Laboratory evaluation of verbutin as a synergist of acaricides against larvae of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae).

Synergistic effects of verbutin, a member of aryl alkynyl derivatives, to three commonly used acaricides were evaluated with the modified Food and Agricultural Organization Larval Packet Test (FAO-LPT) against both susceptible and resistant strains of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae). These tick strains demonstrated various levels of resistance to coumaphos (2.5-8.2x), permethrin (57.9-711.7x), and amitraz (3.5-177.5x). Verbutin alone was more toxic to tick larvae than piperonyl butoxide (PBO), a conventional pesticide synergist. With synergism ratios (SRs) ranging from 1.5 to 6.0, verbutin was more potent than PBO (SR = 0.9-1.6) in synergizing coumaphos toxicity to resistant tick larvae. Similarly, verbutin (SR = 1.8-15) was also found to be more potent than PBO (SR = 0.9-2.5) in synergizing amitraz in resistant tick strains. Both verbutin and PBO significantly increased permethrin toxicity to larvae of all tick strains tested, and there was no significant difference between the two synergists. SRs ranged from 2.1 to 4.4 and from 2.1 to 3.6 for PBO and verbutin, respectively.