Comparison of the first infecting stages of two Metarhizium anisopliae sub-species on ticks [abstract]

Entomopathogenic fungi are promising biocontrol agents against arthropod pests. In recent years it was found that they are also highly efficient in controlling ticks. The present study focused on the pre-penetration events of tick-fungus interaction: attachment, germination and appressorium formation. For this purpose we compared the virulent of two subspecies of the fungus Metarhizium anisopliae: M. anisopliae var. anisopliae (MAan) and M. anisopliae var. acridum (MAac). MAan was highly virulent to eggs, larvae and adults of Boophilus annulatus and to most stages of Rhipicephalus sanguineous and Hyalomma excavatum, while MAac demonstrated low virulence to almost all stages and species tested. We developed a new quantitative method for removing adhered spores from ticks’ cuticle using organic solvents and found that conidia of high and low virulence fungi adhered equally to ticks-cuticle. What’s more, no correlation was observed between the tick susceptibility to fungus and number of adhered spores. Fungi germination and appressorium formation was tested on ticks’ cuticle extracts. The lipids extracts from the most susceptible tick, B. annulatus, stimulated spore germination of high virulent MAan, but not of low virulent MAac. Only extracts from engorged H. excavatum females, the most resistant tick tested, inhibited MAan development. Low virulent fungus MAac did not germinate or formed appressorium on any ticks’ extract. The pathogenicity process of MAan was investigated in details toward B. annulatus eggs. The histopathology investigation of B. annulatus eggs infected with MAan demonstrated all fungi developmental phases. In conclusion, adhesion is a non-specific process while germination and appressorium formation depends on cuticle compounds. Understanding the mechanism of infection on all of its stages is highly important for the use of fungi against ticks and other pests.