Utilising Ecology and Molecular Techniques to Identify Alternative Hosts, Green Bridges, and Population Structure of a Serious Aphid Pest

Nasonovia ribisnigri (Mosley, 1841) is the most damaging pest on outdoor lettuce crops during the summer months. Control measures to ameliorate yield loss are hampered by the cryptic ecology of this pest aphid, with low numbers trapped, and an autecology of feeding on concealed young leaves. A lettuce cultivar introduced in the 1980’s provided single gene resistance (Nr-gene) against N. ribisnigri. However, over reliance of this cultivar led to resistance-breaking biotypes arising during 2007.
This multifaceted thesis aimed to address unknown areas of N. ribisnigri ecology. Over 50 years of UK suction trap data were studied to understand how the changes in climate and agricultural practises have influenced population trends. Different trapping methodology was incorporated to improve successful capture of this cryptic aphid to help improve current forecasting models. Various semi-field and field experiments were established to identify alternative summer host plants that could be used by N. ribisnigri as a green bridge between secondary hosts and lettuce. Additionally, the project aimed to assemble a draft genome of N. ribisnigri to provide insight into the level of gene flow between populations in England, and to study the potential resistance-breaking mechanism in lettuce cultivars containing the Nr-gene.
Firstly, population trend analysis revealed significant declines in the abundance of N. ribisnigri in the UK since 1965. Secondly, multiple trapping methods elucidated the low dispersive behaviour of N. ribisnigri. Thirdly, the importance of green bridge vegetation in connecting occurrence of this host-alternating species from its primary host to the lettuce crop was expounded with field surveys and gut content analysis. This work suggests N. ribisnigri is using an intermediate secondary host plant prior migration onto outdoor lettuce later in the year. Next, the first draft reference genome and transcriptome for N. ribisnigri were de novo assembled. Subsequently, population genetic analysis revealed a low level of gene flow between populations in England and highlighted a divide between the East and West populations of N. ribisnigri. Finally, RNA-seq analysis of susceptible and resistant N. ribisnigri biotypes revealed a single candidate resistant gene likely responsible for the resistance-breaking mechanism in lettuce cultivars containing the Nr-gene.
The work in this thesis has already been applied by growers, this may be extended to improve the monitoring and field control of N. ribisnigri. Promising avenues have been opened for further research and development which may be extended to other invertebrate pests, including dietary analysis to identify previous host plants in aphids, techniques in population genetics, and analysis of resistance mechanisms.