Key points
- Honeydew is the sugary waste excretion produced by many different sap-sucking insects. It can appear on any plant infested with sap-sucking insects, outdoors or indoors.
- A black, soot-like fungus grows on outdoor honeydew-covered surfaces. This is called sooty mold. It does not infect plants, though shading of the leaves from heavy mold growth can stress or stunt a plant.
- Sooty mold is often more noticeable than honeydew, but the presence of either suggests that a plant pest outbreak is present and may need management. Both residues will wear off over time once pest populations are under control.
What is honeydew?
Photo: Joe Boggs, OSU Ext.
- Sap-sucking insects need to ingest a lot of sap (phloem) in order to extract the dilute proteins and nutrients they need. The result is a lot of undigested sugar-water that passes through the insect, which is excreted as waste. This clear, sticky liquid is called honeydew.
- It will drip or be squirted onto surfaces near or below the feeding insect. Since many sap-sucking insects feed in groups, the honeydew buildup can be rapid and substantial before it is detected.
- Leaves or other surfaces coated in honeydew will look glossier than normal and will be sticky to the touch.
- Pests that create honeydew include aphids, adelgids, planthoppers (including spotted lanternfly), leafhoppers, whiteflies, mealybugs, psyllids, and soft scale insects. Although many of these insects can fly or jump, they need to remain fairly sedentary while feeding, and therefore are likely to be found close to honeydew residue.
What is sooty mold?
- Several similar-looking fungus species comprise the group referred to as sooty mold. They get their name from the black, soot-like coating of fungal growth that appears atop honeydew deposits.
- These fungi are not plant parasites and do not directly cause plant disease.
- Indirectly, they can stress or stunt plants by blocking light from the leaves (interfering with photosynthesis), resulting in slowed growth or premature leaf shed.
- Sooty mold also can grow on inanimate surfaces coated in honeydew, such as bark, outdoor furniture, walls, cars, or sidewalks.