The control of the southern cattle tick, Boophilus microplus (Canestrini), in Mexico and many other countries relies on chemical
acaricides. Boophilus microplus has developed resistance to all major classes of
acaricides in recent years. To gain a better understanding of the resistance and to develop resistance management strategies that benefit both Mexican ranchers and USDA's cattle
fever tick eradication program (CFTEP), the authors used larval bioassay techniques to determine levels of resistance to
permethrin and
amitraz and then evaluated synergism between these two
acaricides in one susceptible laboratory tick strain and four resistant strains originating from Mexico and Brazil. To examine mechanisms of resistance to
permethrin in these strains, the frequency of a mutated
sodium channel gene was determined using a PCR assay. The tick strains from Mexico and Brazil demonstrated 49.4- to over 672.2-fold resistance to
permethrin, and up to 94.5-fold resistance to
amitraz. While the San Roman strain from Mexico was the most
permethrin-resistant strain, the Santa Luiza strain from Brazil was the most
amitraz-resistant strain. A significant correlation was found between the
permethrin resistance ratio and the allelic frequency of the
sodium channel mutation. Significant synergism between
permethrin and
amitraz was found when one
acaricide was tested in the presence of another. Synergism ratios ranged from 1.5 to 54.9 when
amitraz was tested as a synergist for
permethrin. Similar synergism ratios were obtained when
permethrin was tested as a synergist for
amitraz.
Permethrin caused virtually no mortality in the San Roman strain, even at the highest concentration (3294 microg cm(-2)). Adding
amitraz (11.0 microg cm(-2)) to
permethrin led to a dramatic increase in larval mortality, even at very low concentrations of
permethrin.