none. Prevalence in lepidopteran hosts outbreaks of the winter moth, Operophtera brumata Linnaeus (Lepidoptera: Geometridae), have recently become widespread on heather moorlands in northeast Scotland. These outbreaks represent remarkable phenomena given the poor nutritional quality of the dominant moorland host plant Calluna vulgaris (Linnaeus) Hull (Ericaceae). Winter moth performance on C. vulgaris was compared with that on the other available moorland host plant, Vaccinium myrtillus (Linnaeus) (Ericaceae), and parasitism levels were measured in two moorland outbreak populations. Larval densities in the field were generally higher on Calluna than on Vaccinium. However, larvae showed better survival, more rapid development and greater pupal weights on Vaccinium than on Calluna. Feeding trials indicated that Vaccinium was more digestible than Calluna. No evidence of parasitism was found in a high-altitude moorland winter moth outbreak population from mainland Scotland, but the parasitoid Phobocampe neglecta (Holmgren) (Hymenoptera: Ichneumonidae) was present in 27% of larvae from a lower-altitude moorland outbreak on Orkney. The results indicate that the presence of Vaccinium and the opportunity to escape from parasitism may facilitate O. brumata outbreak formation in high-altitude moorland sites. However, neither factor can provide a full explanation for the present phenomenon.