Involvement of esterases in diazinon resistance and biphasic eVects of piperonyl butoxide on diazinon toxicity to Haematobia irritans irritans (Diptera: Muscidae)

Resistance to insecticides remains a major problem for the successful control of the horn Xy, 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 Xy. There are only a few reported cases of horn Xy 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 Xies. PBO-synergized diazinon products are also available for horn Xy 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 eVect of PBO on diazinon toxicity to horn Xies using a Wlter paper bioassay technique. These bioassays in both the susceptible and diazinon-resistant horn Xy strains revealed a biphasic eVect of PBO on diazinon toxicity to horn Xies. PBO inhibited diazinon toxicity when the PBO concentration used was high (5%), and no eVect was observed when PBO concentration was intermediate (2%). However, at low concentrations (1% and lower), PBO signiWcantly synergized diazinon toxicity. We demonstrated that enhanced esterase activity was associated with survivability of horn Xies 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 signiWcant eVect on the overall esterase activity in the horn Xy. The observed synergistic eVect of PBO at lower concentrations on diazinon toxicity to horn Xies 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 detoxiWcation of diazinon, and reduced diazinon toxicity at high PBO concentration by inhibiting the bio-activation of diazinon. Published by Elsevier Inc.