An Analysis of House Fly Resistance to Insecticides in California1

The pattern of insecticide resistance in house flies, Musca domestica L., from 6 areas of California was found to reflect the differing chemical-control practices in each area. DDT resistance continues to be high in all areas despite more than 10 years of nonuse of this compound for fly control. It shows positive correlation with diazinon resistance but not with malathion resistance, and thus its prolonged stability was likely enhanced by diazinon pressure. Dieldrin resistance also remains high, but shows no correlation with organophosphorus resistance. The non-identity of diazinon and malathion resistances was clearly discernible. High resistance to diazinon (20.7×) in the Anaheim strain was accompanied by only moderate resistance to malathion, and high resistance to malathion in the Moorpark strain was accompanied by only moderate resistance to diazinon (7.1×). The highest degree of resislance observed (at the LD90 level) by topical application of organophosphates and carbamates was: malathion >60×, diazinon 25×, ronnel 14×, fenthion 10.1×, dimethoate 4.2×, dimetilan 3.7× and Hercules 8717-E [m-(2-propynyloxy) phenyl methylcarbamate] 3.5×.