Toxicity of cypermethrin and diazinon to Haematobia irritans (Diptera : Muscidae) in its American southern range

Synthetic pyrethroids are considered currently less effective than organoph osphate insecticides for controlling Haematobia irritans on cattle in its A merican southern range. To test the validity of this hypothesis, horn fly p opulations from 95 cattle herds in central-northern Argentina and southern Brazil were evaluated to determine 50% lethal concentrations (LC50) to cype rmethrin and diazinon, respectively. Horn flies were exposed to filter pape rs and glass vials treated with cypermethrin and diazinon, respectively, an d LC50 values were determined using probit analysis. Two and 78 fly populat ions showed LC50 values indicating susceptibility and resistance to cyperme thrin, respectively, in comparison to historical LC50 values obtained when the horn fly populations were susceptible to cypermethrin. Five fly populat ions had an undetermined resistance status to cypermethrin. The diazinon te sts showed no obvious indications of H. irritans resistance. Thirty nine pe rcent of farmers still rely on pyrethroids to control horn fly populations on cattle which represents a decreasing proportion from a few years ago whe n horn fly control was carried out using pyrethroids almost exclusively. Or ganophosphates could provide control for horn flies resistant to pyrethroid s; however, the reliance on chemicals alone to control H. irritans is not c onsidered a proper approach for the future. (C) 2001 Elsevier Science B.V. All rights reserved.