Abstract: Metarhizium spp. is an important biocontrol resource for the management of underground pests. While it is known that the control efficiency as entomopathogenic fungi is often affected by other microorganisms, the underlying mechanism is unclear. In this study, we examined the inhibitory effect of Acinetobacter gyllenbergii, a secondary endosymbiont isolated from Melanotus cribricollis larva, on the activity of Metarhizium pingshaense that effectively infects wireworm. The results showed that both the fermentation broth and volatiles of A. gyllenbergii showed significant inhibitory effects on spore germination and production of M. pingshaense.The fermentation broth of A. gyllenbergii significantly inhibited mycelium growth of M. pingshaense in seven days, while the volatiles of A. gyllenbergii did not affect mycelium growth of M. pingshaense. Nine effective volatile compounds of A. gyllenbergii were identified by GC-MS analysis. Seven compounds, inclduing α-pinene, aniline, nonanal, 2,4-di-tert-butylphenol, limonene, 1-dodecene and dimethyl trisulfide, had significant inhibitory effects on the activity ofM. pingshaense. Therefore, the endosymbiotic bacteria ofM. cribricollis larva showed a strong inhibitory effect on the growth of M. pingshaense.

Key words: Melanotus cribricollis, secondary endosymbiont, volatile, antimicrobial activity, biological control.