They are called Thatching Ants because they construct a mound or thatch of plant material. They clean up needles, plant materials, twigs and leaves. They are located around small trees, shrubs or rocks. They eat honeydew from aphids, mealybugs, scale insects, etc. that are found on trees and shrubs.
Thatching Ants should be left alone if they are not bothering you with any gardening or other activity because of the benefit they provide in the environment. If they are in your way, then we can take care of them. They can be aggressive; however, and will pinch you and squirt formic acid in the wound, which feels like a sting. They are not wood destroying and normally don't set up a colony inside the house, but we have had occasions where they have moved into a structure. One species we have found does excavate wood in landscape timbers. That is one of many reasons not to use Railroad Ties in your yard.
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Courtesy of Washington State Extention