The interaction between migration and disease in the Fall Armyworm, 'Spodoptera frugiperda'

Abstract

Every year billions of insects undertake long-distance seasonal migrations, moving hundreds of tonnes of biomass across the globe and providing key ecological services. Yet we know very little about the complex migratory movements of these tiny animal migrants and less still about what causes their populations to fluctuate in space and time. Understanding the reason for these population level changes is important, especially for insect species that are agricultural pests and disease vectors. One possible driver of large scale population dynamics in migratory insect species is disease. Migration is a stressful and energetically-costly behaviour. Fighting off, or living with, infections is also costly. In migratory animals that have been exposed to disease this may lead to potential trade-offs between investment in migration and investment in the resistance and tolerance mechanisms associated with infection. Using a combination of rotational flight mills, bioassays and molecular techniques, this thesis uses the fall armyworm, Spodoptera frugiperda, and its associated baculovirus, S. frugiperda multiple nucleopolyhedrovirus (SfMNPV), as a model system to describe the trade-off between migratory effort and disease susceptibility, and how this affects disease dynamics at a geographic scale. After a general introduction to the topic (Chapter 1), Chapter 2 uses an inter-species analysis to describe the insect flight patterns associated with migratory behaviour in three species of migratory noctuid moth, linking these with previous work on the upregulation of genes associated with the migratory syndrome and providing evidence of sex-biased dispersal in the fall armyworm. Chapter 3 builds on these results by quantifying the impact of infection on migratory flight behaviour, and provides the first evidence of Bateman’s principle in insect migrants by demonstrating that males and females exhibit different developmental and physiological responses to infection, and adopt different flight strategies following virus exposure. To understand how this affects susceptibility, Chapter 4 quantifies the effect of flight effort on resistance to infection, showing that prolonged bouts of flight results in an increase in disease loads but only in populations with low levels of background infection. This provides evidence that the trade-off between flight effort and resistance is context dependent and possibly phenotypically plastic. Finally, Chapter 5 contextualises these laboratory results by investigating fluctuations in disease load across the United States of America. Findings from this study show that the host-pathogen system is relatively stable over large geographic distances and time periods of up to two years. Where variation does occur, there is evidence for ‘escape’ from infection but that this is often associated with the cost of reduced resource availability in males. Overall the work demonstrates key physiological and behavioural adaptations that enable insects to engage in long-distance migration when faced with competing costs of flight and disease resistance.