Ants around the outside perimeter of their home are often there because the ants are feeding on “honeydew” on foundation plants. Honeydew is the term for the anal excretions of certain insects that suck plant juices: like aphids, whiteflies, and scale insects. Honeydew can be seen as sticky, sugary droplets on the plant, or in the case of houseplants, on the floor or table surfaces below the plant. Ants love to eat honeydew and will actually tend or take care of aphids and other insects that produce honeydew. The ants offer protection from predators and the aphids reward the ants with sweet honeydew droplets. In the case of outside plants, eliminating the aphids on the plants will usually get rid of the ants.
But the honeydew attraction can come from inside as well. This past weekend, I discovered that two of my indoor houseplants were infested with ants, which meant that they were also infested with plant pests. The plants were on two different floors of the house and were infested with two different types of ants! The ants were there because they had discovered “honeydew” on the surfaces of the leaves.
It’s my own fault. I knew that these two houseplants were infested with scale insects. I could see the flattened scales on the leaves and stem. I think the plants were infested when I bought them and it just took several months over the winter for the infestation to build up enough to be noticed. I was waiting for a nice warm day to take the plants outside where I could treat the foliage. But the unusually warm early spring started the ants foraging and the ants beat me to the plants.
The downstairs plant in front of the sliding glass doors to the patio was infested with large carpenter ants that were trailing in from the patio. The upstairs plant was infested with tiny acrobat ants that found their way in around the window frame. Neither group of ants was nesting inside the house, but sometimes you need an expert to determine that for sure.
How did the ants from outside find the plants inside? Ants forage randomly all the time looking for food. Often one or two ants will find their way into a home. That’s easy for the ant. Every home has plenty of little cracks and openings that an ant can get through. They often enter around a door or window. An ant or two inside isn’t a crisis, but a whole bunch of ants inside means something is going on.
Since houseplants are usually located near a window, the ants don’t have far to go once inside to detect the honeydew droplets. In this case, the ants were entering the house directly at the backside of the plant against the window where I couldn’t see the ant trail. The activity had apparently been going on for some time before I noticed it. If the occasional ant forages inside but doesn’t find food, it will wander back outside if it can. If the ant does find suitable food (like a plant infested with honeydew), it will return to the nest and recruit other ants to share in the bounty. Soon there is a trail of ants from the outside to the inside to the infested plant. Sometimes the ants will move from the plant to other areas (like the kitchen) in the house.
Getting rid of the ants on the plants isn’t such a problem. Once the honeydew-producing insects are gone, the ants will stop foraging to the plant. Just be sure to wash all surfaces — leaves, table, and floor of honeydew droplets (not such an easy job, the stuff is sticky!) Also clean surfaces where you saw ant trails to remove the attractant pheromone. The important thing is to know where your ants are coming from. If you think they might be nesting inside your house, or if you keep seeing them inside after you have cleaned up the honeydew, you should call a pest control professional for a thorough inspection.