Effects of carbon dioxide and nitrogen enrichment on a plant–insect interaction: the quality of Calluna vulgaris as a host for Operophtera brumata

Abstract

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[ratio ]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[ratio ]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[ratio ]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.