Infection dynamics of Metarhizium anisopliae in Rhipicephalus microplus and the role of tick traits in susceptibility

Tick-borne diseases (TBDs) pose a major threat to global livestock production, particularly in tropical and subtropical regions, and their burden is projected to increase worldwide in the coming decades as a result of climate change. In Brazil, tick infestations are responsible for economic losses estimated at about USD 3.24 billion annually. Growing resistance to chemical acaricides necessitates alternative control strategies such as biological agents. This study assessed the pathogenicity of a local strain of Metarhizium anisopliae (PR1) against adult Rhipicephalus microplus ticks using the Adult Immersion Test at conidial concentrations of 10⁷, 10⁸, and 10⁹ conidia/mL, each tested in triplicate alongside positive and negative controls. All concentrations caused complete tick mortality within 14 days, with higher concentrations accelerating tick death and completely inhibiting oviposition and larval hatchability. The estimated LT₅₀ and LT₉₀ values for the 10⁹ conidia/mL treatment were 2.46 [2.09-2.75] days and 3.63 [3.25-4.29] days, respectively. In contrast, untreated ticks exhibited only 6.7 % mortality and laid an average of 1567 ± 314 eggs, of which 84 % hatched. A significant positive correlation was observed between tick weight and the time to death at the highest concentration, suggesting heavier ticks resisted infection longer, potentially due to higher energy reserves or thicker cuticular. These results underline the potent in vitro pathogenic and reproductive-suppressing effects of M. anisopliae strain PR1 under laboratory conditions, while emphasizing the need for further field validation before considering its application in broader vector management programs.