Abstract
To investigate the kdr (knockdown resistance) resistance-associated gene mutation and determine its frequency in pyrethroid-resistant horn fy (Haematobia irritans) populations, a total of 1,804 horn flies of 37 different populations from all Brazilian regions (North, Northeast, Central-West, Southeast, and South) were molecular screened through polymerase chain reaction (PCR). The kdr gene was not detected in 87.08% of the flies. However, the gene was amplified in 12.92% of the flies, of which 11.70% were resistant heterozygous and 1.22% were resistant homozygous. Deviation from Hardy-Weinberg equilibrium (HWE) was found only in 1 ranch with an excess of heterozygous. When populations were grouped by region, three metapopulations showed significant deviations of HWE (Central-West population, South population and Southeast population). Tis indicates that populations are isolated one from another and kdr occurrence seems to be an independent effect probably reflecting the insecticide strategy used by each ranch. Although resistance to pyrethroids is disseminated throughout Brazil, only 48% of resistant populations had kdr flies, and the frequency of kdr individuals in each of these resistant populations was quite low. But this study shows that, with the apparent exception of the Northeast region, the kdr mechanism associated with pyrethroid resistance occurs all over Brazil.
Highlights
The horn fly, Haematobia irritans irritans, is considered one of the most economically important cattle ectoparasites due to its widespread distribution and effect on cattle
According to SINDAN (2005) there are 91 products for horn fly control registered for cattle use in Brazil
Inadequate and excessive pyrethroid product applications have lead to the current resistance problem in Brazil (BARROS, 2004)
Summary
The horn fly, Haematobia irritans irritans, is considered one of the most economically important cattle ectoparasites due to its widespread distribution and effect on cattle. To minimize economic losses to cattle producers, insecticides are used for fly control This practice leads to the development of insecticide resistance, which has been detected to active ingredients of all chemical classes commercially available up to the 1970’s, except insect growth regulators. Guerrero et al (1997) developed a PCR assay to test individual horn flies for the presence of the kdr pyrethroid resistance-associated sodium channel mutation. Untreated cattle herds were brought to corral or cattle chutes depending on their infestation levels as well as proximity to the collection sites. No records from these animals were available.
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