Using genetic analysis to determine the effects of disease on the spatial dynamics of the invasive fall armyworm, Spodoptera frugiperda, in Africa.

Withers, Amy and Wilson, Kenneth and Jones, Christopher and Smith, Judith and Pearson, Aislinn (2022) Using genetic analysis to determine the effects of disease on the spatial dynamics of the invasive fall armyworm, Spodoptera frugiperda, in Africa. PhD thesis, Lancaster University.

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Abstract

Invasive crop pests are a global problem, resulting in food shortages, severe economic burdens, and devastating environmental losses. Many invasive insects are migratory with strong flight ability which further enables their rapid spread. Disease can control pests through occurring naturally and as biocontrol agents. However, the presence of naturally occurring diseases and potential flight routes of invasive pests are unknown when an invasive species first reaches an area. This thesis uses a combination of field collections, molecular biology, and behavioural experiments to fill some key knowledge gaps in our understanding of the invasive fall armyworm (Spodoptera frugiperda) in Africa. The fall armyworm is a migratory crop pest that first arrived in Africa in 2016 and rapidly spread across the continent. Chapter One provides a general introduction to the topics covered in this thesis, focusing on migratory insects to explore what is currently known about pest control, how disease interacts with migration and what genetic analysis can tell us about these topics. Chapter Two, uses fall armyworm larvae collected in six African countries to identify the presence of microbial natural enemies. It then models how environmental factors may influence the distribution and prevalence of the viral natural enemy, Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV). To improve current understanding of population structure of the fall armyworm in Africa, Chapter Three uses a combination of molecular approaches, and investigates whether this can explain variation in the distribution of SfMNPV. Chapter Four takes an in-depth look at the effects SfMNPV has on the fall armyworm during flight, using RNA sequencing to reveal what occurs at the molecular level that could lead to changes in migratory capacity. It compares the molecular responses of males and females, highlighting similarities and differences across many key areas including metabolism, immunity, and reproduction. Using four genes identified by RNA sequencing, Chapter Five focuses on gene expression of the immunity related Toll-pathway. This furthers our knowledge of how the fall armyworm responds to SfMNPV and flight and helps to unravel why males and females respond differently. Finally, Chapter Six is a general discussion chapter that brings these findings together to discuss and consolidate the knowledge that this research has contributed to the field. Overall, this thesis greatly contributes to current understanding of fall armyworm in Africa, and what molecular changes are triggered by flight and disease in a migratory insect pest.