Phloemfeeding

Phloemfeeding refers to the feeding of certain plant-sucking insects on the phloem tissue of vascular plants, using piercing-sucking mouthparts that deliver slender stylets into sieve elements. Common phloem feeders include aphids, psyllids, whiteflies, scale insects, mealybugs, and some leafhoppers and planthoppers. These insects typically reside on or within the phloem-rich tissues such as young shoots and new leaves, where they access the nutrient-rich sap that flows through sieve tubes.

Insects access phloem sap by inserting their stylets into sieve elements and secreting saliva that can contain enzymes and effector molecules aimed at countering plant defenses and keeping the phloem conduit unobstructed. The sap is high in sugars, particularly sucrose, and often limited in certain amino acids and minerals, which influences feeding behavior. Phloem flow and plant defenses such as callose deposition at sieve plates can create barriers, prompting feeding in short bursts or along protected routes. As a byproduct of sap ingestion, many phloem feeders excrete honeydew, a sugary liquid that can attract other organisms and support secondary fungal growth.

Nutrition for phloem feeders is supported by obligate bacterial symbionts; for example, aphids harbor Buchnera aphidicola, which supplies essential amino acids missing from phloem sap. This mutualism is crucial for insect development and reproduction.

Phloem feeding can reduce plant vigor, alter growth, and cause symptoms such as leaf curling and stunting. Phloem-feeding insects are also important vectors of plant pathogens, including luteoviruses and poleroviruses, and certain bacteria. Plants respond with phloem-specific defenses, and management often combines monitoring, biological control, systemic insecticides, and resistant varieties.