Honeydew: A mixture of sugars and other plant-derived chemicals excreted by some species of aphids and by some species of insects in families related to aphids. When these insects feed on plants, honeydew drips from them onto plant leaves or onto the ground. Fresh honeydew may be fed upon by other insects as an energy source. Ants of many species are avid feeders on honeydew, and they may even guard the aphids against predators and parasitoids to protect this energy source. Unconsumed honeydew on plant leaves promotes growth of a black fungus called sooty mold, which may become so dense that it interferes with metabolism of the plant. Contrast with nectar.