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Honeydew is asugar-rich stickyliquid that is secreted byaphids, somescale insects, many othertrue bugs, and some other insects as they feed onplant sap. When their mouthpart penetrates thephloem, the sugary, high-pressure liquid is forced out of theanus of the insects, allowing them to rapidly process the large volume of sap required to extract essential nutrients present at low concentrations. Honeydew is particularly common as a secretion inhemipteran insects and is often the basis fortrophobiosis.[1] Somecaterpillars ofLycaenidae butterflies and some moths also produce honeydew.[2] In addition to various sugars, honeydew contains small amounts ofamino acids, other organic compounds, and inorganicsalts, with its precise makeup affected by factors such as insect species, host plant species, and whether a symbiotic organism is present.[1][3]
Honeydew-producing insects, such ascicadas, pierce phloem ducts to access sugar rich sap; the excess fluid released by cicadas as honeydew is called "cicada rain".[4][5] The sap continues to bleed after the insects have moved on, leaving a white sugar crust called manna.[6]Ants may collect, or "milk", honeydew directly from aphids and other honeydew producers, which benefit from the ants' presence due to their driving away predators such aslady beetles or parasitic wasps—seeCrematogaster peringueyi. Animals and plants in a mutually symbiotic arrangement with ants are calledMyrmecophiles.
In Madagascar, some gecko species in the generaPhelsuma andLygodactylus are known to approachflatid 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.[7]
Honeydew can causesooty mold on manyornamental plants. It also contaminates vehicles parked beneath trees, and can then be difficult to remove from glass and bodywork. Honeydew is also secreted by certain fungi, particularlyergot.[8] Honeydew is collected by certain species of birds, mosquitoes,[9][10]wasps,stingless bees,[11] andhoney bees, which process it into a dark, stronghoney (honeydew honey). This honey is highly prized in parts ofEurope andAsia for its reputed medicinal value.Parachartergus fraternus, aeusocial wasp species, collects honeydew to feed to their growing larvae.[12] Recent research has also documented the use of honeydew by over 40 species of wild, native, mostly solitary bees in California.[13]
Honeydew is anexcretion, because it is unused food that is expelled through the insect'sanus.[14] The food is plant sap, very rich in sugar but low in protein, so excess sugar must be excreted. While honeydew is often called "secretion" because sugary liquid does not sound like a typicalexcreta, "secretions" come from specializedglands.
Like excreta in general, honeydew is harmful to the insect, e.g., promoting fungal growth, attracting enemies, and clogging surfaces. Many insects coat excreted honeydew droplets with wax secreted by setae and associated glands around the anal opening.[15]
InNorse mythology,dew falls from theash treeYggdrasil to the earth, and according to theProse Edda bookGylfaginning, "this is what people call honeydew and from it bees feed."[16]
InGreek mythology,méli, or "honey", drips from the Manna–ash (Fraxinus ornus), with which theMeliae, or "ash treenymphs", nursed the infant godZeus on the island ofCrete[17] (as in theHymn to Zeus byCallimachus).[citation needed]
Honey-dew is referenced in the last lines ofSamuel Coleridge's poemKubla 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.
Ommatissus lybicus is attracted to certaincultivars of thedate palm tree. The honeydew producing insects preferred theMedjhool variety to theDeglet Noor inIsrael, where they have been observed in theArava Valley. Very dense insect populations may have some adverse effects. Different methods of controlling the insects, including natural and chemical, have been studied.[18]
Ineucalypt forests, production of both honeydew nectar and manna tends to increase in spring and autumn.Eucalyptus can produce even more manna than honeydew nectar. Thesugar glider eats both, licking the nectar from branches. Other species attracted to the nectar include thefeathertail glider,brush-tailed phascogale, andbrown antechinus. Most trees are not able to produce sap if the phloem duct becomes damaged by mechanical processes.[6]
The acorn weevil (Curculio glandium) habitually bores into the youngacorns ofoak trees. This injury can cause the tree to release a sweet honeydew, thought to attract wasps that can parasitize the weevil within its acorns.[citation needed]Honey bees sometimes collect this substance, in addition to honeydew from aphids feeding in oak forests, and use it to produce oakhoney, an unusual varietal sold by specialistbeekeepers.[19][20]
Twoscale insects in theSinai,Trabutina mannipara andNajacoccus serpentinus, feed onTamarisk trees. They secrete a sugary nectar that turns white when it hardens.[21]
Thehoney made produced when bees harvest the honeydew of sap-eating insects such asaphids is known ashoneydew honey orforest honey. It is notably darker and more viscous than typical honey. Honeydew honey typically contains moremelezitose. It is prized in parts of Europe andNew Zealand.[22][23]
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