Chlordimeform [N′-(4-chloro-2-methylphenyl)-N, N dimethylformamidine], BTS-23376 [1, 3-di-(4-chloro-2-methylphenylimino)-2-methyl-2-azapropane] and BTS-27419 [1, 3-di-(2, 4-dimethylphenylimino)-2-methyl-2-azapropane] cause terminal detachment of the cattle tick, Boophilus microplus, from cattle or mice. A simple laboratory method of screening such ‘detaching agents’ is described in which mice are used as host animals.
Freshly moulted female B. microplus, fed as nymphs on cattle, were allowed to attach inside protective rings of Perspex fixed to anaesthetized mice and their rate of detachment was measured following topical application of acetone solutions of candidate chemicals to the ticks. No appreciable detachment was detected in 120 min when an organophosphorus compound (chlorpyrifos), a carbamate (carbaryl), organochlorine compounds (DDT and lindane) or a synthetic pyrethroid (allethrin) were tested in this way at doses of 2.5, 0.25 and 0.025 μg/tick.
However, graded time-detachment and dosage-detachment relationships were determined quite readily for chlordimeform, BTS-23376 and BTS-27419. As there were differences between these detaching agents in rate of action and in the variance of tick response to them, estimates of their relative potency depended on the time lapse after treatment and the response level at which the comparisons were made. BTS-27419 was 30 and 16 times and BTS-23376 was 14 and 3.6 times as effective as chlordimeform at the median detachment dose after 30 min and 60 min respectively.