Does the use of engorged adult ticks of Rhipicephalus microplus as substrate modifies the acaricidal behavior of Metarhizium anisopliae?

Entomopathogenic fungi (EPF) is one of the most promising alternatives to regulate tick populations. However, these fungi may lose acaricidal effectiveness over time, due to the storage period and/or successive cultivation on artificial media. It is known that using arthropod pests as a substrate for EPF could potentially alter their acaricidal behavior over time, however, studies using ticks for this purpose are scarce. Hence, the aims of this research were: (1) to isolate strains of Metarhizium anisopliae from paddocks of cattle farms, (2) to evaluate the effect of engorged adult ticks of Rhipicephalus microplus as a substrate on the tickicide behavior of Metarhizium anisopliae strains, and (3) to determine the lethal time of each M. anisopliae strain to kill 50% (LT50) and 99% (LT99) of engorged ticks. First, the natural acaricidal effect of 10 strains of M. anisopliae sensu lato isolated from soils of cattle farms on R. microplus was evaluated. Second, the influence of substrate (R. microplus, Galleria mellonella and Sabouraud dextrose agar [SDA]) on the acaricidal activity and virulence index (lethal time) of each strain during four generations was evaluated. Strains MaV69, MaV60 and MaV67 showed more than 90% mortality at day 20 post-treatment. The use of engorged adult ticks as substrate increased the virulence of five M. anisopliae s.l. strains. Larvae of G. mellonella and SDA as substrate did not modify the acaricidal behavior of the 10 strains evaluated. Seven of ten EPF decreased their LT50 and three decreased their LT99 as an index of their virulence enhancement.