Interactions between an invasive and endemic species of armyworm in Africa and its implications for pest management

Invasive species, such as the fall armyworm (Spodoptera frugiperda; FAW), can disrupt ecosystems and threaten food security. To combat this, integrated pest management (IPM) requires a comprehensive understanding of species interactions. This thesis, through a combination of behavioural experiments, field surveys, and chemical ecology experiments, aims to fill crucial knowledge gaps in the interactions between armyworms, including the FAW, native African armyworm (S. exempta; AAW), and cotton leafworm (S. littoralis; CLW). The invasion of FAW in Africa in 2016 has underscored the urgency of this research. Chapter 1 provides a general introduction, focusing on the overlap between species distributions and host plants and direct and indirect interactions. Direct interactions are then studied in chapters 2-4, and indirect interactions in chapters 5 and 6. Larval contest studies confirmed FAW's highly cannibalistic behaviour and showed FAW's predates AAW and CLW. To improve the current understanding of the field relevance of this, field surveys in a field in Tanzania provided the first evidence for FAW and AAW cooccurrence in the field, indicating interactions between FAW and AAW are non-competitive, although more data is needed. No-choice reciprocal crosses and dissections found no evidence for hybridization between FAW, AAW, and CLW. Dynamic headspace collection and olfactometer studies showed larvae respond to intraspecific and interspecific herbivory; FAW larvae preferred infested maize plants while CLW larvae preferred non-infested maize plants. EAG and further olfactometer studies identified 29 EAG-active compounds that were used to make bioactive blends. Finally, chapter 7 is a general discussion chapter that consolidates the contributions to the field and suggests further work. Overall, this thesis improves understanding of the interactions between native and non-native insects and how understanding these could improve IPM.