Exploring novel biocontrol strategies to manage wireworms, a major soil-dwelling pest insect in potatoes and maize

Abstract: Wireworms, the larvae of click beetles (Coleoptera: Elateridae), are globally the most important soil-dwelling pest insect in potatoes and cause tremendous damage in seed maize production. Due to their complex and long belowground development, no sufficient control measure is available. The implementation of trap crops has been shown to reduce wireworm damage in various crops by distracting larvae from the main crop, but with this strategy pest densities are not reduced. Entomopathogenic fungi (EPF) of the species Metarhizium brunneum have previously been shown to possess a high pathogenicity against wireworms under laboratory conditions, but lack control efficacy under field conditions. While many control approaches focus on the robust larval stage, management of adults has so far mostly been limited to pheromone trapping.
Larval control: We explored novel biocontrol approaches for wireworms within field and
greenhouse experiments. The potential of a perennial attract-and-kill strategy combining EPF
and trap crops to reduce wireworm populations and crop damage is evaluated in a three-year
field experiment. For the characterization of best trap crop candidates, wireworm larvae are
collected, and seasonal feeding preferences investigated via molecular gut-content-analysis. To further optimize the combination of EPF and trap crops, we studied the rhizosphere competence of M. brunneum on promising trap crop candidates. To better understand the interaction of EPF, trap crops and pest larvae, the combination of EPF application and trap crops were tested in different substrates in greenhouse experiments to optimize control efficacy. In a one-year field experiment in potatoes wireworm abundance and damage were reduced by a combined application of EPF and trap crops by 50.8 % and 42.5 %, respectively. The combination had the highest control effect when pest densities were high. Presence of trap crops improves establishment and efficacy of M. brunneum spores compared to an application on bare soil. We observed better rhizosphere competence of certain plant candidates in greenhouse experiments and molecular gut content analysis will identify preferably consumed trap crops. These findings will contribute to find an optimized trap crops mixture for wireworm control. We expect the implementation of trap crops combined with an EPF application to be a resilient and sustainable control strategy for wireworm biocontrol, since it has no negative effects on the environment. Depending on the trap crops used, it even may provide additional benefits on top of the biocontrol effect, e. g. increase in-field biodiversity or facilitate nitrogen fixation.
Beetle control: In order to add another component to current control approaches, we
identified attractive plants for female A. sputator beetles to manage their oviposition choice.
Attractive plants could be avoided before sensitive crops or used to concentrate and control
eggs and emerging larvae in oviposition strips along field margins. For the identification of
attractive plants, we conducted behavioral olfactometer experiments and Electroantennography (EAG) experiments. To reduce stress on test insects, we developed an optimized olfactometer setup and identified at least two promising plant candidates. Attractive plant candidates for adult click beetles could be used to complement existing control approaches by managing oviposition sites, resulting in a strengthened control of pestilent wireworms.