SUMMARY Sheep have been dipped in various concentrations of aldrin and dieldrin and the rate of loss of insecticide on the wool has been determined using a biological assay method. In one experiment with aldrin, the biological assay method was confirmed by chemical analysis. Depletion is rapid immediately following dipping for a period of 2–4 weeks, after this it is much slower. It has been shown that when dipping is carried out at the normal time, i.e. May to June, the chemical persists for a period of 3–4 months. When sheep are dipped in late July and August, however, the period of protection is much greater. The distribution of insecticide over the fleece has been examined; the area with least pick‐up of insecticide is in the tail region where the wool is coarse. Weather and rate of wool growth could not be correlated with rate of insecticide loss.

No abstract