Honeydew (secretion)

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.

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