By Donald Lewis
Iowa State University Extension
I can tell it's springtime by the number of “"big, black ants" that I see. “"Big, black ant" is not a technical description, of course, but it is a fairly common and accurate way for people to describe the insects they see walking on the kitchen counters, crawling around inside the dishwasher or trundling up and down the old tree trunk next to the house.
The description of “"big, black ant" nearly always refers to carpenter ants, even though other species of ants are nearly as large and black, also. Carpenter ants are much feared, usually more so than is warranted. On occasion, carpenter ants do ruin window frames, door sills, deck boards and other things made of lumber. However, in almost all cases of severe damage there is also a persistent and long-standing moisture problem. Too much moisture in lumber leads to both wood decay and carpenter ants. It's hard to say which lets you down faster.
Not all carpenter ants inside the house indicate an indoor infestation. If you see a few carpenter ants indoors, especially in springtime, these nuisances could be accidental invaders that have wandered in from an outdoor nest location. Research indicates the ants are capable of wandering long distances; the nest may not even be near the house. Common outdoor carpenter ant nest sites include hollow trees, dead limbs, stumps, old firewood and landscape timbers. Carpenter ants chew sandpaper-smooth tunnels and chambers in wood for their nests. Shredded fragments of wood that look like coarse sawdust are ejected from the nests, along with dead ants and bits of dead insects that the carpenter ants have eaten.
Carpenter ants are commonly found nesting inside older trees that are hollow or in trees with dead limbs and branches. The nests are made in wood that had already started to decay because of excess moisture let in through an opening to the outside such as mechanical injuries, knots, cracks, holes and old insect tunnels.
Carpenter ants in trees are not directly harmful to the tree. They are only taking advantage of an existing situation. The opening to the outside, the moisture, and the soft, decayed wood already existed. The ants came second. The ants may extend the damage into sound wood and an injured tree may be further losing structural strength. Still, ant control is not essential for the tree's health.
Control of carpenter ants inside a tree may be warranted, however, as one way to reduce invasion of the ants into adjacent structures. Insecticides labeled for use on trees in the landscape can be applied (dusted or sprayed) directly into the nest cavity. A treatment is not likely to permanently rid a tree of carpenter ants; retreatment every year or so may be necessary.
Plugging or sealing tree cavities or treating tree wounds with wound dressings is not advised. Such treatments are unnecessary and will not eliminate nor prevent decay or carpenter ant activity.
Cutting down an otherwise viable landscape tree in the hopes of avoiding ant problems in the home is an extreme response and is not recommended. If someone suggests removing trees from your landscape to control ants, remember that complex problems rarely have such simple solutions. Check and double-check with qualified arborists before making drastic and irreversible assaults on your landscape plantings.
A carpenter ant picture from the USDA is online at https://bugwoodcloud.org/images/768x512/1435184.jpg.
A guide to common ants of Iowa, with drawings, is online at https://hortnews.extension.iastate.edu/guide-common-ants-iowa.