Understanding the population dynamics of host-natural enemy interactions in spatially heterogeneous habitats may help mitigate the negative effects of environmental change.\ud The aim of this thesis is to investigate the role of spatial processes in determining the distribution and abundance of two co-occurring Lepidoptera species of differing dispersal capabilities, and how this may affect interactions with natural enemies of differing transmission modes. Operophtera brumata is a resident species, attacked by a single parasitoid and three viral pathogens, all with differing transmission strategies; Abraxas grossulariata is a new coloniser, and may have escaped its parasitoid enemies. Both species feed on the same hostplant, Calluna vulgaris, in the Orkney isles, Northeast Scotland.\ud \ud Natural enemy prevalence was rarely related to host density of either species at any scale. Significant host density dependence in infection was detected only among\ud horizontally transmitted pathogens infecting O. brumata, and only during high density outbreaks. Parasitoid prevalence was significantly negatively related to O. brumata host density, although only during outbreak years, either due to satiation or competition from pathogens. Conversely, two years following an outbreak, parasitoid prevalence was found to be positively related to host density. There was no evidence that O. brumata escapes parasitoid attack at higher elevations, although newly colonising A. grossulariata populations may be susceptible to microclimatic variation, mediated by topography. Such susceptibility may be a result of unfavourable conditions at the northward margin of the species’ range. A genetic signal of recent colonisation was detectable among A. grossulariata populations within Orkney when compared to\ud mainland Britain. Genome scans also suggest that A. grossulariata may be under selective pressure from pathogenic infections, possibly due to their low genetic\ud diversity. Local adaptation of O. brumata to individual patches was not apparent. The implications of these findings and potential areas of future research, are discussed
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