Effect of piperonyl butoxide on diazinon resistance in field strains of the sheep blowfly, Lucilia cuprina (Diptera : Calliphoridae), in New Zealand

Pretreatment of first instar larvae of 28 resistant strains of Lucilia cupr ina (Wiedemann) with the inhibitor of microsomal oxidases, piperonyl butoxi de, resulted in a biphasic response to the phosphorothioate insecticide dia zinon. Analysis of the data revealed a complex response in which both syner gist-dependent and independent effects occurred. The responses varied marke dly from strain to strain. A laboratory susceptible strain and field strain s with resistance factors of less than 20-fold exhibited, in the presence o f piperonyl butoxide, an increased LC50 with respect to diazinon whereas th ose strains with > 20-fold resistance were synergized by the compound. We c onclude tentatively that microsomal mixed-function oxidases play a contribu tory role in the development of resistance and that the variation in synerg ist effect from strain to strain may be attributed, at least in part, to th e two-fold effect of these enzymes on phosphorothioate insecticides such as diazinon.