RESUMEN
Development of pyrethroid resistance in Haematobia irritans in Santa Fe province, Argentina, resulted in an increased use of pyrethroid insecticides, probably due to lack of suitable alternative treatments. We explored the efficacy of mixtures of cypermethrin and piperonyl butoxide (PBO) against pyrethroid-resistant H. irritans. Groups of 25 Holstein cows each, naturally infested with cypermethrin resistant H. irritans were assigned to treated or control groups in April, September, October and December 1997. Cattle in treated groups were medicated with pour-on oil formulations of 5% cypermethrin (dose = 4 mg per kg of body weight) with 5% or 10% PBO in April, and with a mixture containing 5% of both components thereafter. Efficacy was tested for 21 days after treatment. A treatment of 5% cypermethrin pour-on without PBO was evaluated in October 1997. Samples of horn flies were obtained before September, October and December treatments and exposed for 2 h to filter papers impregnated with different cypermethrin concentrations to determine the 50% lethal concentration (LC50). No difference in efficacy was found between cypermethrin pour-on formulations with 5% or 10% of PBO (more than 94% efficacy on day 21 after treatment). Efficacy of 5 % cypermethrin-5% PBO mixture decreased rapidly in the successive treatments (less than 40% efficacy was observed on day 21 after the December treatment), and the period after treatment with an efficacy higher than 95% was 14 days for the treatment carried out in April, 10 days in September; 7 days for the treatment performed in October and 4 days for the December treatment. The LC50 of cypermethrin was 36.6 microg per cm2 in September and increased to 116.6 and 226.1 microg per cm2 in October and December, respectively. It is concluded that the addition of PBO to cypermethrin did not provide a treatment that would give a long term control of pyrethroid resistant-horn flies.
