IN-VITRO AND IN-VIVO EVALUATIONS OF A STRAIN OF BOOPHILUS-MICROPLUS (ACARI, IXODIDAE) SELECTED FOR RESISTANCE TO PERMETHRIN

A strain of Boophilus microplus (Canestrini) was selected for resistan ce to permethrin by pressuring larvae with increasing doses (range, 0. 05-0.35% [AI]) through successive generations (generations F-2-F-7). A t the beginning of the selection process (F-2), the pyrethroid resista nt (PR) strain was 5.4 times more resistant to permethrin than the pyr ethroid susceptible (PS) strain, and the level of resistance increased in each successive generation of the PR strain, reaching a resistance factor (resistance factor) of 20.9 in the F-7 generation. Thus, in on ly 5 generations the level of resistance in the PR strain was increase d by 4-fold. The efficacy of permethrin was evaluated by dipping cattl e infested with the PS strain and cattle infested with the F-7 generat ion of the PR strain in a vat at 0.057% (AI) (Coopers Animal Health, M undelein, IL; recommended concentration). Evaluation of the PS strain showed that untreated cattle produced significantly (P < 0.05) more fe males with a higher index-of reproduction than treated cattle. Additio nally, biological factors (female weight, egg mass weight, and percent egg hatch) associated with the untreated females were all significant ly (P < 0.05) greater than treated females. Conversely, results obtain ed from the PR strain showed that tick numbers, index of reproduction value, and biological parameters of females in the untreated group wer e not significantly different (P > 0.05) from those of the treated gro up. The overall control achieved in the PS strain (99.2%) was signific antly higher (P < 0.05) than that of the PR strain (35.7%). The result s of the study as they relate to the United States Boophilus Eradicati on Program are discussed.