Thermal limitations of Metarhizium anisopliae efficacy: selection for application on warm-blooded vertebrates

Temperature is one of the main obstacles for on-host applications of entomopathogenic fungi for ectoparasite control. The effects of temperatures typical of the body surfaces of warm-blooded animals on the germination, growth and virulence of four strains of Metarhizium anisopliae toward engorged Rhipicephalus (Boophilus) annulatus females were evaluated. The M. anisopliae strains studied can be divided according to their thermal characteristics: (1) strains which germinate (90-100%), grow and infect ticks similarly at 25, 30 and 35°C; and (2) strains which recover their ability to germinate relatively quickly following a thermal shock (37 or 40°C for 6-48 h) before incubation at a favorable temperature. These latter strains could recover their infectivity after a short thermal shock (6 h at 37-40°C), but not after more prolonged exposure to these temperatures (48-72 h). These two thermal characteristics do not interact, but reflect the efficacy of strains used to control ectoparasites on warm-blooded vertebrates.