Acromyrmex charruanus

Quick Summary

• Difficulty: Expert
• Origin & Habitat: Central Uruguay, specifically the Florida Department, found in Eucalyptus globulus plantations nesting within thatch-mound nests of Acromyrmex heyeri at elevations around 224m ^[1].
• Colony Type: Socially parasitic, workerless inquiline. Queens live permanently inside host colonies of Acromyrmex heyeri and possess no worker caste of their own ^[1][3].
• Size & Growth:
  • Queen: 6.64-7.23 mm total length ^[1].
  • Worker: No workers present, species is workerless ^[1].
  • Colony: Single queen supported by host colony workers ^[1].
  • Growth: Not applicable, colony produces one batch of reproductives then dies (semelparous) ^[1][3].
  • Development: Not applicable, species does not produce workers. (Development time from egg to alate (winged reproductive) has not been documented. Alates are present in host nests during February ^[1].)
• Antkeeping:
  • Temperature: Not independently studied, follow requirements for host species Acromyrmex heyeri.
  • Humidity: Not independently studied, maintain conditions suitable for host leafcutter fungus gardens.
  • Diapause: Unknown, based on temperate South American climate, host may require winter rest period.
  • Nesting: Requires established thatch-mound or artificial nest of host species Acromyrmex heyeri with active fungus garden ^[1].
• Behavior: Passive and dependent on host workers. Queens are tolerated by host colonies and do not fight with the host queen ^[1]. Males and queens attempt to fly immediately when nest is disturbed, suggesting regular mating flights occur rather than in-nest mating ^[1].
• Common Issues: requires a mature colony of the host species Acromyrmex heyeri, which is difficult to obtain and maintain., host colony will likely die after the parasite reproduces, making this a single-use display rather than a permanent colony., no workers means the queen cannot forage or care for brood, entirely dependent on host workers., extremely rare in collections and likely protected given limited known range., reproductive timing is fixed to austral fall (february), outside this window, no alates will be present.

Social Parasitism Biology

Acromyrmex charruanus is an obligate inquiline social parasite, meaning it must live permanently within the nest of its host species, Acromyrmex heyeri ^[1][2]. Unlike temporary parasites that eventually kill the host queen, A. charruanus is queen-tolerant, the host queen remains alive and continues to produce workers that serve the parasite ^[1]. The parasite has lost the worker caste entirely, genetic and morphological analysis of 144 individuals from two parasitized colonies found no evidence of workers ^[1][3]. The parasite queen produces only alate (winged) sons and daughters. The species reproduces semelparously: after producing a mass of reproductives during the austral fall (February), the colony collapses and the host nest dies ^[1][3]. This makes the species essentially impossible to maintain long-term in captivity, as the colony has a built-in expiration date after its single reproductive event.

Host Requirements and Colony Collapse

To keep A. charruanus, you must first maintain a healthy colony of Acromyrmex heyeri, a leaf-cutting ant that builds large thatch-mound nests ^[1]. The parasite only invades established colonies, there is no known method for founding a colony from a single queen in captivity. Once established, the parasite drains host resources to produce its own reproductives. Observations suggest parasitized host colonies cannot recover from the mass rearing of parasite brood, both known parasitized colonies were found empty with collapsed mounds three months after parasite alates were observed ^[1]. The fungus gardens of parasitized colonies appear tattered and reduced compared to healthy host colonies ^[1]. This means any attempt to keep A. charruanus will inevitably result in the death of the host colony, making this species unsuitable for standard ant keeping.

Morphological Distinctions

Queens of A. charruanus are easily distinguished from their host A. heyeri queens by their much smaller size (6.6-7.2 mm total length vs larger host queens), darker brown coloration, and abundance of long, recurved, coarse hairs covering the body ^[1]. They possess relatively longer appendages, mandibles, and antennal scapes compared to their body size, along with a shorter gaster ^[1]. Males are the smallest known in the genus Acromyrmex and can be distinguished from host males by their dark color, long coarse setae, and quadrate heads ^[1]. These morphological differences help field biologists identify parasitized colonies during the February survey period when parasite alates are present.

Reproductive Timing and Mating

Unlike most Acromyrmex species that reproduce in spring (October-December), A. charruanus has evolved a diametrically opposed mating season in the austral fall (February) ^[1]. This temporal separation prevents hybridization with the host. When parasitized nests are opened, alate reproductives immediately attempt to fly, indicating they perform regular mating flights rather than mating inside the nest ^[1]. The sex ratio varies by colony, one colony produced 74 gynes and 119 males (ratio 0.62), while another produced 7 gynes and 6 males (ratio 1.17) ^[1]. After mating flights, the colony system collapses, with an estimated parasitism rate in the wild of only 2% of host nests ^[1].

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References