No abstract

Summwry[82P]Coumaphos, a phosphorothionate, was rapidly absorbed and metabolized by cattle tick larvae dipped in aqueous suspensions of the acaricide. One metabolite was shown by chromatographic, spectroscopic, and cholinesterase inhibition evidence to be the oxygen analogue, a more potent in vitro cholinesterase inhibitor than the parent compound. Other water-soluble~ metabolites were only tentatively identified. Sublethal doses were metabolized mainly to water-soluble materials and a small quantity of oxygen analogue whereas lethal doses, producing complete mortality in 2-3 hr, yielded almost equal amounts of the oxygen analogue and water-soluble materials.In vivo inhibition of cholinesterase in treated larvae was dose-dependent and preceded death.

Dosage-mortality tests on larval ticks have indicated that the Mt Alford and Gracemere strains of B. micro pius differ from the Biarra and Ridgelands strains respectively, not only as previously reported by exhibiting markedly greater resistance to both chlorpyrifos and diazinon, but also by exhibiting moderately greater resistance in the Mt Alford strain to bromophos-ethyl and dioxathion and in the Gracemere strain to carbophenothion and coumaphos. Their responses to other chemicals were similar to those of the Biarra and Ridgelands strains respectively.Tests of acetylcholinesterase (AChE) activity and kinetic studies of inhibition of AChE in Mt Alford and Gracemere larvae indicated the presence of enzymes with relatively insensitive components similar to those of Biarra and Ridgelands AChE respectively.The penetration of [I4C]chlorpyrifos was similar in Mt Alford, Gracemere, Biarra and Ridgelands larvae, but of the chlorpyrifos metabolized considerably less was recovered as the oxygen analogue in Mt Alford and Gracemere larvae. This difference appeared to be due to greater hydrolytic activity. It was concluded that the greater resistance of these strains is due to enhanced detoxication.The previously described Mackay strain, although maintaining its pattern of resistance, was shown to have changed to one now exhibiting a reduced capacity for detoxication and a Ridgelands type of AChE. The Silkwood strain found nearly three years later was shown to be similar to the changed Mackay strain.

Strains of the cattle tick resistant to organophosphorus compounds were first found at Ridgelands in central Queensland in 1963 (Shaw and Malcolm 1964; Shaw 1966; Roulston, Stone, Wilson, and White 1968). Resistance of a different type was subsequently found at Biarra in south-eastern Queensland in 1966 (Roulston and Wharton 1967; Wharton 1967; Shaw, Cook, and Carson 1968). Ridgelands and Biarra strains of ticks both exhibit resistance to a wide range of organophosphorus and carbamate chemicals but differ in that resistance levels are higher and resistance extends to a wider range of chemicals in the Biarra strain. Biochemical investigations have shown that resistance in both strains is due to the presence of an acetylcholin-esterase system which is relatively insensitive to inhibition by organophosphorus chemicals (Lee and Batham 1966; Roulston, Schnitzerling, and Schuntner 1968; Schuntner, Roulston, and Schnitzerling 1968)

Amitraz, 1, 5-di(2, 4-dimethylphenyl)-3-methyl-1, 3, 5-triazapenta-1, 4-diene, labelled with 14C in the 2-methyl groups was applied to B. microplus larvae by an immersion technique. The chemical penetrated readily but never appeared in large amounts internally due to rapid cleavage to N-2, 4-dimethylphenyl-N'-methylformamidine. The expected complementary cleavage product 2, 4-dimethylformanilide was not produced in equivalent quantity. However, large amounts of polar metabolite(s) were produced. Small quantities of 2, 4-dimethylaniline and an unidentified non-polar metabolite were also produced. Of the identified chemicals only amitraz and N-2, 4-dimethylphenyl-N'-methylformamidine were toxic to larvae. Piperonyl butoxide applied simultaneously with amitraz had only a slight effect on metabolism but had a three-fold synergistic effect. SKF 525-A similarly applied had a negligible effect on both metabolism and toxicity.