Soil matters: impacts of farming practices on natural regulation of root pests of field vegetables
Delabre, Céline Marie Coralie
Agricultural intensification and over-reliance on pesticides have had serious detrimental effects on ecosystems. Agroecology research is attempting to restore healthier agroecosystems, capable of delivering a wide range of ecosystem services, including natural regulation of crop pests by their natural enemies. This study explores the impacts of farming practices on the cabbage root fly Delia radicum, whose larvae cause serious damage to brassica crops. Two long term experimental rotations, comparing organic and conventional practices, were monitored over two seasons for fly eggs and pupae as well as their natural enemies. Additionally, a paired soil survey was conducted across the UK and Ireland to investigate varied commercial organic and conventional practices, their impact on soil as habitat supporting plant growth, as well as on soil-based natural regulation. Organic practices in the experimental rotations had an overall positive impact, reducing fly eggs in both sites and pupal numbers in one site, compared to pesticide treated plots, as well as increasing activity density of their potential epigeal and belowground predators. Experimental field soils were also used to further investigate the potential regulation through insect pathogens using a model pest, but no consistent significant differences between management types were detected. Additionally, potential bottom up control through plant-soil-pest interactions was also tested by growing brassica plants in those same soils whilst inoculating them with D. radicum eggs. Too few pupae were extracted to be able to conclude decisively, but data did not point towards a reliable enhanced pest suppression in organic soils. Paired commercial soils were impacted by local management, with organic soils being more biologically active. Unlike experimental field soils, organic commercial soils were more suppressive overall for the model pest, potentially due to entomopathogenic nematode presence. Brassicas grown in organic commercial soils developed significantly larger root systems, without a reduction of top biomass, but inoculation experiments once again did not reveal any clear difference in pest survival between organic and conventionally managed soils. In line with current research, chemically based management was shown to have a detrimental effect on soil biological activity and pest antagonist communities when compared to organic management. Under the adequate management, soil can help foster functional biodiversity to help effectively deliver a wide range of ecosystem services including pest regulation. As a habitat, soil is unfortunately often overlooked in conservation biocontrol studies, even though it is an integral part of sustainable and resilient agroecosystems. This thesis attempts to highlight the importance of also including soil in natural pest regulation studies along with aboveground landscape elements that are more easily manipulated.