Submitted to: Pesticide Biochemistry and Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/10/2006
Publication Date: 1/5/2007
Citation: Li, A.Y., Guerrero, F.D., Pruett, J.H. 2007. Involvement of esterases in diazinon resistance and biphasic effects of piperonyl butoxide on diazinon toxicity to Haematobia irritans irritans (Diptera: Muscidae). Journal of Pesticide Biochemistry and Physiology. 87:147-155 Interpretive Summary: The horn fly, Haematobia irritans irritans (L.), remains one of the important ectoparasites affecting pastured cattle in the United States and many other countries. Control of the horn fly relies heavily on chemical insecticides, particularly synthetic pyrethroids and organophosphates (OPs). Resistance to pyrethroid insecticides in this pest in the U.S. and other countries resulted in control failures and horn flies continue to cause significant economic losses to ranchers. The OP insecticide diazinon has been increasingly used to control pyrethroid resistant horn flies. However, horn flies resistant to diazinon have been reported recently in the U.S. and Mexico. To sustain the use of diazinon for horn fly control, a study was conducted at the USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory, in Kerrville, Texas, to investigate the mechanism of diazinon resistance and to evaluate the effect of a synergistic compound, piperonyl butoxide (PBO), on diazinon toxicity to horn flies. Results of toxicity bioassays and biochemical analysis of esterases in both resistant and susceptible horn flies revealed a metabolic mechanism of resistance and a new biphasic mode of PBO interaction with diazinon. Enhanced activity of esterases was found to contribute to horn fly resistance to diazinon. PBO had no effect on esterase activity in horn flies. PBO inhibited diazinon toxicity to horn flies at higher concentrations, and enhanced diazinon toxicity at lower concentrations. We suggested that the synergistic effect of PBO at lower concentrations was achieved by facilatating the penetration of diazinon through horn fly cuticle. The knowledge generated from this study may help develop new synergized diazinon formulations for the control of resistant horn flies.
Technical Abstract: Resistance to insecticides remains a major problem for the successful control of the horn fly, Haematobia irritans irritans (L.), one of the most important pests of cattle in many countries including the United States. The organophosphate (OP) insecticide diazinon has been used to control pyrethroid resistant populations of the horn fly. There are only a few reported cases of horn fly resistance to diazinon in the United States and Mexico. Piperonyl butoxide (PBO) has been used successfully as a synergist of pyrethroid insecticides to control horn flies. PBO-synergized diazinon products are also available for horn fly control in the United States, although PBO is known to inhibit the bio-activation of certain OP insecticides including diazinon. A study was conducted to evaluate the effect of PBO on diazinon toxicity to horn flies using a filter paper bioassay technique. These bioassays in both the susceptible and diazinon-resistant horn fly strains revealed a biphasic effect of PBO on diazinon toxicity to horn flies. PBO inhibited diazinon toxicity when the PBO concentration used was high (5%), and no effect was observed when PBO concentration was intermediate (2%). However, at low concentrations (1% and lower), PBO significantly synergized diazinon toxicity. We demonstrated that enhanced esterase activity was associated with survivability of horn flies exposed to diazinon alone. PBO has been shown to inhibit esterase activity in other insect species. However, results of biochemical assays with esterases from this study suggest that PBO did not have significant effect on the overall esterase activity in the horn fly. The observed synergistic effect of PBO at lower concentrations on diazinon toxicity to horn flies could not be explained by reduced esterase activity due to PBO inhibition. It is likely that PBO synergized diazinon toxicity at lower concentrations by facilitating penetration of diazinon through the cuticle and/or inhibiting the oxidative detoxification of diazinon, and reduced diazinon toxicity at high PBO concentration by inhibiting the bio-activation of diazinon.