Several ant species “herd” the small pear-shaped insects called aphids and “milk” them for a sweet fluid that they excrete after feeding on sap from roots or leaves.
They are sometimes called dairying ants. Ants that “milk” root-eating aphids often dig them chambers to serve as “barns.”
The ants protect their aphids, carry them from plant to plant, and gently stroke them with their antennae to get them to release droplets of the liquid, called honeydew. Some species of yellow ants called Acanthomyops feed almost exclusively on honeydew harvested from roots by aphids.
Aphids harm houseplants and some crops by sapping them of their fluid and sometimes by coating the leaves with so much honeydew that mold grows, inhibiting photosynthesis.
Agricultural extension agents recommend some steps to counter aphids on houseplants without using commercial insecticides. First, isolate new plants until it is obvious that they are not infested, or remove the infested leaves.
If an infestation is advanced, try swabbing or spraying the leaves with rubbing alcohol or soapy water, repeating at intervals of a few days to get rid of eggs and new hatchlings; rinse off the soap.
Or buy ladybugs, which feed on mites, not plants.