Unravelling Fall Armyworm-Stemborer Interaction Mechanisms in Maize Cropping Systems in Makueni, Kilifi and Bungoma Counties
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Date
2024-01
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Kenyatta University
Abstract
Fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae), which recently invaded African nations, currently coexists with the major resident stemborers like Busseola fusca (Lepidoptera: Noctuidae) and Chilo partellus (Lepidoptera: Crambidae), severely damaging maize crops. Following their co-inhibition on maize cropping systems, exploitative and interference intraguild interactions occur between these lepidopteran pests. These interspecific interactions may impact population dynamics and niche displacement of these pests in maize crops as they continue to establish in different cropping systems. A number of field surveys have reported co-existence of S. frugiperda and stemborers in maize fields in Africa, but the actual occurrence, magnitude and exact mechanistic basis of their interaction remain largely unknown. To understand the underlying mechanisms of their interactions in maize cropping systems in Kenya, this study purposes to investigate plant-mediated intraspecific and interspecific interactions, predation in laboratory and semi-field settings, and larval field occurrences of S. frugiperda and the two stemborer species. In addition, reverse transcription quantitative PCR (RT-qPCR) was performed to quantify the levels of expression of defense genes following damage by the insect pests. The results of larval feeding assays used to evaluate interactions between competing herbivore pests showed that the initial feeding of S. frugiperda had negative impact on subsequent stemborer larval feeding and survival, indicating that S. frugiperda may have induced mechanisms that prevent the establishment and survival of competing species. Similarly, S. frugiperda larvae attained higher growth parameters (larval weight, length, and survival) and relatively faster rate of development than stemborers after co-existing on maize crop for 15 days, indicating S. frugiperda’s ability to outcompete stemborers in a co-inhabiting environment. Predation experiments revealed that S. frugiperda’s 2nd-6th instars preyed on both B. fusca and C. partellus larvae at various developmental larval stages. Spodoptera frugiperda preyed on stemborers at considerably higher rates than it cannibalized its conspecifics (P < 0.001). Presence of stemborers considerably reduced S. frugiperda conspecific cannibalism when compared to the absence of heterospecific stemborer species (P = 0.04). In the three altitudinally different agroecological zones, field surveys revealed that S. frugiperda larvae were substantially more prevalent than stemborers (P < 0.001). Neonates of C. partellus and B. fusca consumed less leaf area on plants initially fed on by S. frugiperda and more leaf area on plants initially exposed to stemborer and undamaged plants. Furthermore, there was high expression of pathogenesis related protein-5 (pr5), maize proteinase inhibitor (mpi) and lipoxygenase 3 (lox-3) genes upon S. frugiperda feeding compared to C. partellus and B. fusca feeding, a demonstration of a mechanism that the invasive species utilizes to deter colonization and performance of other competing species. Results from this study demonstrated that the invasive S. frugiperda exhibits clear competitive advantages over resident stemborers and could displace them within the maize cropping systems in Kenya. Furthermore, this study revealed the S. frugiperda employs interference and exploitative competition through predation and competitive plant-mediated interactions respectively, as some of the mechanisms to outcompete resident stemborers. These findings provide essential information for use when developing pest management strategies for these lepidopteran pests in maize cropping systems.
Description
A Thesis Submitted In Partial Fulfillment of the Requirements for the Award of the Degree of Master of Science (Biotechnology) in the School of Pure and Applied Sciences of Kenyatta University, January 2024.
Supervisors
1. George Ochieng’ Asudi
2. Daniel Munyao Mutyambai
3. Fathiya Khamis