Cross-resistance patterns among Lucilia cuprina (Diptera: Calliphoridae) resistant to organophosphorus insecticides

Strains of Lucilia cuprina (Wiedemann) have been characterized as having low, intermediate, or high levels of esterase-mediated hydrolysis Of the organophosphorus insecticide, chlorfenvinphos. These levels correlate respectively with susceptibility to organophosphorus insecticides, malathion resistance, or diazinon resistance. Diazinon and chlorfenvinphos are diethyl organophosphorus insecticides having 2 ethoxy groups attached to their central phosphorus atom, whereas malathion is a dimethyl organophosphorus insecticide having 2 methoxy groups attached to its phosphorus atom, and, unusually, malathion also has 2 carboxylester bonds in addition to the phosphoester bonds that define organophosphorus compounds. We tested larvae for resistance to diazinon and also assessed representative malathion-resistant and diazinon-resistant L. cuprina strains at the adult stage for resistance to 12 organophosphorus insecticides, including analog pairs differing only in respect to their dimethyl-diethyl status. Two malathion-resistant strains have low-level cross-resistance to diazinon (3 to 4-fold), 4 diazinon-resistant strains have high-level diazinon resistance (11 to 16-fold), and 2 strains with a combined (malathion plus diazinon) resistance type also have high-level diazinon resistance (17 to 18-fold) relative to 3 organophosphorus insecticide-susceptible strains. One of the diazinon-resistant strains showed approximate to 2 times greater resistance factors toward diethyl organophosphorus insecticides than their dimethyl analogs while (leaving aside malathion to consider only the majority which have no carboxylester groups) a malathion-resistant strain showed 2-5 times greater resistance factors toward the dimethyl organophosphorus insectides than their diethyl analogs. The diazinon-resistant strain showed no resistance to 2 di-isopropyl organophosphorus compounds or to 2 organophosphorus insecticides which are asymmetric about the phosphorus atom (optically active). The malathion-resistant strain showed only slight resistance (<3-fold) to either the di-isopropyl or optically active organophosphorus insecticides, including the di-isobropyl analog of malathion. These cross-resistance patterns parallel those of certain organophosphorus insecticide-resistant strains of Musca domestica L., in which diazinon and malathion resistances also are proposed to be esterase mediated, reinforcing other biochemical data suggesting a general mechanism among the higher Diptera.