Honeydew is a sugar-rich sticky liquid, secreted by aphids and some scale insects as they feed on plant sap. When their mouthpart penetrates the phloem, the sugary, high-pressure liquid is forced out of the gut's terminal opening. Honeydew is particularly common as a secretion in hemipteran insects and is often the basis for trophobiosis. Some caterpillars of Lycaenidae butterflies and some moths also produce honeydew. Honeydew can cause sooty mold — a bane of gardeners — on many ornamental plants. Honeydew is also secreted by certain fungi, particularly ergot.
Honeydew is collected by certain species of birds, wasps, stingless bees and honey bees, which process it into a dark, strong honey (honeydew honey). This is highly prized in parts of Europe and Asia for its reputed medicinal value. Parachartergus fraternus, a eusocial wasp species, collects honeydew to feed to their growing larvae.
Ants may collect, or "milk," honeydew directly from aphids and other honeydew producers, which benefit from their presence due to their driving away predators such as lady beetles or parasitic wasps — see Crematogaster peringueyi. Animals and plants in a mutually symbiotic arrangement with ants are called Myrmecophiles
In Madagascar, some gecko species in the genera Phelsuma and Lygodactylus are known to approach flatid plant-hoppers on tree-trunks from below and induce them to excrete honeydew by head nodding behaviour. The plant-hopper then raises its abdomen and excretes a drop of honeydew almost right onto the snout of the gecko.
Religion and mythology
In Norse mythology, dew falls from the ash tree Yggdrasil to the earth, and according to the Prose Edda book Gylfaginning, "this is what people call honeydew and from it bees feed."
In Greek mythology, méli, or "honey", drips from the Manna–ash, (Fraxinus ornus), with which the Meliae, or "ash tree nymphs", nursed the infant god Zeus on the island of Crete, (as in the Hymn to Zeus by Callimachus).
Honey-dew is referenced in the last lines of Samuel Coleridge's poem Kubla Khan, perhaps because of its mythological connotations:And all who heard should see them there, And all should cry, Beware! Beware! His flashing eyes, his floating hair! Weave a circle round him thrice, And close your eyes with holy dread, For he on honey-dew hath fed, And drunk the milk of Paradise.
In the Hebrew Bible, while the Israelites are wandering through the desert after the Exodus, they are miraculously provided with a substance, manna, that is sometimes associated with honeydew. Exodus 16:31 provides a description: "it was like coriander seed, white, and the taste of it was like wafers made with honey."