Rover Ants (Brachymyrmex patagonicus), an emerging pest species

As if we didn’t already have enough pest ants to worry about, here is a relatively new one. The rover ant Brachymyrmex patagonicus, a tiny South American species, has been working its way under the radar across the southern United States. Its presence is now large enough that pest control companies are reporting a sudden increase in requests. According to gardeners I’ve talked to, these ants emerged in huge numbers here in Tucson about 5 years ago. Given the interest in this species, I thought I’d post a summary of what we know of this emerging pest and how to reliably identify it.

Distribution in North America. The map below is adapted from MacGown et al (2007), with additional data on the southwestern cities taken from my own collections, credible rumors from pest control folks, and from Dale Ward’s Ants of the Southwest. Brachymyrmex patagonicus appears to be spreading in the southeast and has recently arrived in various western cities. I would not be surprised if it is already established in San Diego and Los Angeles. If you have seen this ant in a location not indicated below, please drop a comment or send me an email.

Biology. We do not know much about this species. Colonies nest in soil and leaf litter, often under covering objects such as stones. They tend hemipterous insects for honeydew and visit nectaries. In Tucson, mating flights occur year round, even in mid-winter on warmer nights. Alates are attracted to lights, generally appearing shortly after dusk, and their reproductive output is impressive. I’ve been running blacklights in urban Tucson for a couple of years, and B. patagonicus alates consistently outnumber those of all other ant species combined. Here is a mating pair:

The ecological effects of the Brachymyrmex invasion remain largely unstudied. (If you’re an aspiring myrmecologist looking for a thesis project, this one’s ripe for the picking!) The subjective consensus of myrmecologists is that B. patagonicus won’t become a major pest in the way that, say, fire ants and Argentine ants are, but they do enter houses and might be regarded as a small nuisance for homeowners. In Tucson, B. patagonicus co-exists with native Solenopsis, Pheidole, Dorymyrmex, Camponotus, Forelius, and Pogonomyrmex without any obvious effect. On the negative side, they appear to have displaced a native Monomorium in my yard, but that is a single observation and is purely anecdotal.

Identification. All Brachymyrmex species have only nine antennal segments in the worker caste, reduced from the 11-12 segments typical of most other ant genera. This trait makes identification to genus relatively simple with sufficient magnification. That, and the fact that Brachymyrmex are really small. Few species break 3mm in length, and B. patagonicus is usually between 1 and 2mm.

To confirm that an ant is Brachymyrmex patagonicus, look for this combination of characters:

  • Eyes relatively large
  • Several long erect hairs present on the mesosoma
  • Body usually medium to dark brown in color
  • Appressed hairs on the abdominal tergites relatively sparse, so that the integument appears smooth and shining

For comparison, here is a photo of Brachymyrmex obscurior, a similar species that is also introduced from the tropics. Notice the smaller eyes and the denser covering of appressed hairs on the gaster:

Another commonly encountered Brachymyrmex in North America is the native species B. depilis. Notice the small eyes, the light color, the lack of standing hairs, and the dense covering of appressed hairs on the gaster:

One final nomenclatural note. Brachymyrmex patagonicus in the U.S. has sometimes been referred to as “B. musculus” based on an earlier misidentification. However, Argentine Brachymyrmex expert Estela Quirán recently confirmed the match between South American B. patagonicus and the North American populations, so the taxonomy of this new invader appears stable for now.

Resources:
MacGown, J.A., Hill, J.G., Deyrup, M.A. 2007. Brachymyrmex patagonicus (Hymenoptera: Formicidae), an emerging pest species in the southeastern United States. Florida Entomologist: Vol. 90, No. 3 pp. 457–464.

Quiran, E.M., J.J. Martinez, Bachmann, A.O. 2004. The Neotropical genus Brachymyrmex Mayr, 1868 (Hymenoptera: Formicidae) in Argentina. Redescription of the type species, B. patagonicus Mayr, 1868; B. bruchi Forel, 1912; and B. oculatus santschi, 1919. Acta Zoologica Mexicana 20: 273-285.

Mississippi pest control extension sheet on B. patagonicus.

Joe MacGown’s in-depth page on B. patagonicus

Specimen images from Antweb and from Joe MacGown.

56 thoughts on “Rover Ants (Brachymyrmex patagonicus), an emerging pest species”

  1. You can see an up-to-date range map by querying the species at iNaturalist.org – it includes a wider range than shown here. Updating the page here would be a never-ending process, so perhaps just look at iNaturalist. Of course, some of the iDs there may be wrong.

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