A.C.L. Churchill, A. Rot

The fungus Metarhizium anisopliae (Hypocreales: Clavicipitaceae) is an efficient biological control agent of various arthropods, actively penetrating host cuticles via penetration pegs, colonizing the host body, and causing host death. Laboratory assays against various hosts (ticks and insect) have revealed that while some hosts are highly susceptible to the fungus (LT50’s of 3– 7 days), other hosts are resistant to infection. Results of previous studies of the factors involved in host susceptibility have suggested that cuticular compounds from susceptible and resistant hosts may differentially influence fungus development. This work describes the microscopic development of a green fluorescent protein (GFP)-expressing M. anisopliae strain on the susceptible hosts Rhipicephalus annulatus (tick, Acari: Ixodidae), and Galleria mellonella (greater wax moth, Lepidoptera: Pyralidae), and the resistant host Hyalomma excavatum (tick, Acari: Ixodidae). Conidia were observed germinating on all hosts examined. However, the fungus was observed penetrating host cuticles only on susceptible hosts and never on resistant hosts. Moreover, growth of germinating conidia and the hyphae that developed from them was inhibited on resistant hosts and the conidia died within a few days. These observations and previous studies strengthen our hypothesis that differences in cuticle composition are a major factor in determining host susceptibility to fungal infection. Understanding the differential induction of virulence factors of the fungus M. anisopliae by distinct host cuticular extracts will enable better utilization of the fungus in pest control. [L]