depletion rate together with the appreciably lower percentage removal were the factors involved in maintaining the even deposition of insecticide with the progress of dipping. Had more sheep been dipped the end result would probably be different. 6. Continued immersion of sheep in an emulsion dip will result in a sudden and rapid complete depletion of insecticide below the level of biological effectiveness. Using a suspension dip it will take longer to reach this point. 7. Using a non-returnable spray as the means of applying an insecticide a suspension spray must be mixed at 2 ½ times the concentration of a dip and an emulsion of anionic type at about 5 times the concentration of a dip to obtain the same concentration of insecticide on treated sheep. 8. Attention is directed to Laudani's observation that the amount of insecticide absorbed by wool is dependent not only on the concentration of the active ingredient in the dip wash but also on the absolute amount present. Full consideration must therefore be paid to the capacity of a dip or bath. 9. The third group of animals was treated with dust at a rate of 17 gm. of the active ingredient per head, i.e. the amount absorbed by each sheep dipped in the suspension, in order to facilitate comparison of larvicidal efficacy of a dust and a suspension. The bio-assay tests with first instar maggots of Lucilia cuprina revealed an average protection of relatively shorter duration, 29•4 weeks with the considerably heavier deposit (30•6 gm.) in the case of emulsion, than the 27•4 weeks with the suspension (16 • 9 gm.) and 29 • 8 weeks for the dust (17 gm.). The dust, therefore, gave the best protection of the three formulations per unit of insecticide absorbed by the fleece. 10. The grease content of the fleece was shown to exert a marked influence on the degree of efficacy of the insecticide deposited from an emulsion. This does not apply, however, in the case of the dust or the suspension. The loss of larvicidal activity is proportionate to the level of wool grease which is not affected by dipping or spraying. 11. The quality of the wool has no effect on the protecting properties of any of the formulations. 12. The use of B.H.C. or any compound possessing good powers of diffusion in the wool is recommended for the long term protection of woolled sheep against blowfly strike. Such compounds may be used in the form of dusts or wettable powders but the present investigation suggests that their use as emulsions is contraindicated.