Susceptibility to two pyrethroids in Boophilus microplus (Acari : Ixodidae) populations of northwest Argentina - Preliminary results

Cattle are treated 6-12 times yearly to control Boophilus microplus ticks i n the east zone of the Argentinean infested region, while 1-4 treatments ar e applied for tick control in the west zone. In the 1970s resistance to org ano-phosphate acaricides was found in the east zone, but not in the west zo ne. However, a shift to synthetic pyrethroids (SP) was made through all inf ested regions. Currently, indications of resistance to SP in the east zone, but not in the west zone, are provoking to a switch to formamidine acarici des, During 1998 a total of 147 B. microplus engorged females were collecte d from 20 beef cattle ranches from the west zone of the Argentinean infeste d region. Individual progenies of these ticks were tested ('larval packet t est') with cypermethrin and deltamethrin, and their LC 50 and LC 90 were co mpared to those estimated for the Milagro susceptible strain. No evidence o f resistance to these SP was found. Due to sampling restraints the results are presented as preliminary. Nevertheless, a shift away from use of SP for control of B, microplus in the west zone appears to be unjustified and sho uld be independent of the resistance circumstances observed in the east zon e of the Argentinean tick infested region. (C) 2000 Elsevier Science B.V. A ll rights reserved.