Attract and kill kept simple: Fungus colonized barley kernels in cover crops for microbial wireworm control

Abstract: The multiannual belowground development of wireworms is characterized by a sequence of active and inactive phases. During the former, wireworms are found feeding and foraging in upper soil levels. It follows that a control measures against wireworms may hit its target only when applied during active phases of the pest insect.Wireworms typically show two peaks of activity per year, one in springtime and a second one in late summer. Considering this seasonal activity, we developed a simple “attract and kill” approach, which was tested in a semi-field pot experiment. Metarhizium-colonized, sterile barley kernels were applied into the upper substrate layers of the pots in late summer, immediately before sowing of summer oat as a cover crop. The CO2-emitting roots of the oat seedlings should attract artificially released late instar larvae of Agriotes obscurus into fungus infested upper substrate layers during their late summer activity peak. After hibernation in deeper substrate layers, wireworms were assumed to migrate upwards again during springtime, facing the risk of getting infected a second time, right before planting of potatoes as the main crop. Establishment of the fungus was estimated by counting colony forming units in substrate samples. The efficacy of the control measure was evaluated by assessing the mortality of recaptured wireworms from the pots. We were able to confirm establishment of the fungus in the substrate after a few weeks and to show up to 70% of wireworm mortality, depending on fungus inoculum dose applied. The simple yet promising biocontrol strategy against wireworms is currently tested in a follow-up experiment under field conditions.