Abstract 1. As species shift their geographic distributions, new feeding interactions with natural enemies such as parasitoids, and resources such as host plants, may be established, and existing interactions may be severed.
2. The leaf mining moth Phyllonorycter leucographella (Zeller, 1850) (Lep.: Gracillariidae) first colonised the southern United Kingdom in the mid 1980s associated with its ancestral host plant Pyracantha coccinea M. Roem. (Rosaceae), which is widely cultivated in the U.K. The moth has since spread northwards to central Scotland and has been recorded feeding on a novel host plant, Crataegus monogyna L.
3. The combined effects of latitude and time since colonisation on parasitoid community responses to the arrival of this novel host were investigated across its U.K. range. The response of parasitoids to colonisation of C. monogyna was also investigated.
4. Both the observed richness of parasitoid species associated with P. leucographella, and the proportion of P. leucographella parasitised declined with latitude and towards the current range margin. A combination of a latitudinal gradient in parasitoid and alternative host species richness is likely to lead to the trends in species richness and parasitism observed.
5. Experimental host patches exposed to parasitism beyond the current range margin of P. leucographella experienced low levels of parasitism consistent with range-margin populations, indicating an instantaneous response by native parasitoids to availability of the novel host. Parasitism levels and numbers of associated species in the U.K. were similar to those observed in the species’ native range in Turkey.
6. The host plant switch to C. monogyna was not associated with an altered parasitoid assemblage, but rates of parasitism were significantly higher on the novel host plant.
7. Alterations in the incidence and frequency of victim-enemy interactions as species shift their geographic ranges may be key in determining rates of range expansion and the impact invading species have on ecological communities.