The rate of loss of DDT on cattle in relation to protection from reinfestation by the cattle tick Boophilus microplus

Abstract

Australian Illawarra Shorthorn cattle were sprayed with 0.5% w/v DDT on 31 consecutive occasions at intervals of 4 weeks. Spraying was rotated amongst three groups of cattle. The rate of loss of DDT was determined on the barrel for all occasions and concurrently on the flank and escutcheon for the last 16 occasions. A protective period, defined as the minimum period cattle were protected against reinfestation by the cattle tick, was determined concurrently from observations made on the flank and escutcheon. A cyclic change in the rate of loss of DDT occurred on all sites. The rate was highest in summer and lowest in winter, and was in antiphase with corresponding changes in the protective period. The magnitude of the change of rate of loss of DDT was greater on the flank than on either the barrel or the escutcheon. The rate of loss from the three sites was always in the sequence flank > escutcheon > barrel. The mean deposits of DDT present on the flank and escutcheon when protection had just ceased were respectively 0.29 mg/g hair and 0.054 mg/sq. in., and the protective periods did not differ significantly. The protective period on the barrel was not measured, but it was calculated from data on the DDT deposits to be longer than on either of the other sites, even allowing for the fact that laboratory tests showed that DDT on barrel hair was relatively less effective against larvae than similar weights of DDT on flank hair.