Pathogenicity of Metarhizium flavoviride Ma130821 Against the Larvae of Metabolus flavescens Brenske Feeding on Different Host Plants

Various strains of entomopathogenic fungus Metarhizium flavoviride are potential insect control agents. However, there are few systematic studies on their pathogenicity against larvae of pests in the family Scarabaeidae. This study evaluates the pathogenicity of M. flavoviride Ma130821 against larvae of Metabolus flavescens Brenske feeding on maize and tobacco, respectively, and examines the infection process and aspects of the host immunological response. The present results showed that M. flavoviride Ma130821 caused 76.67%–100.00% mortality of M. flavescens larvae within 18 days. When hyphal bodies infected the hemocoel of a host, this strain inhibited the normal protein metabolism through the consumption of protein nutrients and then activated the immunity by increasing the amounts of hemocytes. The infection also initiated the humoral immunity response by increasing the content and enzymatic activity of phenoloxidase (PO). After inoculation, the content and enzymatic activity of PO presented a significant increase and reached the peaks at 72 or 96 h after inoculation (HAI), indicating that immune responses of hemolymph in infected larvae were more intensive at middle/late stage of infection. However, the growth of hyphal bodies was not recognized by the host's immune system as invaders when they reached a high density. The amount of total hemocyte and the content and enzymatic activity of PO all decreased significantly. M. flavoviride Ma130821 appears to affect the larvae's immune system and results in decreased immunity. Our results demonstrate that M. flavoviride Ma130821, with high virulence to larval M. flavescens by immune responses of hemolymph, could be provided efficient entomopathogenic fungi for pest control.