Veterinary Parasitology 94 (2000) 117–125 In vitro assessment of Metarhizium anisopliae isolates to control the cattle tick Boophilus microplus Ana Paula Guedes Frazzon a , Itabajara da Silva Vaz Junior a,b , Aoi Masuda a,c , Augusto Schrank a,c , Marilene Henning Vainstein a,d,∗ a Programa de Pós-graduação em Biologia Celular e Molecular (PPGBCM/CBiot), Universidade Federal do Rio Grande do Sul, Campus do Vale, P.O. Box 15005, 91501-970 Porto Alegre, Brazil b Departamento de Patologia Cl´ ınica Veterinária, Universidade Federal do Rio Grande do Sul, Campus do Vale, P.O. Box 15005, 91501-970 Porto Alegre, Brazil c Departamento de Biologia Molecular e Biotecnologia, Universidade Federal do Rio Grande do Sul, Campus doVale, P.O. Box 15005, 91501-970 Porto Alegre, Brazil d Departamento de Microbiologia, Universidade Federal do Rio Grande do Sul, Campus do Vale, P.O. Box 15005, 91501-970 Porto Alegre, Brazil Received 2 May 2000; accepted 26 July 2000 Abstract Metarhizium anisopliae is a filamentous fungus used for tick control. The in vitro effects of 12 M. anisopliae isolates on engorged Boophilus microplus females were analysed. The most pathogenic isolate (E6S1) caused a 100% death rate when 10 7 spores/ml were used to infect ticks. Isolates of M. anisopliae taken from experimentally infected ticks proved to be more pathogenic than fungus maintained on culture media. A comparison between dsRNA mycovirus-free and infected M. anisopliae isolates suggested that, in general, virus free isolates were more infective. The results showed that the biological control of B. microplus by M. anisopliae infection might constitute an additional method to integrated tick control management. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Boophilus microplus; Ticks; Metarhizium anisopliae; Entomopathogenic fungi; Biological control; dsRNA mycoviruses ∗ Corresponding author. Present Address: Centro de Biotecnologia, Universidade Federal do Rio Grande do Sul, Campus do Vale, P.O. Box 15005, 91501-970 Porto Alegre, Brazil. Tel.: +55-51-316-6060; fax: +55-51-319-1079. E-mail address: mhv@dna.cbiot.ufrgs.br (M.H. Vainstein). 0304-4017/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0304-4017(00)00368-X