The effectiveness of two new Metarhizium brunneum formulations against wireworms in potato cultivation

In the absence of officially approved agrochemicals for the use against wireworms (Agriotes spp.) in German potato cultivation, the entomopathogenic fungus genus Metarhizium has been regarded as promising biocontrol agent. However, previous mycoinsecticide formulations of M. brunneum could not achieve consistent effectiveness in field use. Within the scope of the project AgriMet, the effectiveness of two new formulations of M. brunneum was evaluated over three years in the field, greenhouse and laboratory. The application of a soil granule and/or a wettable powder (dry product) of M. brunneum during potato planting aimed to reduce potato tuber damage by controlling wireworms. During field trials in the Lower Saxony (Germany), the combination of both formulations led to an effectiveness between 1 % and 13.6 % compared to the untreated control. The single application of the AgriMet-Granule and the AgriMet-Dry Product was less successful. The standardised greenhouse set-up in pots with A. obscurus indicated that an application rate of at least 300 kg ha^-1 of the AgriMet-Granule, respectively 1×10¹³ conidia ha^-1 of the AgriMet-Dry Product was necessary to effectively reduce tuber damage (44-54 %), while surprisingly no larval mortality was recorded. The laboratory experiments showed, that only the AgriMet-Dry Product provided a lethal potential against wireworms, whereas the AgriMet-Granule showed deficits in terms of product quality. However, the effectiveness of the AgriMet-Dry Product against A. obscurus, A. sputator and A. lineatus resulted in significant differences in mortality indicating a species-specific virulence of the M. brunneum isolate used. Additional influencing factors became apparent through analysing multiple soil parameters at the respective field site such as temperature, moisture, colony-forming units of Metarhizium spp. and wireworm density, underlining the complexity of wireworm control using a microbial control agent. Especially the low soil temperatures during spring application pose a serious challenge to the development of an effective control strategy against wireworms.