Abiotic, biotic and intrinsic factors limiting the effectiveness of two new Metarhizium brunneum formulations against wireworms in potato cultivation

Wireworms ( Agriotes spp.), the soil-dwelling larvae of click beetles (Coleoptera: Elateridae), are a major pest in German potato cultivation. As there are currently no officially approved agrochemicals to protect against tuber damage, biological alternatives such as the use of entomopathogenic fungi are urgently needed. This study investigated the effectiveness of two new formulations of Metarhizium brunneum (AgriMet-granule and AgriMet-powder) for their effectiveness against wireworms under field, greenhouse, and laboratory conditions. Both formulations showed limited effectiveness in reducing tuber damage in field trials (13.6%), with key influencing factors identified as high pest pressure, low soil temperatures during application (9.8-14.7 °C), limited fungal proliferation (max. 959.9 CFU g -1 ) and poor product stability. Greenhouse tests revealed an increase in effectiveness (44-54%) under controlled conditions and at higher application rates (300 kg ha -1 or 1×10 13 conidia ha -1 ), but these levels are neither economically nor technically feasible for practical use. Laboratory bioassays with A. obscurus , A. lineatus and A. sputator confirmed species-specific virulence of the applied M. brunneum isolate JKI-BI-1450, with high mortality observed only in A . obscurus (82%), which considerably limits the control potential against several wireworm species abundant in German arable soils. These findings highlight key challenges for inoculative biocontrol of wireworms using Metarhizium spp. and emphasize the need for biocontrol strategies that consider biotic, abiotic and intrinsic factors to sustainably reduce wireworm populations. • AgriMet formulations reduced potato tuber damage by only 13.6% in field trials. • Low soil temperatures (9.8-16.5 °C) limited fungal proliferation and field effectiveness. • Greenhouse effectiveness (44-54%) required extremely high application rates, which are neither economically nor technically not feasible. • Virulence of the applied M. brunneum isolate differed among Agriotes obscurus , A. lineatus and A. sputator. • Practical application requires improved formulations and an optimized application strategy adapted to the crop rotation.