Isotopic enrichment in a phloem-feeding insect: influences of nutrient and water availability

The isotopic enrichment between an animal and its diet can vary among and within living systems, but the sources of variation are not yet fully understood. Some studies have found that diet quality or an animal's nutritional status can influence the degree of trophic enrichment, while others have dismissed nutrition as a contributing factor. We evaluated the effects of nutrient and water availability on carbon and nitrogen isotopic enrichment in a specialized plant-herbivore system. Aphids are largely sedentary and rely exclusively on nitrogen-poor phloem sap of their host for nutrition. We grew potato aphids [Macrosiphum euphorbiae (Aphididae)] on an accepted host, pumpkin [Cucurbita pepo L. (Cucurbitaceae)], in a glasshouse environment. Twelve pumpkin plants growing under high- and low-watering regimes were inoculated at 4 weeks of age with aphids. During the course of the experiment we collected leaves, phloem sap, aphids and honeydew (i.e., aphid exudates). We found no trophic enrichment between aphids and their phloem sap diet, but significant carbon enrichment of honeydew relative to aphids (2.5 per thousand) and phloem sap (2.1 per thousand). Honeydew was also enriched in nitrogen compared to the phloem sap (1.2 per thousand). Watering treatment had a substantial impact on trophic enrichment. Correlations among tissues, an indication of uniform trophic enrichment among samples, were significant only for the carbon isotopic composition, and then only for plants and aphids grown in the low-water treatment. Diet quality also influenced the degree of isotopic enrichment; trophic enrichment for both carbon and nitrogen isotopic composition increased as diet quality (C/N) declined. We conclude that the degree of trophic enrichment is variable due, in part, to diet quality, but that the scale of variation is small.