Brachymyrmex

Workers have nine antennal segments with no antennal club, distinguishing them from most ants which have more segments. The petiole is concealed by the gaster in dorsal view. Workers are minute (maximum 2.5 mm in B. patagonicus), with soft metasoma and simple morphology that makes detailed observation difficult. The genus lacks the hairy legs and distinct antennal funiculus proportions that characterize some related taxa.

Native range: naturally disturbed habitats in South America, particularly Argentina and Paraguay. Introduced range: urban and residential environments, buildings, and structures; occurs in areas with specific vegetation types, canopy cover levels, and soil bulk density. Invasive populations thrive in human-modified landscapes across the southern United States.

Native: Neotropical region from United States to Argentina and Chile, including Caribbean islands; specifically Argentina and Paraguay for B. patagonicus. Invasive range: B. patagonicus introduced to southern USA (first reported 1976, now 15 states from Georgia to California), Bahamas, continental Asia, Spain, Martinique; indoor occurrences in England, Netherlands, and Japan with uncertain outdoor survival. Some Brachymyrmex species introduced to Japan and Madagascar.

Nests are small and contain hundreds of workers. New queens and males are produced in nests. Dispersal occurs primarily through nuptial flights rather than colony budding; no isolation by distance suggests strong dispersal ability.

B. patagonicus exhibits a multicolonial structure unusual for invasive ants: each colony occupies a single nest with strong aggression toward non-nestmates, including biting and formic acid spraying that escalates to fighting with locked mandibles. Colony boundaries are maintained through nestmate recognition mediated by cuticular hydrocarbon chemical differentiation. Aggression levels between colonies correlate with genetic difference. Foraging trails from different colonies can occur in extremely close proximity (few centimeters) with mutual tolerance at trail level but aggression between individual workers. Workers do not bite or sting humans.

B. patagonicus is an ecologically dominant invasive species that reaches extremely dense populations (colonies every 2.5 meters in some areas). Unlike most invasive ants that eliminate intraspecific competition through supercolonial cooperation, this species maintains high densities despite strong territorial aggression between colonies. Acts as a natural enemy of aphids: B. patagonicus has been observed subduing adult crapemyrtle aphids (Sarucallis kahawaluokalani).

B. patagonicus is a significant nuisance pest infesting buildings in urban environments across its invasive range. Workers are small, do not bite or sting, and do not mechanically transmit disease. The species has spread rapidly since its first detection in the United States in 1976, becoming established in 15 states.

• Wasmannia auropunctata (little fire ant)Similar tiny size and yellow coloration; distinguished by W. auropunctata's painful sting and different antennal segmentation.
• Plagiolepis alluaudi (Alluaud's little yellow ant)Similar minute size (<2 mm), yellow color, and invasive status in urban environments; distinguished by P. alluaudi's extreme polygyny and unicolonial structure versus B. patagonicus's monogyne multicolonial organization.
• Paratrechina longicornis, Pheidole megacephala, other tramp ant speciesShare global invasive status and urban habitat preference; distinguished by Brachymyrmex's unique nine-segmented antennae and concealed petiole.

• invasive species
• urban pest
• Formicinae
• rover ant
• multicolonial
• monogyne
• aphid predator
• nine antennal segments
• cuticular hydrocarbons
• nestmate recognition

• BugGuide
• Wikipedia
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• During Quarantine, an Entomologist Takes a Closer Look at a New Invasive Ant Species
• Crapemyrtle Aphid: A Cosmopolitan Pest of a Popular Ornamental Tree
• crapemyrtle aphid natural enemies - Entomology Today
• Abiotic and biotic factors affecting the distribution of Solenopsis invicta Buren, Brachymyrmex sp., and Linepithema humile (Mayr) in East Baton Rouge Parish, Louisiana
• The underdog invader: Breeding system and colony genetic structure of the dark rover ant (Brachymyrmex patagonicus Mayr)
• Divide and conquer: Multicolonial structure, nestmate recognition, and antagonistic behaviors in dense populations of the invasive ant Brachymyrmex patagonicus