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 fro resistance to permethrin by pressuring larvae with increasing doses (range, 0.05-0.35% [AI]) through successive generations (generations F2-F7). At the beginning of the selection process (F2), the pyrethroid resistant (PR) strain was 5.4 times more resistant to permethrin than the pyrethroid 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 F7 generation. Thus, in only 5 generations the level of resistance in the PR strain was increased by 4-fold. The efficacy of permethrin was evaluated by dipping cattle infested with the PS strain and cattle infested with the F7 generation of the PR strain in a vat at 0.057% (AI) (Coopers Animal Health, Mundelein, IL; recommended concentration). Evaluation of the PS strain showed that untreated cattle produced significantly (P < 0.05) more females with a higher index of reproduction than treated cattle. Additionally, biological factors (female weight, egg mass weight, and percent egg hatch) associated with the untreated females were all significantly (P < 0.05) greater than treated females. Conversely, results obtained from the PR strain showed that tick numbers, index of reproduction value, and biological parameters of females in the untreated group were not significantly different (P > 0.05) from those of the treated group. The overall control achieved in the PS strain (99.2%) was significantly higher (P < 0.05) than that of the PR strain (35.7%). The results of the study as they relate to the United States Boophilus Eradication Program are discussed.