Phloem nutrition of detached cabbage leaves varies with leaf age and influences performance of the green peach aphid, Myzus persicae

Aphids mainly feed on the phloem sap of host plants and their performance is determined by nutrients and defensive metabolites in their diet. Plant leaves are connected with each other by phloem bundles and thus plant resistance to aphids may require the leaves be intact. Excised plant tissues usually have different resistance levels to aphids compared with intact plants; however, the underlying mechanism remains unknown. In this study, we found that the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), strongly preferred detached young cabbage leaves, but exhibited a poorer performance on these leaves than on detached mature leaves. Compared with other leaves, detached young leaves contained higher levels of arginine, asparagine, methionine, and valine, whereas young leaves showed a very poor phloem nutritional quality, reflected by significantly lower amino acid concentrations in aphid honeydew. In contrast, honeydew produced by aphids feeding on detached old leaves and mature leaves contained higher total amino acid concentrations and lower total sugar contents, indicating higher phloem nutritional quality. Because honeydew excretion rates were comparable among aphids feeding on different leaves, the poorer performance of aphids on detached young leaves may be explained by the lower nutritional quality and food quantity that aphids had ingested. On the other hand, levels of most glucosinolates were the highest in detached young leaves and the lowest in detached old leaves, suggesting that the higher attractiveness of young leaves for aphids is not likely determined by glucosinolates. These results suggest that total amino acids that aphids can ingest play a key role in determining aphid performance. Future studies on plant resistance to aphids should focus on both phloem nutritional quality and aphid feeding rate, and should avoid using excised leaves or detached plant tissues. Host plant leaves are connected with each other by phloem bundles and thus plant resistance to aphids, determined by nutrients and defensive metabolites in their diet, may require the leaves be intact. Myzus persicae (Hemiptera: Aphididae) had a poorer performance on young compared to mature detached cabbage leaves, which may be due to lower nutritional quality and nutrition quantity that aphids ingested. Future studies on plant resistance to aphids should avoid using excised leaves or detached plant tissues.