Pogonomyrmex Mystery: What Are Those Spots?

While visiting a local park this week, I stopped by to visit a Pogonomyrmex nest I have been watching over the years.

black-spots-around-harvester-ant-nest

I almost immediately noticed the black spots around the nest entrances.

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Here's another entrance about 20 inches from the first two. There was a noticeable blackening around the hole in comparison to the surroundings. Otherwise, the ants seemed to be active and doing fine.

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This is a closer view.

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Looks kind of shiny.

My first thought was ant feces. Studies have shown that some ants mark their nest areas with feces, for example a study by Grasso et al. (2005).

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My other guess is that someone tried pouring something into the nest (as it is a public place.)

What do you think? Have you seen this before?

Grasso, D. A., Sledge, M. F., LE Moli, F., Mori, A., and Turillazzi, S. 2005. Nest-area marking with faeces: a chemical
signature that allows colony-level recognition in seed harvesting ants (Hymenoptera, Formicidae). Insectes Sociaux 52:36–44. (.pdf available for viewing)

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