Background: Intensive application of chemical insecticides is required for aphid pest control . Among the biorational alternatives, entomopathogenic fungi (EPF) are the most sustainable biocontrol agents; those of the order Hypocreales attack and cause fungal disease in arthropod hosts, with variations in host susceptibility attributed to both fungal and host characteristics. We evaluated inter- and intra-species variations in Metarhizium spp. virulence and differences in fungal disease progression on adult and nymph stages of the green peach aphid, Myzus persicae (Sulzer), a parthenogenetically reproducing insect species.

Results: Minor interspecies diversity was detected between the generalist Metarhizium species examined. Interestingly, significant intraspecies diversity was observed between Metarhizium brunneum isolates Mb7 and MbK. Infected adult aphids demonstrated similar disease progression for both isolates, mortality rates of more than 80% and 5-fold reduction in fecundity. However, nymph mortality was detected only following MbK infection, with 50% mortality and significant reduction in molting rates. Confocal laser-scanning microscopy demonstrated the variation in the disease stages of conidial adhesion and hemocoel colonization on each examined day post-inoculation for each isolate. Significantly faster disease progression was observed in MbK-infected vs. Mb7-infected nymphs, the latter demonstrating a higher percentage of uninfected nymphs accompanied with aphids' molting.

Conclusions: The observed intraspecies variation suggests that altered conidial adhesion to the nymph cuticle is a major factor affecting virulence. We prove the role of nymph ecdysis as a defense mechanism disrupting fungal infection. As significant differences were observed between closely related isolates, this study emphasizes the importance of appropriate isolate selection for biological control.