Cypermethrin pour-on synergized with piperonyl butoxide: effects on Haematobia irritans (Diptera : Muscidae) natural populations resistant to cypermethrin

Development of pyrethroid resistance in Haematobia irritans in Santa Fe pro vince, Argentina, resulted in an increased use of pyrethroid insecticides, probably due to lack of suitable alternative treatments. We explored the ef ficacy of mixtures of cypermethrin and piperonyl butoxide (PBO) against pyr ethroid-resistant H. irritans. Groups of 25 Holstein cows each, naturally i nfested with cypermethrin resistant H. irritans were assigned to treated or control groups in April, September, October and December 1997. Cattle in t reated groups were medicated with pour-on oil formulations of 5% cypermethr in (dose = 4 mg per kg of body weight) with 5% or 10% PBO in April, and wit h a mixture containing 5% of both components thereafter. Efficacy was teste d for 21 days after treatment. A treatment of 5% cypermethrin pour-on witho ut PBO was evaluated in October 1997. Samples of horn flies were obtained b efore September, October and December treatments and exposed for 2 h to fil ter papers impregnated with different cypermethrin concentrations to determ ine the 50% lethal concentration (LC50). No difference in efficacy was foun d between cypermethrin pour-on formulations with 5% or 10% of PBO (more tha n 94% efficacy on day 21 after treatment). Efficacy of 5% cypermethrin-5% P BO mixture decreased rapidly in the successive treatments (less than 40% ef ficacy 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 treat ment carried out in April, 10 days in September; 7 days for the treatment p erformed in October and 4 days for the December treatment. The LC50 of cype rmethrin was 36.6 mu g per cm(2) in September and increased to 116.6 and 22 6.1 mu g per cm(2) in October and December, respectively. It is concluded t hat the addition of PBO to cypermethrin did not provide a treatment that wo uld give a long term control of pyrethroid resistant-horn flies. (C) 1999 E lsevier Science B.V. All rights reserved.