Implications for management of wireworms: insect pest suppressiveness of soils with plant functional diversity

Abstract

Plant species diversity in agricultural systems has been shown to provide multiple ecological and agronomic benefits. How it affects the soil microbial communities, especially the guild of entomopathogens capable of controlling insect pests, has not been studied. We used the set-up of the “Experimentation in Ecosystem Research”, at Jena, Germany, to study the role of plant species diversity in facilitating insect pest suppressiveness in soils via entomopathogens such as insect-killing fungi (EPF) and nematodes (EPN). The aim was to gain insights for improving the natural control of wireworms and other pests by supporting the levels of pest antagonistic organisms in the soil. Soils were sampled from plots with a fixed number (from zero to 16) of plant species from different functional groups. Samples were baited with mealworm Tenebrio molitor larvae and the mortality (k-factor) was determined. A supplementary study was conducted to relate the findings to a soil pest, wireworm (Agriotes sp.), using potting soil and a high and low dose of entomopathogenic fungi and nematodes. Field-collected wireworms were subjected additionally to a maximum challenge test, using a Metarhizium strain naturally occurring in the soil from which the wireworm larvae were collected. Our results showed that the increase in the number of plant species at the growing site can significantly increase the killing power (k-factor) of the soil. Plots with legumes had significantly higher k-values than plots without. EPF occurred significantly more often in plots with legumes than in plots without. Wireworms survived entomopathogen treatments where all mealworms died, but developed mycosis in the maximum challenge test. These findings need to be incorporated into cropping system design as a component in maximizing ecosystem services via ecostacking.