Outbreaks of winter moth, Operophtera brumata L. (Lepidoptera:Geometridae), have recently become common on moorlands in Scotland. This thesis describes the ecology of moorland O. brumata populations, and tests the hypothesis that outbreaks are caused by increased host-plant quality. The principal moorland host, Calluna vulgaris (L.) Hull, common heather, is of low nutritional quality for O. brumata. Experimental nitrogen-enrichment of C. vulgaris enhances O. brumata development, although there is no evidence that outbreaks occur on nitrogen-enriched C. vulgaris in the field. Calluna vulgaris nutritional quality is not significantly affected by growth in carbon dioxide-enriched atmospheres. There is no evidence that moorland O. brumata populations are specialised to feeding on C. vulgaris, and no evidence that synchrony between larval emergence and C. vulgaris budburst affects larval survival and development. Compensatory feeding may enhance the ability of O. brumata to utilise C. vulgaris. Life history and metabolic differences are shown between moorland and non-moorland O. brumata populations. These differences are best explained as physiological adaptations to local climate, and are unlikely to be caused by nutritional specialisation of O. brumata populations to different hosts. Evidence is presented that winter weather conditions strongly affect O. brumata abundance in high-altitude moorland outbreak sites, and that O. brumata escapes from parasitism at such sites. The relative importance of "top-down" factors, such as natural enemies, and "bottom-up" effects, such as host quality, in determining O. brumata abundance is discussed. It is argued that, in high-altitude moorland sites, the interaction between winter weather conditions and the "top-down" effect of natural enemies is a more important determinant of outbreak potential than the "bottom-up" effect of host quality. The effects of future environmental changes on this system are considered.

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