Integrated control of Rhipicephalus microplus: Assessment of the use of chemical acaricides and Metarhizium anisopliae in cattle from a tropical climate region

This study evaluated the use of the entomopathogenic fungus Metarhizium anisopliae as part of an integrated strategic control program for Rhipicephalus microplus in dairy cattle raised under tropical conditions. All experiments were conducted using an oil dispersion (OD) formulation containing aerial conidia of M. anisopliae (ESALQ E9). In the first experiment (laboratory assay), engorged females of two strain were immersed in different concentrations (1 × 10⁷, 5 × 10⁷, and 1 × 10⁸ conidia/mL). Subsequently, the reproductive biology of the ticks was evaluated, and the percent control was calculated. The second experiment assessed the efficacy applied using two spraying systems: a backpack sprayer and a power sprayer. For animal treatment in both experiments in field trials, 5 L of the fungal suspension at a concentration of 1 × 10⁸ conidia mL⁻¹ was applied to each animal. Thirty naturally infested Holstein × Gyr crossbred cows were allocated into three groups: backpack sprayer, power sprayer, and untreated control. Tick counts were performed through 28 days post-treatment to calculate efficacy. The viability of the conidia was also evaluated after application using the different sprayers. In the third experiment evaluated a strategic control protocol combining a synthetic chemical acaricide with M. anisopliae. Cows with different Holstein × Gyr genetic compositions (5/8 Holstein × 3/8 Gyr and 3/4 Holstein × 1/4 Gyr) were monitored for 126 days under field conditions. The study compared groups receiving alternating chemical and biological treatments with those receiving only chemical treatments or serving as controls after day 63. In the first experiment (laboratory conditions), fungal formulation reduced (p ≤ 0.05) both the quantity and viability of eggs produced by engorged females, resulting in efficacy exceeding 90% at the highest concentration tested, in both strains. In the second experiment, both spraying systems proved suitable for applying the fungal formulation, with no significant differences between application methods. Efficacy against R. microplus reached approximately 60% between 7 and 14 days post-treatment, regardless of the equipment used. Conidial viability was not affected (p > 0.05) by any of the spraying systems. In the strategic control study, alternating four applications of a synthetic acaricide with two applications of M. anisopliae resulted in a 33.3% reduction in chemical acaricide use. This integrated approach effectively maintained low tick burdens in 5/8 Holstein × 3/8 Gyr cattle, whereas control was less consistent in animals with a higher proportion of Holstein genetics (3/4 Holstein × 1/4 Gyr). In conclusion, integrating M. anisopliae into strategic tick control programs is feasible under tropical field conditions and enables a meaningful reduction in chemical acaricide use. This approach represents a promising tool for sustainable tick management and may contribute to delaying the selection of acaricide-resistant R. microplus populations.