Selection of Metarhizium spp. Brazilian isolates to control Rhipicephalus microplus ticks: in vitro virulence tests and conidiogenesis

This study aimed to select Metarhizium spp. isolates to control Rhipicephalus microplus ticks by analyzing their in vitro virulence against R. microplus eggs, larvae, and engorged females as well as their ability to produce aerial conidia on potato dextrose agar (PDA). After the treatment of R. microplus eggs with the highest fungal concentration (108 conidia.ml-1), most of the eleven tested isolates reduced the larval hatching compared to the control group, except M. anisopliae s.l. ARSEF 2211 and ARSEF 3641. M. anisopliae s.l. isolates ARSEF 729, ARSEF 760, ARSEF 929, and ARSEF 3643 exhibited the best results in the egg bioassay. In the bioassay with larvae, the entomopathogenic fungal isolates yielded average larval mortality ranging from 0.1% to 98.9% and from 23.9% to 99.9% five and fifteen days after the treatment, respectively. ARSEF 552, ARSEF 729, ARSEF 929, and ARSEF 3643 yielded the highest larval mortality. Analysis of the bioassay with R. microplus engorged females found that the different isolates negatively impacted the egg mass weight, larval hatching percent, egg production index, and nutritional index. The percent of tick control ranged from 5.32% to 70.83%, and the best tick control rates were caused by M. anisopliae s.l. ARSEF 3643 (70.83%), ARSEF 3641 (62.87%), and ARSEF 729 (64.27%). The highest conidiogenesis on PDA was observed for M. anisopliae s.l. ARSEF 3641 and M. pingshaense ARSEF 552. The isolates ARSEF 729 and ARSEF 3643 are considered promising candidates for field applications against R. microplus ticks.