Identifying ants with large thatch mounds (Joshua, Portland, Oregon, USA)


I'm trying to decide whether to exterminate the ants living in grass/twig/leaf mounds around my property. If they are not native I intend to try to eliminate them.
I live about 30 minutes outside of Portland, Oregon at about 1500ft elevation. My land is mostly wooded but with grassy, sunny areas with orchards trees and garden areas. Over several acres there are at least 6 ant mounds each 6-24" tall and 12-24" across in the grass or at the edges of the woods.

The ants have red heads and near black bodies. They are about 1/4" long (not less than that but not close to 1/2"). From looking over some ID keys online here's what I know to look for that describes these ants:

Petiole with 1 node and is very distinct (spiky)
Thorax is uneven
Head is red and thorax and abdomen are black or very dark brown
Antennal clubs are indistinct (or absent)
Eyes are black
No stinger is visible
Seem to be two (maybe three) black dots between the eyes and much smaller than the eyes.

If there's a good resources that you can point me toward to do my own research that would be great too.

I hope that you might be able to help.

Thanks much,
Joshua (Portland, Oregon, USA)

Dear Joshua,

Thank you for the very detailed and helpful description of the ants you are finding on your property. This really helps to narrow down the ants you are likely finding. From your notes on the ants themselves and their mounds it sounds like you likely have Formica (wood ants). You can see images of many of the species here and here.

These are likely native species and although their nests can be unsightly on a well-manicured lawn they are no threat to you, your children or pets, or home. The species you are finding probably belongs to the "Rufa group".

To identify ants you find on your property in the future, I would recommend "Ants of North America: A Guide to the Genera" by Brian L. Fisher and Stefan P. Cover.

Thanks for contacting us and keep enjoying your native ant fauna,
Corrie Moreau & the AntAsk Team

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