The effect of selected bacteria on the virulence of Metarhizium novozealandicum C14 to Costelytra giveni larvae (Scarabaeidae)

The ability of disease-causing bacteria to synergise infection by the entomopathogen Metarhizium of larvae of Costelytra giveni, a New Zealand native scarab, has been previously demonstrated, but the role of soil bacteria in infections is unknown. Bacteria isolated from both dead C. giveni larvae and soil were assessed for their effect on the virulence of Metarhizium novozealandicum C14 towards C. giveni larvae. Seven bacterial isolates showed synergism with M. novozealandicum C14 against second instar larvae, but no synergism was found against third instar larvae. No apparent disease was noted with C. giveni larvae independently challenged with the bacteria isolates. An isolate of Yersinia enterocolitica (isolate 6-1) in combination with M. novozealandicum C14 caused higher larval mortality than the other bacterial isolate-entomopathogen combinations against second instar larvae. Using plate assays, Y. enterocolitica 6-1 was found to produce more chitinase, but not protease, compared with the other bacterial strains suggesting a role of chitinase in synergism. This study suggests that the combination of Y. enterocolitica (isolate 6-1) and M. novozealandicum C14 could provide effective biocontrol of second instar grass grub larvae.