Sticky substances on the leaves and stems of house plants are an indication that pests are making a meal of the plant and leaving their trail behind. Prompt removal of an infestation can protect the plant from further damage.
A sticky substance found on a house plant is often honeydew, an excretion left behind by pests, usually scales or aphids, which live and feed on the plant.
The most common house plant pest that leaves a sticky trail is scale. These tiny, round or oval-shaped pests appear on the plant as brown or grey bumps and will usually be found on stems, undersides of leaves and along the veins. A heavy infestation can result in yellow and dropping leaves and stunted growth.
Aphids, which are unusual on houseplants, are small insects that congregate around flower or leaf buds.
If caught early, most house plant infestations can be controlled by washing with a mild detergent solution (1/2 tsp non-bleach dish detergent per quart of room temperature water). For heavier infestations, an insecticide labelled for use on scales or aphids and safe for houseplants can be used on the plant. Heavily infested plants that are already dropping leaves or showing signs of stunted growth should be discarded.