Submitted to: Revista de Medicina Veterinaria
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/3/1997
Publication Date: N/A
Interpretive Summary: The horn fly, Haematobia irritans, is a recently introduced pest of Argentina. Cattle producers are concerned about the rate of resistance increasing with continued use of pyrethroid insecticides. In Santa Fe Province, Argentina, the efficacy of a 5% pour-on of cypermethrin in 7 successive treatments between March 1995-May 1997 was determined. The results showed a decrease in susceptibility and by October 1996 the treatment was not protecting cattle 7 days after treatment. Exposing these flies to specific tests showed resistance ranging from 16.8 to 36.5 fold, definitely indicating expected product failure.
Technical Abstract: Field and laboratory work were performed to evaluate the presence of Haematobia irritans populations resistant to cypermethrin in the province of Santa Fe. To this aim the efficacy of a 5% pour-on cypermethrin was assessed in 7 successive treatments (March 1995-May 1997) in a farm where all cattle were treated when the mean number of H. irritans was close or higher than 200 flies per cattle. The evolution of this horn fly population was compared to a population of control cattle never treated with ectoparasiticides by counting the flies once a week in both cattle group. H. irritans of 4 different sites were exposed to filter papers treated with different concentrations of cypermethrin and the mortality was compared with the corresponding mortality of a susceptible strain to obtain the resistance ratio (RR). The results showed a progressive decrease of the cypermethrin efficacy. By October 1996 the pour-on treatment was unable to protect the majority of cattle 7 days after treatment. In the 1995-96 horn fly season the treated cattle had a mean of 45 H. irritans vs. 123 for control cattle; however, the means were of 129 and 158, respectively, in the fly season 1996-97. The exposure of flies to filter papers treated with cypermethrin showed that the 4 populations were resistant to this pyrethroid, with RR ranging from 16.8 to 36.5, including H. irritans from a farm where cattle were never treated with pyrethroids.