Leaves with and without sooty mold accumulation from magnolia scale feeding

Leaves with (on left) and without sooty mold accumulation from magnolia scale (Neolecanium cornuparvum) feeding
Photo: Sarah Vanek, Bugwood.org

Updated: August 15, 2022

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?

left with sticky honeydew on it
Honeydew on a hawthorn leaf caused by calico scale feeding
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.

Black sooty mold darkens honeylocust bark
Photo: Joe Boggs, OSU Ext.

Twice-stabbed lady beetle (beneficial) on a crapemyrtle leaf that is heavily coated with black sooty mold. This is an example of natural biological control. 
Photo: Mengmeng Gu, Texas A&M AgriLife Extension Service, Bugwood.org

Management of honeydew and sooty mold

  • Find the pest responsible for the honeydew and reduce their population. If natural enemies (beneficial insects such as lady beetles) aren’t keeping the population under control, intervention with a pesticide might be needed. Use the least-toxic approach to suppress the pest so natural enemies can manage the rest.
  • Discourage ants. Often the presence of a large number of ants traveling along plant stems suggests a pest population is present. Ants feed on honeydew and will defend the insects against their natural enemies, not unlike herders tending and protecting their livestock. Managing the ants allows natural pest control measures to be more successful, especially if you are avoiding the use of pesticides. You can disorient foraging ants outdoors by blasting trails with water. If insufficient, bait stations can be used to weaken the colony.
  • As the production of honeydew slows and stops, the sooty mold will gradually die off and disappear as both it and its food source weather away. New growth also will help conceal unsightly leaves.
  • For plants shedding leaves laden with sooty mold or honeydew, there is no harm in composting the fallen leaves.
  • If sooty mold covers vegetable or fruit surfaces, they are still edible; simply wash off the residue. On hard surfaces, soap and water should be sufficient to remove both honeydew and sooty mold.

Spotted lanternflies excreting honeydew
Video: Dr. Mike Raupp

Related information

Author: Miri Talabac, Horticulturist & CoordinatorHGIC 2022