Isotopic enrichment in a phloem-feeding insect: influences of nutrient and water availability.
Sagers, Cynthia1, Goggin, Fiona2, 1 University of Arkansas, Fayetteville, AR2 University of Arkansas, Fayetteville, AR
ABSTRACT- Recent authors have questioned the utility of stable isotope methodologies in field studies of trophic ecology because the degree of isotopic enrichment between animal and its diet is extremely variable. Driving these concerns is an apparent trend in which diet quality and an animal's nutritional status alter the level of enrichment. We evaluated the influence of nutrient and water availability on isotopic enrichment in a plant-aphid system. Aphids are largely sedentary and rely exclusively on nitrogen-poor phloem sap of their host for nutrition. Twelve pumpkin plants (Cucurbita pepo L.) grown in a glasshouse under high and low watering regimes were inoculated with aphid stem mothers at four weeks of age. During the course of the experiment we collected leaves, phloem sap, aphids and honeydew (i.e., aphid exudates). We found no trophic enrichment between aphid and its phloem sap diet, but substantial carbon enrichment between honeydew and aphid (2.5 ppt). Watering treatment had a substantial impact on trophic enrichment; correlations among tissues, an indication of constant trophic enrichment, were significant only for the carbon isotopic composition of tissues 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, in this case, the scale of variation is small.
Key words: aphid, stable isotope, trophic enrichment
All materials copyright The Ecological Society of America (ESA), and may not be used without written permission.