Efficacy of Metarhizium anisopliae conidia in oil-in-water emulsion against the tick Rhipicephalus microplus under heat and dry conditions

Abstract

Environmental stress conditions compromise the effectiveness of microorganisms for use in biological control. We investigated the thermotolerance of seven Metarhizium isolates from Central Brazil to 32 °C, and the thermotolerance of conidia of Metarhizium anisopliae s.str. IP 119 (Metchnikoff) (Hypocreales: Clavicipitaceae) and M. robertsii formulated in oil-in-water emulsion to 45 °C. We also evaluated the efficacy of oil-formulated conidia of M. anisopliaes.str. IP 119 against the tick Rhipicephalus microplus (Canestrini) (Acari: Ixodidae) under stress conditions (32 °C; 75% RH). The conidial germination on tick’s cuticle was assessed by Scanning Electron Microscopy. Conidia of M. anisopliaes.str. IP 119 formulated in oil-in-water emulsion caused significant control (59.4%) of R. microplus, even when treated ticks were incubated under stress (32 °C; 75% RH). Metarhizium spp. incubated at 32 °C had both decreased radial growth and conidial production. Conidia of IP 119 formulated in oil-in-water emulsion were more tolerant to 45 °C than conidia suspended in water. An oil-in-water emulsion protected M. anisopliaes.str. conidia against heat and dry conditions and may enhance its efficacy to control ticks by improving the tolerance of fungi against adverse environmental factors.