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The fungus gardening ant, Trachymyrmex septentrionalis, is an abundant and widespread member of the New World ant tribe Attini.  Ants in this tribe are remarkable in that all species have established a symbiotic relationship with a specific fungus species, whereby they cultivate underground gardens of the fungus as food for their colonies.  While most fungus-growing ants live in the tropics, Trachymyrmex septentrionalis is one of a few that has radiated up into the temperate zones of the United States. 

Trachymyrmex septentrionalis has the widest range of any North American attine ant and the only one occurring east of the Mississippi river.  It occurs in arid areas from Texas across the southeast, but also in areas of notable cold, as far north as Long Island, and west to central Illinois and southern Ohio.  This species is unique among the ants in tribe Attini in that colonies over most of the range go dormant in the winter and those furthest north have only a short active season (4-5 months).  In the south where temperatures remain greater than 64oF (18oC), T. septentrionalis colonies remain active all winter. 

While their geographic range is large, its distribution is patchy.  Trachymyrmex septentrionalis and the symbiotic fungus they cultivate in their nests require arid, sandy soils.  Further north, T. septentrionalis occurs exclusively in open habitats with pure sand soil.  In the southeastern United States, Trachymyrmex septentrionalis are very common in multiple habitats, preferring open areas with light shade.  Seal and Tschinkel (2006, 2010) describe T. septentrionalis as a “characteristic animal” of the arid longleaf pine forests along the Gulf coast of the United States.  In these forests T. septentrionalis colonies occur in large numbers, with up to 1000 nests per hectare.  They are important species in recycling nutrients in the nutrient-poor soil in this habitat.

Trachymyrmex septentrionalis are smallish, red-brown ants with well-developed eyes.  The workers are all fairly similar in size.  Their colonies are fairly small, about 300-400 individuals in a mature nest.  They create fairly simple underground nests with one or several chambers.  However they move these chambers seasonally, digging deeper in warmer seasons and avoiding areas where the water table level is too high to permit a range of depth.  Their digging behavior is thought to track ideal and specific temperature and humidity conditions for the fungus gardens on which they are dependent.  Especially in the spring and fall when workers are in high gear excavating their nest, their colonies can be easily located by the distinctive and visible crescent-shaped excavation mounds. 

Trachymyrmex septentrionalis forage for a variety of debris they use to fuel their fungus gardens.  This includes dead plant matter, insect frass, and occassionally living plant leaves that they cut into movable sizes.  Winged reproductive male and female ants leave the nest on nuptial flights during summer months to start new colonies.  The females, which become the new colony queen, carry a small piece of fungus from the old colony’s garden.  After she mates, she uses this to inoculate the new nest’s fungus garden.

The name septentrionalis refers to the seven brightest stars in the Northern Hemisphere’s Great Bear constellation, a nod to the northern distribution of this species. 

(MacGown 2014; Rabeling et al. 2007; Seal and Tscinkel 2006; 2010; Seal Gus and Mueller 2012)

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© Dana Campbell

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