There is a sticky residue on some of the leaves of my houseplants and what appear to be little bugs under some of the leaves. What can I do to get rid of this problem?
— Toby Jackson, Chicago
It sounds like your houseplants have an infestation of aphids. Aphids feed on sap from the plants and excrete excess sap as honeydew, which leaves a sticky residue like you describe. Black sooty mold can also begin to grow on the honeydew. This is not only unsightly but also inhibits photosynthesis, which weakens the plant. Some aphids attack plants in outdoor gardens, but natural predators generally keep their populations under control. These natural predators are missing from home environments, so aphid populations can quickly build up if they are not controlled.
Aphids are tiny, pear-shaped (no larger than 1/8-inch long) insects. They are piercing and sucking insects that come in a range of colors including pink, black, yellow, green, orange, red and brown. They tend to prefer new growth, which can become completely covered by the aphids. Aphids can also be found in the center of new shoots and under leaves, which can cause the leaves to curl. Most aphids do not lay eggs indoors, but adult females can give birth to young without mating. Young aphids look like adults and begin feeding right away. They molt approximately four times before maturing, so the small, white, shed skins on countertops and tops of the containers’ growing medium are also signs of an aphid infestation.
If you have space, try to isolate infested houseplants. This will reduce the spread of the aphids to other plants. Try to remove aphids manually by spraying them off in the shower or sink. A spray bottle filled with water is another way to rinse them off. It can take some patience to get at all the aphids on the underside of leaves. Another technique is to rub them off with your fingers or to use a cotton ball soaked with a bit of rubbing alcohol or soapy water. You may want to test the rubbing alcohol on a couple of leaves and monitor for a day or two to make sure it does not damage the foliage. You can also try turning the plant upside down and dipping the foliage in a bucket of water and swishing it around to remove the aphids if it has delicate foliage that could be damaged by other methods of control. Secure the growing medium in the pot with a cardboard disk or newspaper to prevent it from falling out while the pot is upside down. Heavily infested parts of the plant can be pruned out if they do not ruin the plant’s form. This time of year is good for pruning houseplants.
Another option to consider is an insecticidal soap. Insecticidal soaps work by contact, so you need to spray all parts of the plant infested with aphids to kill them. Closely monitor your plants in the coming weeks to make sure all the aphids have been removed. You should expect to do some follow-up treatments to completely eradicate them.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
