Calluna vulgaris L. (Hull) is not one of the usual hosts of the winter moth, Operophtera brumata L., but outbreaks have caused extensive damage to heather moorland in Scotland in recent years. This study investigated the potential role of environmental change in such outbreaks by rearing O. brumata larvae on C. vulgaris plants grown in open‐top chambers for 20 months with enriched CO2 (600 ppm) and nitrogen supply (average 52·5 kg N ha−1 yr−1) in factorial combination. This prolonged exposure to elevated CO2 caused no change in shoot growth, photosynthesis or foliar C∶N ratio of C. vulgaris, even with increased N supply, indicating that the absence of response was not due to N limitation. Increased N supply itself resulted in increased shoot growth and a decrease in tissue C∶N ratio. Phenolic content did not change in response to either CO2 or N enrichment, contrary to the predictions of the carbon/nutrient balance hypothesis. In line with the absence of plant response, there was no effect of CO2 on the development of Operophtera brumata on C. vulgaris, and so continued increase in atmospheric CO2 concentration is unlikely to affect directly O. brumata outbreaks on heather moorland. Operophtera brumata showed increased larval development, growth rate and pupal weight on N‐treated plants, correlated both to the decrease in foliar C∶N ratio, and to the increase in shoot extension which was predictive of survivorship. Thus, increased atmospheric N deposition, or increased rates of mineralization in a warmer environment, might increase the severity of O. brumata outbreaks on C. vulgaris. Since the combination of high N availability and disturbance of heather canopy by herbivory is known to result in increased dominance of grasses, it is suggested that this could lead to further degradation of moorland in upland Britain.

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.

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