Climate change is causing many species to expand their ranges northwards, potentially impacting on the communities that they join through direct and indirect interactions. This thesis focuses on temporal and spatial variation in interactions in a single study system, lepidopteran hosts feeding on stinging nettles and their parasitoids, through collection of caterpillars fi'om sites in the north west of England. Quantitative food webs revealed two distinct sub-compartments for moths and butterflies. Strong links between the two most abundant butterfly species, Aglais urticae and A. io and their two parasitoid species, a tachinid, Pelatachina tibialis, and an ichneumonid, Phobocampe conjilsa, indicated potential for indirect interactions. The range-expanding species, A. io, was not found to be experiencing enemy release in recently colonised areas. The resident species, A. urticae, experienced higher parasitism rates at sites recently colonised by A. io than at sites where both species had been present for a long duration. At the site level, presence and parasitism rates of the butterfly hosts were not related to habitat features, however, both host species experienced higher parasitism with low connectivity to the other host species. Whilst parasitism rates by Ph. conjilsa were higher in more isolated nests, parasitism by P. tibialis was higher in well connected nests. With reference to the natural history of the parasitoid species, it is proposed that Ph. conjilsa uses isolated hosts to avoid P. tibialis, a potentially superior competitor. Finally, it was found that coexistence of the two host species and the two parasitoid species in this system is likely to persist through differential host use by the parasitoids, mainly mediated through differing phenologies. This thesis provides new and valuable information on the spatial and temporal variations in the interactions between co-occurring common lepidopteran species and their parasitoids in the context of climate change.