Thatching Ant Mound

2014 April 9

I was walking in the woods to the northwest of the house with the girls[1] on June 2, 2013, when we quite literally stumbled across this big anthill. As in, Rosie tripped over it. So, I made a note of the location, and came back later with a camera and a ruler. The mound was a mixture of dirt and bits of dead grass, with live grass growing in it. It stood about six inches tall, and was about a foot wide.

This mound is characteristic of ants in the Formica genus, which are also known as “wood ants” or “mound ants”.

It was a coolish day, so I had to dig into the mound a bit to find some ants.

They of course did not take kindly to this, and decided to let me know they were displeased. Luckily, they don’t have actual stings, and while the bite is a bit painful, it’s nothing that one can’t shrug off long enough to get a picture.

I think they are likely to be ants in the Formica rufa species group, which includes the Thatching Ant, Formica obscuripes. The ID features are kind of subtle, though, and I didn’t actually get pictures to show them well enough to tell if they are that exact species or not. So I’m mainly basing this on the range maps, the nest style, and the fact that the Thatching Ant is the only one in this species group that has a common name, and is therefore probably the most, well, common [2].

These ants are pretty fiercely predatory, and are also commonly found farming aphids for their honeydew. In fact, here are some that I previously photographed tending some scale insects on one of our plum trees. They are not at all shy about biting, either. If you wanted to smear someone with honey and stake them out on an anthill, this would be one of the kinds of anthills that would be appropriate to use.

I gather that the mound really is mostly above-ground. See this paper for more details about their nests, and possible reasons why they are built like that. Rather than digging down underground like other ants do, these ants dig down a bit to get dirt, but then bulk it up with leaf litter and dead grass. This might work out pretty well around here, where we have deep snow. An underground nest is likely to get flooded when the snow melts, and could take a long time to warm up in the sun, while the surface nest will stay relatively dry and warm up right away. So, this mound isn’t an indication of a massive, deep nest – what we see is pretty much all there is.

[1] If I remember correctly, Sam and Rosie were hunting fairies at the time.

[2] BugGuide says that the common name of “Thatching Ant” is only associated with the one, most common species in this group, and that the others “don’t have common names”. I don’t think this is realistic, though, and in fact I think it kind of flies in the face of how common names are even used. As far as I’m concerned, the whole idea of a common name is that it is something that can be used casually to refer to an insect well enough that another person will have a good idea of what it is, but without worrying too much about the exact species definition. By this way of thinking, the name “Thatching Ant” would apply equally well to all of the generally-similar members of the genus that build these thatch-pile nests, and not just the particular species Formica obscuripes

2 Responses
  1. James C. Trager permalink
    April 9, 2014

    This genus of ants is a group of particular interest to me, and I’ve done some taxonomic research on them. So, your pictures are good enough for me to identify the ants of this post as Formica ulkei, recognized by shininess, concave rear margin of head, color pattern, and mound with lots of earth in it. This is not taxonomically or nest-structurally (if you will) a thatching ant, as they have the expossed psrt of the mound almost exclusively of twigs, stems and, or conifer needles.
    Not as sure about this one, because characters are subtle, as you say — But I’m about 90% sure that the ants tending scale on your plum are the thatching ant’s close relative F. obscuriventris, the most abundant of the thatching ant group in eastern US.

  2. April 9, 2014

    Thanks very much! It’s good to know for certain what species this is. It will be interesting to track down the similar ants we find around our house, and find out just how many of its relatives we actually have around here.

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