Genetic Diversity and Ecology of Lepidopteran StemBorers in Rwanda

Abstract

This thesis examines ecology, host interactions, genetic diversity, and evolutionary patterns of Lepidopteran stemborers in the agroecological landscapes of Rwanda. It emphasizes on their impacts on cereal and sugarcane stemborers and wild species, their ecological dependence on wild host vegetation, and their responses to climate and landscape changes. Recent studies reported that stemborers have started to shift from their natural habitats and attacking new host plants that were not used to be hosts before. The research was motivated by high speed increasing landuse change by agriculture intensification and urbanization in changing climate that modify natural habitats of stemborers. In addition to this curiosity, less information on stemborer population in Rwanda is available and do not clearly show how environmental gradients, vegetation mosaics, and and these changes reshape stemborers community structure and genetic composition. The field-based ecological results revealed a substantial species richness, host-plant diversity, and habitat-driven distribution patterns. Surveys yielded 18 stemborer genera recorded from 56 host plant species. The most interesting points is that the highest abundance and diversity of stemborers were concentrated in urban and peri-urban wetlands of Kigali City, highlighting the ecological role of human-modified habitats. Poaceae hosts accounted for over 60% of all larvae collected, and common species exchangeable from wild to crop vis versa for survival, where Napier grass (Pennisetum purpureum) served as the primary refugia during dry. The specific survey conducted in sugarcane plantation shows that sugarcane support mass population of Eldana Saccharina Walker, 1865. The population vary by season and first population start in May, a month of transition between heavy rain season (March-early May) to dry late May to August. The population reached peak in August. The infestation severity varied across sugarcane varieties with the most affected was green and red sugarcane varieties that whitepowdery sugar industrial variety. Sanitation practices of old-leave removal in sugarcane plantation have shown a critical effect in reducing population dynamics of sugarcane stemborers, suggesting this practice as pesticides-free method to control stemborers in sugarcane plantation system. For molecular ecology of key stemborer species through genetic analyses of Sesamia nonagrioides (Lefèbvre, 1827) and E. saccharina, Ethiopian population as earliest studied populations in Africa xxii were used as model to understand Rwandan population’s genetic diversity and diververgence. Mitochondrial COI sequencing showed high genetic diversity in selected model species that were associated with wetland vegetations and these from maize and sugarcane. Among these species, wetland associated species and these associated with crops were standing alone as connected sisters that share common ancester but evolved differently in different time. There were no shared haplotypes between population of Rwanda and model population from Ethiopia. The results suggest that over time, landscape history, wetland degradation, climate gradients, and agricultural expansion are shaping contemporary genetic differentiation, and concludes that region-specific biological control strategies, improved surveillance, and cross-habitat management are essential, as genetically distinct populations may respond differently to control measures. Observed variations in the distribution of pest and non-pest stemborer species, together with differences in genetic structure among key model species, indicate that no single management strategy is suitable across landscapes and that continued habitat fragmentation will likely destabilize ecosystems, leading to host range expansion or local extinctions. These patterns reflect broader environmental change, where habitat fragmentation at the local scale reduces connectivity and gene flow, while climate change at regional and global scales drives shifts in species distribution, community reorganization, and biodiversity loss. Together, these processes highlight the need for multi-scale, integrative research to understand stemborers species responses to thesehh combined effects and to support the development of context-specific, climate-smart integrated pest management strategies for sustainable agriculture