Thatching ants are beneficial to trees.

From: Nancy
Subject: Thatch ants Washington State

We have a red-headed thatch ant colony more than six feet across near
large fir, cedar and big leaf maples. I am told variously that these
ants are beneficials, do not damage homes, are only moderately keeping
of aphids on ornamentals, and while they can bite if provoked, are
not dangerous. All this being said, my question is: Do they in any
way damage the trees, particularly the conifers, adjacent to their
nest? I would prefer to take the live-and-let-live approach and leave
them to enjoy their metropolis. We enjoy watching them and will leave
them alone if they aren't damaging the structure of the very large
trees near their mound.

Thank you in advance for your help.

Dear Nancy:

Thank you for your message to the ant blog. It sounds indeed like the ants you describe are the famous thatching ant, Formica obscuripes. The species abounds in some parts of the western conifer forests, and curiously, also extends south to the sagebrush plains of New Mexico and east to the prairies and old fields of Wisconsin and Michigan! Though it does befriend aphids, this may be considered the plant-to-ant equivalent of the cost of feeding a standing army. F. obscuripes is considered a beneficial insect over all, because of its prodigious predation of plant-eating insects, including such pests as spruce budworm. We applaud your live-and-let-live attitude, and are very pleased you chose to ask before acting.

Best regards,

James C. Trager of the Ask Ant Team

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