Resistance to organophosphates in red spider mite, Tetranychus urticae, from English hop gardens

Two strains of Tetranychus urticae, collected in 1969 from hops in Kent (strain RK) and Wisbech, Cambridgeshire (RW), showed a similar pattern of resistance to organophosphates, high to parathion (> 100 ×), low to azinphos-ethyl (7 ×) and intermediate to demeton-S-methyl (21–27 ×); they were both fully susceptible to tetradifon, dicofol and formetanate. RK showed greatei than 50 × resistance to fourteen other organophosphates; exceptions were azinphos-methyl (11 ×), phosphamidon (21 ×) and methidathion (31 ×). Resistance in RK was shown to be controlled by a single major gene, incompletely dominant with parathion but nearer to intermediate in expression to demeton-S-methyl and azinphos-ethyl. A test for allelism showed that the major genes for O.P. resistance in RK and the Dutch strain Baardse reside at the same locus or are closely linked. The results are compared with other work on T. urticae and on Panonychus ulmi.