Horn fly (Diptera : Muscidae) resistance to organophosphate insecticides

Insecticidal ear tags impregnated with organophosphate (OP) insecticides we re used each year from 1989 to 1998 at Rosepine, LA. Weekly fly counts were conducted to evaluate control efficacy of the treatments, and bioassays we re conducted at least twice per year to measure fly susceptibility to OP an d pyrethroid insecticides, Between 1989 and 1992, the efficacy of 20% diazi non-impregnated ear Lags was reduced from > 20 to just 1 week of control. A high risk; of control failure was observed when a resistance frequency of approximately 5% was measured in pre-season bioassays. Resistance to diazin on, fenthion, ethion, pirimiphos-methyl, and tetrachlorvinphos was observed . Esterase activity toward cr-naphthyl acetate was significantly higher in flies collected at Rosepine in 1997 than in flies from a laboratory colony and from a susceptible field population. (C) 2001 Elsevier Science B.V. All rights reserved.