Plant identity, but not diversity, and agroecosystem characteristics affect the occurrence of M. robertsii in an organic cropping system

Abstract

Organic farmers rely largely on cultural practices, such as crop rotation and cover cropping, and on biological control to manage pests. Winter cover crops add diversity to agroecosystems and can benefit soil conservation and health, retention and supply of soil nutrients, regulation of arthropods, and crop yields. Within the framework of a three-year experiment to investigate ecosystem services and disservices associated with 12 winter cover crop treatments during the transition of a small grain – corn – soybean production system to organic management, we examined the effects of cash crop and crop management, cover crop species and diversity, arthropods, and soil properties on Metarhizium spp. in a single field site in central Pennsylvania, USA. We used sentinel insect assays with Galleria mellonella L. to determine the relative occurrence of Metarhizium spp. in soil samples from treatment plots. M. robertsii was the only species detected, and therefore Metarhizium species diversity was not related to cover crop diversity. Detection of M. robertsii was lower in cover crop monocultures and mixtures containing brassicas compared to those with legumes. M. robertsii was detected more frequently in the standing cover crops in the corn phase of the rotation than in the soybean phase, but there was no difference in soil samples in the corn and soybeans after cover crop termination. In samples from cover crops, 33% of the variation of percentage mortality of sentinel G. mellonella by M. robertsii was explained by positive correlations with cover crop biomass in the previous fall, the biomass of weeds in the current season (spring), soil moisture, Ca, and timing of sampling in days after planting, and by negative correlations with number of days since disturbance prior to soil sampling and percent sand. In samples from corn and soybeans, 23% of the variation of percentage mortality of sentinel G. mellonella by M. robertsii was explained by positive correlations with soil labile C, electrical conductivity, and activity-density of ground beetles (Carabidae), and by negative correlations with increasing number of days after planting relative to time of sampling, percent sand, and activity-density of mites. The complex interactions of multiple biotic and abiotic factors that shape the soil community requires further research to develop an understanding of how to manage production systems and practices to promote and conserve biological control in the soil.