Honeydew is a sweet sticky substance, high in sugars and amino acids, that is excreted by various insects, including aphids, leafhoppers, and some scale insects. These insects puncture the phloem vessels in certain plants in order to eat the sap. But because sap contains low amounts of the compounds necessary to build proteins, the insects have to consume large amounts in order to satisfy their nutritional requirements.
The remainder of this sap is excreted from the alimentary canal as honeydew. Some sources say it is “secreted” by insects or, as Wikipedia so delicately puts it, “When [the insect’s] mouthpart penetrates the phloem the sugary, high-pressure liquid is forced out of the gut’s terminal opening.” However you say it, honeydew is what’s left at the end of the digestive tract.
The droplets collect on the leaves and needles of the plants that secrete the sap. Other insects, including honey bees and ants, often use honeydew as a source of food. Honey bees collect and store the honeydew just like nectar.
In some areas of the world, honeydew honey is quite common. Areas of Germany, Greece, and Bulgaria produce large quantities of it. Forest plantations of a single species produce more honeydew than a regular forest because a monoculture tends to produce large populations of the insects that feed on it.
Honeydew honey is said to be very dark with a strong flavor. Although it is very popular in some parts of the world—and commands a premium price—it is not a favorite among North Americans. Honeydew honey is less acidic than floral honey, contains more protein, and is lower in both fructose and glucose. It has also been found to contain varying amounts of gums, dextrins, and plant pigments depending on its source. And because the pH is higher, honeydew honey may contain dead mold spores that grew on the honeydew before it was cured into honey.
Because honeydew honey is higher in indigestible components, it is thought to cause dysentery in bees. And since no pollen is generated during a honeydew flow, bee colonies may suffer from a lack of protein in the hive. Because of these issues, beekeepers usually remove all honeydew honey from the hive before overwintering and often provide a pollen-substitute as well.