Nutrition Outweighs Defense: Myzus persicae (Green Peach Aphid) Prefers and Performs Better on Young Leaves of Cabbage

Plant leaves of different ages differ in nutrition and toxic metabolites and thus exhibit various resistance levels against insect herbivores. However, little is known about the relationship between leaf ontogeny and plant resistance against phloem-feeding insects. In this study, we found that the green peach aphid, Myzus persicae Sulzer, preferred to settle on young cabbage leaves (Brassica oleracea L. var. capitata) rather than mature or old leaves, although young leaves contained the highest concentration of glucosinolates. Furthermore, aphids feeding on young leaves had higher levels of glucosinolates in their bodies, but aphids performed better on young leaves in terms of body weight and population growth. The concentration of glutamine in young leaves was the highest, which stimulated aphids feeding when added to the sucrose solution. Phloem sap of young leaves had higher amino acid:sucrose molar ratio than mature leaves, and aphids feeding on young leaves showed two times longer phloem feeding time and five times more dry honeydew excretion than on other leaves. These results indicate that aphids acquired the highest amount of nutrition and defensive metabolites when feeding on young cabbage leaves that are strong natural plant sinks. The higher phloem sap availability of young leaves likely contributes to the attractiveness and suitability for aphids and may compensate the negative effects of glucosinolates on aphids. According to these findings, we propose that phloem sap availability influenced by leaf ontogeny and source-sink status play a significant role in plant-aphid interaction, which desires more attention in future research.