The duration of protection afforded to sheep against blowfly strike by the
treatment of the wool with an insecticide possessing good powers of diffusion is
correlated with the length of wool at the time of treatment. For every additional 1/4 inch in the length of fleece treated by B.H.C. wettable powder at a concentration
of 0•5 per cent gamma, full protection is prolonged by 2 ½ weeks and partial
protection by 6 weeks.