Sticky Leaves on Indoor Citrus: Find the Honeydew Insect

If the leaves of your indoor lemon, lime or orange feel tacky, or the floor under it is sticky, something is feeding on the tree. That shine is honeydew. Sap-sucking insects take in more sugary sap than they can use and excrete the rest, and it drips onto the leaves below.

UC IPM explains in its sooty mold pest note that honeydew comes from aphids, leafhoppers, mealybugs, psyllids, soft scales and whiteflies. Black sooty mould then grows on the honeydew. Treat the insect and both problems fade.

Find the insect: look above the sticky leaves

Honeydew drips downwards, so the insects are usually on the leaves and stems above the stickiest patch. Check leaf undersides, the leaf joints and the bark of young stems with a hand lens.

What you see	Likely insect	Notes from extension sources
Brown or tan bumps on stems and along leaf veins that don’t move	Soft scale	UC IPM says soft scales are rounded, humped, and excrete abundant honeydew. Flipping the cover removes the insect with it.
Flat scale covers, but no stickiness	Armoured scale	UC IPM and Clemson both note armoured scales do not produce honeydew. Look for a second insect.
White, cottony or mealy clusters in leaf joints and shoot crotches	Mealybugs	Clemson notes the citrus mealybug feeds at shoot crotches and is more apt to be a problem on indoor citrus.
Tiny white insects that fly up when you brush the plant; flat discs under leaves	Whiteflies	Clemson suggests shaking a branch and watching for small white adults. Nymphs feed on lower leaf surfaces.
Small soft insects on flower buds and new shoots, with curled young leaves	Aphids	Clemson describes black citrus aphids on flower buds and new foliage, causing cupping and curling.

Clemson HGIC’s citrus insects factsheet adds that ants on a citrus plant are a sign to look closely for whiteflies, aphids or mealybugs, because ants feed on their honeydew.

If you find speckled, pale leaves and fine webbing but little honeydew, consider spider mites instead. Clemson names spider mites and mealybugs as the most troublesome pests of containerized citrus kept indoors over winter.

What to do

1. Isolate the tree from other houseplants. University of Maryland Extension recommends isolating plants with mealybugs so the infestation does not spread.
2. Remove what you can by hand. For light mealybug infestations, University of Maryland suggests dabbing individual insects with a cotton swab dipped in household alcohol, keeping it off the leaves. UC IPM’s scale guide says you can prune off heavily infested twigs on small plants.
3. Choose a spray that fits the pest and the crop. Clemson says horticultural oil controls scales, whiteflies, mealybugs and aphids, and that insecticidal soap works well on whiteflies, mealybugs and aphids. It advises taking indoor citrus outdoors in mild weather to spray, then bringing it back once dry. Do not spray citrus in bloom. If fruit is present, Clemson limits oil to no stronger than 3 tablespoons per gallon of water.
4. Read the label. The product must list citrus or the right crop, the pest and the indoor setting. Our houseplant pest management guide and neem oil vs insecticidal soap comparison explain how to match a product to the pest.
5. Repeat and recheck. University of Maryland notes that several applications are usually needed for mealybugs, and to check the plant each time you water.
6. Wash off the residue. UC IPM says sooty mould can be washed off plants with a strong stream of water or with soap and water. It also notes that fruit covered in sooty mould is still edible once washed with mild soap and warm water.

If a plant is heavily infested, University of Maryland advises that discarding it can be the better choice for mealybugs and whiteflies.

Is sooty mould harming the tree?

UC IPM explains that sooty mould does not infect the plant. It grows on the honeydew, but a heavy coating blocks light, reduces photosynthesis, and can lead to early leaf drop. Clemson’s container citrus factsheet describes it the same way, and adds that managing aphids, whiteflies and soft scales prevents it. Once the honeydew stops, the mould gradually decreases.

Will a fan or more air movement help?

We found no extension guidance showing that a fan prevents scale, mealybugs or whiteflies on indoor citrus, so do not rely on one as a pest treatment. A few related points are supported:

• Sooty mould is not a leaf disease that airflow dries out. UC IPM says the only lasting fix is controlling the insects that make honeydew.
• Dust matters. UC IPM’s whitefly guide notes that dusty conditions favour whiteflies and disrupt their natural enemies. Clemson suggests wiping dust from indoor citrus leaves to help reduce mite and scale problems.
• Avoid cold draughts. RHS warns in its citrus growing guide that cool or fluctuating temperatures, dry air and cold draughts can make citrus drop leaves or fail to flower and fruit. Keep any fan from blowing cold air straight onto the tree.
• Pollination does not need a fan. RHS and Clemson both describe most citrus as self-fertile. Clemson notes tangerines as the exception that needs a second variety.

Prevent the next outbreak

• Inspect new plants before they join your indoor citrus.
• Check plants for scale, mealybug, aphids and whiteflies before bringing them back indoors in autumn, and rinse the foliage outside first, as Iowa State recommends for all houseplants.
• Keep the tree watered and fed but not overfed. Clemson notes that drought-stressed citrus is more susceptible to scale, and that over-fertilizing can increase pest problems.

If your citrus is dropping leaves after a recent move or repot rather than after an infestation, see our indoor citrus transplant shock guide.

Related guides

Other indoor citrus problems we cover: removing thorns from citrus tree, sunburn.

References

• UC IPM: Pest Notes: Sooty Mold
• UC IPM: Scales
• UC IPM: Whiteflies
• Clemson Cooperative Extension HGIC: Citrus Insects & Related Pests (HGIC 2221)
• Clemson Cooperative Extension HGIC: Container Citrus Production (HGIC 1363)
• University of Maryland Extension: Mealybugs on Indoor Plants
• University of Maryland Extension: Whiteflies on Indoor Plants
• RHS: How to grow citrus
• Iowa State University Extension and Outreach: Moving Indoor Plants Outside for the Summer