Aphid honeydew in ecosystems and the environment

Honeydew is a sugar-rich sticky liquid excreted by aphids and other hemipterous insects, such as scale insects, whiteflies, and psyllids, that feed on plant phloem sap. Compared to flower nectar, aphid honeydew contains the same primary sugars (sucrose, glucose, and fructose) as nectar, but also additional oligosaccharides, such as melezitose and erlose, which are synthesised by aphids. Honeydew deposited on leaf surfaces, other surfaces, and in soil plays various roles in the ecosystem, serving as an essential food source for ants, bees, and bumblebees, as well as a supplementary food source for many other animal groups. Bees and ants are important predators and pollinators, particularly in forest ecosystems. Consequently, honeydew produced by aphids and other sap-feeding insects plays a crucial role in maintaining ecosystem functioning. A direct ecosystem service provided by aphid honeydew is a particular honeydew honey, but a substantial part of uni- and multiflower honey may also contain honeydew. Other ecosystem services include enhanced soil microbial activity and reduced nitrogen leaching from forest soil. In urban environments, the honeydew of arboreal aphids can be a significant nuisance, as it coats walls, cars and playgrounds with a sticky layer that collects dust and pollen and supports the growth of sooty mould. Sooty mould growing on honeydew on the leaf surfaces of understory plants can reduce photosynthesis and plant growth. We conclude that extensive multi-trophic network mappings are needed to better quantify how honeydew links above-ground and below-ground food webs, particularly nutrient transfer pathways and cascading effects.