It's one thing to have ants in the yard, but once they start nesting in your potted plants, even the most tolerant homeowner will be ready to eradicate them. While ants cause few problems for houseplants by themselves, their presence may indicate another pest problem more deserving of your attention. Keeping an eye out for these little critters in your plants could clue you in to a more serious problem before it's too late.
Ants belong to the same family as bees, wasps and termites, a family of insects that lives in social groups called colonies. Most ants lack wings, although a colony will release a short-lived, winged reproductive generation a few times per year. In North America, ants tend to be small---one inch or less in length---and brown, black or reddish in color. Ants have a "waist" between their body segments and segmented antennae, which distinguishes them from their insect relatives. Because ants live in communicative colonies, where you find one, you may soon find hundreds.
Although some species eat seeds and seedlings, according to the University of Kentucky entomology department, ants cause little direct damage to houseplants. If you see large numbers of ants on your houseplants, the risk to your plants' health likely comes instead from what attracts them: sap-sucking bugs like aphids, scale and mealybugs.
Aphids, scale and mealybugs drain sap from your plants, taking water and nutrients as they do so. In turn, they excrete a sticky, sugary substance called honeydew. Honeydew attracts ants so readily that some ant species form symbiotic "farming" relationships with the pest insects, protecting the pests in exchange for the nutrient-rich honeydew they crave. Extensive infestations of sap-sucking insects can cause serious harm to your houseplants and, eventually, plant death.
Observe your houseplant closely for signs of the insects that attract ants. Although some species are tiny and hard to see, the honeydew will leave a sticky residue on the leaves that you can feel. According to the University of Delaware Cooperative Extension, controlling insect pests that attract ants will usually prevent ant problems as well, as ants begin foraging elsewhere for food. Insecticidal soap or spot treatment with rubbing alcohol controls these pests.
If another insect pest isn't involved, inspect your potting soil for signs of nesting activity. If ants have nested in the pots, transfer the plants to clean pots with fresh potting soil, taking care to not to take the ants with you. If ants continue to enter your home, the University of Delaware Cooperative Extension recommends ant bait traps as the most effective solution against ant invasions.