Abstract
Horn fly susceptibility to insecticides was evaluated in the state of Mato Grosso do Sul, Brazil, from October 2000 to September 2002. Insecticide bioassays (n=57) were conducted in 38 ranches from 14 municipalities throughout the state. Horn flies from wild populations were collected on cattle and exposed to filter papers impregnated with cypermethrin, permethrin, or diazinon and mortality was assessed after two hours. Resistance to cypermethrin was detected in all populations, with resistance ratios (RR) ranging from 27.6 to 91.3-fold. Permethrin bioassays provided apparently low levels of resistance (RR<5), however, resistant flies were found in 96.9% of the populations based on diagnostic concentrations. From both pyrethroid bioassays, resistance was detected in 97.4% of the populations. On the other hand, a high susceptibility to diazinon (RR< or =1.1) was detected in all populations. Pyrethroid products, most cypermethrin (92.3%) and deltamethrin (66.7%), were used in all ranches controlling horn flies (97.5%). Insecticide treatments, usually incorrectly applied, were routinely delivered by manual backpack sprayers in most ranches (84.5%). This profile of insecticide use helps to explain the widespread resistance of horn flies to pyrethroids in the state as well as their high susceptibility to the organophosphate. Inadequate control practices contribute to aggravate the resistance problem and its consequences.
Highlights
The horn fly, Haematobia irritans irritans, is considered a major livestock pest in Brazil
Horn flies from wild populations were collected on cattle and exposed to filter papers impregnated with cypermethrin, permethrin, or diazinon and mortality was assessed after two hours
As part of a comprehensive survey on the status of horn fly susceptibility to insecticides carried out in several regions of the country, the present study focused on the particular status of resistance in the state of Mato Grosso do Sul, Brazil
Summary
The horn fly, Haematobia irritans irritans, is considered a major livestock pest in Brazil. After its entry in the north of the country, horn fly dispersion was facilitated by cattle transporting and eventually reached the state of Mato Grosso do Sul in 1990. Since resistance to distinct insecticide classes may develop in just two to four years in a horn fly population (QUISENBERRY et al, 1984; BARROS et al, 2001), it was not a surprise that complaints regarding reduced product efficacy and control failure had arisen all over the country following several years of chemical control of this livestock pest. As part of a comprehensive survey on the status of horn fly susceptibility to insecticides carried out in several regions of the country, the present study focused on the particular status of resistance in the state of Mato Grosso do Sul, Brazil
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