Conidia and blastospores of Metarhizium spp. and Beauveria bassiana s.l.: Their development during the infection process and virulence against the tick Rhipicephalus microplus

The current study compared the virulence of conidia and blastospores of Metarhizium robertsii (IP 146), M. anisopliae sensu lato (s.l.) (IP 363) and Beauveria bassiana s.l. (IP 361 and CG 307) against unfed larvae and engorged females of Rhipicephalus microplus (Acari: Ixodidae). In addition, the development of fungal propagules on tick cuticle was investigated. Tick larvae were treated with fungal suspensions at 106, 107 or 108 propagules mL−1, and percent mortality was assessed every two days. Engorged females were immersed in fungal suspensions (1.0 × 107 propagules mL−1) for 1 min, and their biological parameters monitored daily. The virulence of conidia and blastospores against larvae varied considerably among the isolates tested. Only females treated with blastospores of IP 146 or IP 361 presented lower nutrient and egg production indices than the control group; the higher percent control was reached when females were treated with blastospores of IP 146 (98%), IP 363 (79%), or IP 361 (93%) in comparison to the groups treated with conidia, 71%, 59%, or 63%, respectively. Engorged females treated with conidia or blastospores of IP 146 or IP 361 were also examined by scanning electron microscopy (SEM). Germination of blastospores of IP 146 and IP 361 was observed on tick cuticle after 4 h incubation at 27 ± 1 °C and RH > 90%, whereas germ tubes from conidia of both isolates were observed at 48 h. Appressoria in developing blastospores of B. bassiana IP 361 were observed after 4 h incubation, whereas no appressoria were seen in developing blastospores of M. robertsii IP 146. Blastospore penetrations by both fungal isolates through natural openings was also evidenced by SEM; fine sections of R. microplus engorged females treated with blastospores of IP 146 or IP 361 confirmed that these isolates penetrated through their cuticle and natural openings. Blastospores might be promising for use in biocontrol of ticks, since they are virulent against R. microplus and present rapid development on their cuticle.