Impacts of maize cropping systems on soil microbiome phytochemistry and trophic interactions between the fall armyworm, Spodoptera frugiperda (J. E. Smith) and its endoparasitoid Cotesia icipe (Fernandez-Triana & Fiaboe)

Abstract

Intensification of staple crop production with synthetic chemical inputs has contributed to negative environmental impacts. Ecological intensification, using diverse maize-cropping systems, has the potential to reverse these detrimental effects and sustainably improve crop productivity. However, the influence of these diverse maize-cropping systems on soil physicochemical properties, the soil and maize-root microbiome, and subsequent plant-insect interactions has not been clearly understood. Diversified maize-cropping systems, such as push-pull cropping systems and maize edible-legume intercropping (MLI) systems, are known to enhance plant defence mechanisms, reducing susceptibility to invasive pests like the fall armyworm (FAW, Spodoptera frugiperda J. E. Smith). This polyphagous pest, originating from the Americas, has become a global concern with its invasion into Africa and subsequent spread worldwide, targeting over 353 host plant species, primarily maize and sorghum, staple crops for most African communities. Despite the widespread damage caused by S. frugiperda, limited research has focused on understanding the influence and mechanisms of diverse maize-cropping systems soil legacy on inducing resistance to pests. Therefore, the goal of this study was to determine the effects of different types of maize-cropping systems (push-pull, MLI, and maize-monoculture) on soil health (the soil's physical and chemical properties), the microbiome (the rhizospheric soil, and the bacterial and fungal communities that live in the roots of the maize plants), and the plant's resistance to damage by the S. frugiperda. This encompassed both direct effects, such as reduced S. frugiperda neonate feeding and larval development, and indirect effects, such as the attraction of the S. frugiperda endoparasitoid, Cotesia icipe (Fernandez-Triana & Fiaboe). Chapter one of this thesis briefly introduces S. frugiperda and emphasises the significance of soil physicochemical properties and the microbiome in diversified maize-cropping systems. It discusses the importance of studying tritrophic interactions in maize-cropping systems, investigated the influence of soil microbiomes on plant-insect interactions, and examined the role of the endoparasitoid, C. icipe in pest management. The chapter also reviews available control methods, suitable farming practices for enhancing soil and plant health, and effective pest management strategies. Finally, it outlines the rationale and objectives of the study. Chapter two outlines the field sampling procedures used on long-term push-pull cropping system farms. The soil physicochemical properties and microbiomes were analyzed using the Bouyoucos hydrometer method and the MiSeq Illumina sequencing platform. The goal was to understand the impact of long-term push-pull cropping systems on the physicochemical properties of soil, rhizospheric, soil and maize-root microbiomes. Soil from push-pull cropping systems had higher levels of pH, electrical conductivity, nitrogen, organic carbon, potassium, phosphorus, calcium, magnesium, and exchangeable acidity, among other parameters, compared to soil from conventional maize-monoculture cropping systems. The study compares the microbiomes relative abundance, diversity, and functional protein pathways with conventional maize-monoculture cropping systems. This chapter emphasizes the less-known belowground interactions within the push-pull, revealing its significance in harboring ecologically important microbial groups that improve soil fertility, decomposition, nutrient cycling, and plant protection compared to maize-monoculture cropping systems. Chapter three examines the impact of crop mixtures, particularly MLI systems, on belowground microbial communities and soil physicochemical properties in maize-cropping systems. By comparing the maize-monoculture cropping system with four different MLI systems, it shows that crop mixtures have a big effect on the rhizospheric soil and the microbial community restructuring around maize-roots. They also facilitate the growth of beneficial fungal and bacterial populations, significantly contributing to agrobiodiversity and belowground ecological services. Chapter four presents the findings on the impact of MLI in comparison with conventional maize-monoculture cropping systems, on soil legacies, maize growth, and the direct resistance of maize plants to S. frugiperda. The chapter includes details on field sampling and soil physicochemical properties, maize plant growth parameters evaluation, and S. frugiperda neonate feeding and larval development. The findings of this study reveals that soil-conditioned by MLI systems enhances nutrient levels and maize growth while reducing S. frugiperda neonates feeding and larval development. Hence, this chapter underscores the positive influence of soil-conditioned by MLI systems on maize plant growth, soil health, and S. frugiperda management. Chapter five discusses how long-term push-pull cropping systems alter the soil and affect plant-soil feedback. It focuses on maize phytochemical profiles, volatile-mediated interactions in plant-insect relationships, and plant growth parameters. The study scrutinizes the behavioural reactions of the S. frugiperda endoparasitoid, C. icipe, to the alterations in volatile profiles resulting from push-pull soil-conditioning, in contrast to conventional maize-monoculture cropping systems. Maize plants grown in soil-conditioned by push-pull had higher soil physicochemical properties, plant biomass, and growth rate. Consequently, S. frugiperda neonates fed less on leaf tissues from soil-conditioned by push-pull in comparison to maize grown in soil-conditioned by maize-monoculture cropping systems. In quantitative and qualitative terms, diverse volatiles were observed in maize plants grown in soil-conditioned by push-pull cropping systems compared to maize-monoculture cropping systems. The volatiles emitted by maize plants grown in soil-conditioned by push-pull cropping systems were significantly more attractive to C. icipe than those emitted by maize plants grown in maize-monoculture cropping systems. Chapter six discusses and concludes the thesis, provides a summary of the study's key findings, explores practical applications of the findings, and offered recommendations for future research endeavors.