Pathogenicity of Metarhizium and Cordyceps isolates against larvae of different Agriotes species and populations in correlation with conidial size and germination

Entomopathogenic fungi offers a promising approach to control Agriotes larvae, although virulence varies for different Agriotes species. Therefore, the first step was to look for a highly virulent fungal strain for different Agriotes species. To achieve this, six Metarhizium and two Cordyceps strains were studied. Several Metarhizium strains were highly efficient and achieved over 90% mortality over four months by dip treatment. The most promising strain to control A. lineatus and A. obscurus was the M. brunneum strain JKI-BI-1450, while M. robertsii strain JKI-BI-1442 was most effective against A. sputator. The Cordyceps strains had no pathogenic effect. In the second step, we investigated whether fungal strain-specific characteristics such as conidial size and germination could be related to the effect of the strains against Agriotes larvae. A correlation could not be confirmed for A. lineatus and A. obscurus. In contrast, against larvae of A. sputator shorter and wider conidia as well as those that germinate later and show a lower germination rate after 96 h were more effective. In the third step, we investigated whether populations of the same Agriotes species, differ in their susceptibility to entomopathogenic fungi. Different populations of A. obscurus larvae showed variable susceptibility. Significant differences in the Restricted Mean Time Lost (RMTL), but not in the final mortality, were determined for different populations of A. sputator larvae. In contrast, the efficacy on A. lineatus was similar among the various populations tested. Our study indicates highly specific and complex interactions between the Agriotes species and the Metarhizium strains.