Gardening glossary
Honeydew
Honeydew is the digestive byproduct of phloem-feeding insects that must process enormous volumes of plant sap to extract the small amount of nitrogen they need. Plant sap is rich in sugars and water but relatively poor in protein, so the insect ingests far more carbohydrate than it can use and excretes the excess as a clear, sticky droplet from its anus.
The first sign for most gardeners is not the residue itself but its consequences. Leaves below an infested area develop a glossy, sticky film that picks up dust and feels tacky to the touch. Floors, windowsills, furniture, or paving stones beneath an infested plant become noticeably sticky. Ants march up and down the plant tending the insects and harvesting the honeydew, often shielding aphids or scale from predators in exchange. Wasps, bees, and flies may also gather to feed on the sugary deposit.
The long-term problem is sooty mold — a complex of dark fungi (Cladosporium, Aureobasidium, Alternaria, and others) that colonise the honeydew layer and turn affected leaves black. Sooty mold does not infect plant tissue directly but blocks light from reaching the leaf surface, reducing photosynthesis and weakening the plant.
Because honeydew is a symptom and not a pest in itself, the only durable solution is to identify and control the producing insect. Inspect the undersides of leaves and stems directly above the sticky residue — aphids, soft scale, mealybugs, or whiteflies will almost always be present. Once the pest is controlled, existing honeydew can be wiped off with a damp cloth or rinsed with a mild soap solution, and sooty mold will gradually slough away with new growth.