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 the 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.
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.
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 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).
- 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 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."
See also 
- Delabie, JHC (2001) Trophobiosis Between Formicidae and Hemiptera (Sternorrhyncha and Auchenorrhyncha): an Overview. Neotropical Entomology 30(4):501-516 PDF
- Maschwitz U, Dumpert K, Tuck KR (1986). "Ants feeding on anal exudate from tortricid larvae: a new type of trophobiosis". Journal of Natural History 20 (5): pp. 1041–1050. doi:10.1080/00222938600770751.
- Koch, H., Corcoran, C., Jonker, M. 2011. Honeydew Collecting in Malagasy Stingless Bees (Hymenoptera: Apidae: Meliponini) and Observations on Competition with Invasive Ants. African Entomology 19 (1): 36-41.PDF fulltext
- Foelling, M; C Knogge and W Bohme. "Geckos are milking honeydew-producing planthoppers in Madagascar". J. Nat. Hist. 2001: 279–284.
- Faulkes, Anthony (Trans.) (1995). Edda. Everyman. ISBN 0-460-87616-3 pp18–19
- Clauss, James Joseph (1993). The Best of the Argonauts: The Redefinition of the Epic Hero in Book 1 of Apollonius's Argonautica. Hellenistic culture and society 10. University of California Press. p. 170. ISBN 978-0-520-07925-0.