Comparison of In Vitro and In Vivo Ectoparasiticide Activity of Experimental Benzimidazole-Carbamate with Permethrin and Amitraz

A series of in vitro and in vivo bioassays were conducted to assess the ectoparasiticide activity of isopropyl-4-nitro-2,6-bis(trifluoromethyl)-1-benzimidazole-carbamate, an experimental benzimidazole-carbamate class compound. This compound was less potent than permethrin against ectoparasiticide-susceptible larvae of the lone star tick, Amblyomma americanum (L.) (Acari: Ixodidae); larvae of the southern cattle tick, Boophilus microplus (Canestrini); and adult stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae) in vitro, but it was significantly more potent than permethrin against the Santa Luiza strain of B. microplus known to possess high-level resistance to amitraz and pyrethroids. In contrast, the benzimidazole-carbamate was substantially more efficacious than permethrin when applied topically onto rats that were infested with A. americanum nymphs. These results suggest that this experimental compound may be a viable candidate ectoparasiticide that retains significant activity against resistant B. microplus and also suggests that the benzimidazole-carbamate chemistry may be useful for addressing the growing problem of resistance in ectoparasites. Keywords: benzimidazole-carbamate, Boophilus , larval immersion microassay, resistance, acaricide