Atta sexdens

Atta sexdens is a leafcutter ant species in the ant family Formicidae (order Hymenoptera), subfamily Myrmicinae [1]. It belongs to the tribe Attini, the fungus-growing ants, and is a member of the genus Atta(established by Fabricius in 1804) [2]. Approximately 15 species are recognized in genus Atta, which are among the most conspicuous Neotropical ants [2]. The original description of A. sexdens was by Linnaeus (1758) under the name Formica sexdens, later placed in Atta (Fabricius, 1804) [1][2].

Sources: [1] Ramalho, M. d. O., Martins, C., Morini, M. S. C., & Bueno, O. C. (2020). What Can the Bacterial Community of Atta sexdens (Linnaeus, 1758) Tell Us about the Habitats in Which This Ant Species Evolves? Insects, 11(6), 332. https://doi.org/10.3390/insects11060332 [2] Maurício Bacci, Scott E. Solomon, Ulrich G. Mueller, Vanderlei G. Martins, Alfredo O.R. Carvalho, Luiz G.E. Vieira, Ana Carla O. Silva-Pinhati, Phylogeny of leafcutter ants in the genus Atta Fabricius (Formicidae: Attini) based on mitochondrial and nuclear DNA sequences, Molecular Phylogenetics and Evolution, Volume 51, Issue 3, 2009, Pages 427-437, ISSN 1055-7903, https://doi.org/10.1016/j.ympev.2008.11.005.

Content

Morphology and Caste System

Atta sexdens exhibits extreme worker variety. Colony members are divided into four main worker castes defined by head width. The four worker castes are:

Gardener-Nurses: tiny workers (head width ~1.0 mm) that tend brood and fungus;
Within-nest Generalists: mid-sized workers (head width ~1.4 mm) performing a variety of nest tasks;
Foragers-Excavators: larger workers (head width ~2.2 mm) that cut, carry foliage and excavate;
Defenders: the largest workers (head width >3.0 mm) specialized in colony defense[3].

Worker body lengths span roughly 3–24 mm, reflecting this caste system. Larger “major” workers specialize in defense, medium workers primarily cut and haul leaves, and the smallest workers process leaf material and tend the fungus garden [3][4]. Leafcutter workers vary over a 300-fold range in body mass, with the largest mainly serving as soldiers and cutters and the smallest as fungus tenders [4]. Virgin queens are much larger (often >20 mm long, ~1 inch) and winged, whereas males are smaller (~8–12 mm) and short-lived [3] . Only the queen and males are sexual reproductives and all workers are sterile females [3].

Sources: [3] Byrne, A. (2002). Atta sexdens. Animal Diversity Web. [University of Michigan Museum of Zoology]. animaldiversity.organimaldiversity.org last used on 11.06.2025 [4] Helanterä H, Ratnieks FL. Geometry explains the benefits of division of labour in a leafcutter ant. Proc Biol Sci. 2008 Jun 7;275(1640):1255-60. doi: 10.1098/rspb.2008.0024. PMID: 18319212; PMCID: PMC2602677.

Colony Structure and Lifecycle

Atta sexdens colonies are founded by a single mated queen. Nuptial flights occur late in the wet season, that would be roughly October to December. queens mate with several males, fly up to ~10–11 km, land, shed their wings, and dig a small initial nest chamber [3]. The queen carries a pellet of the mutualistic fungus from her natal nest into the founding chamber, establishing the first fungus garden [3]. She subsists on her fat reserves and wing muscles until the first brood of workers emerges [3]. The first emerging workers are the tiny gardener-nurses, which tend the fungus and brood [3]. As the colony grows, the queen produces increasingly larger worker castes: within-nest workers, then forager/excavators, and finally defenders [3].

After about 2 years of growth, the colony begins to produce sexual brood (alate queens and males) [3]. At around 6 years old, a mature colony can be enormous: one report found ~1,920 chambers (248 with fungus gardens) and on the order of 5–8 million workers [3]. Colony expansion involves extensive excavation. A colony eventually turns over tens of tons of soil (a single colony can move on the order of 40,000 kg of soil) during nest construction [3]. Only ~2.5% of founding queens succeed in establishing a colony [3]. Surviving colonies and their queens can live for 10 to 15 years or more [3].

Role in Ecosystems

Atta sexdens colonies are major ecosystem engineers. By harvesting foliage and excavating vast underground chambers, they profoundly alter nutrient and carbon cycling [5]. The process of excavation brings nutrient-poor deep subsoil into contact with organic-rich surface layers, greatly increasing soil heterogeneity [5]. A single colony can aerate and turn over tens of thousands of kilograms of soil over its lifetime [3]. Leafcutter ants account for an enormous fraction of plant-eating in Neotropical forests: one estimate is that Atta and related leafcutters consume ~25% of available foliage and collect ~10–15% of standing leaf biomass in their foraging range [5]. Atta ants maintain a mutualistic relationship with the fungus they cultivate. The fungus provides the ants with various types of nutrients, while the ants, in return, feed the fungus and protect it from parasitic fungi of the genus Escovopsis. To defend against these parasites, the ants use bacteria from the genus Streptomyces[3].

Sources: [5] Swanson AC, Schwendenmann L, Allen MF, Aronson EL, on A-L, Dierick D, Fernandez-Bou AS, Harmon TC, Murillo-Cruz C, Oberbauer SF, et al. Welcome to the Atta world: A framework for understanding the effects of leaf-cutter ants on ecosystem functions.. Functional Ecology. 2019;(0(0). doi:https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2435.13319

Agricultural Impacts

Atta sexdens is a notorious agricultural pest. The intense leaf-cutting behavior of A. sexdens can damage young seedlings and mature trees alike [1]. In fact, leafcutter ants globally are estimated to cause agricultural damage on the order of billions of US dollars per year [1]. Chemical controls methods are applied, but control is difficult and often temporary [1].

Symbiosis with Fungi

Like all attine ants, Atta sexdens practices obligate fungiculture. Colonies cultivate a specialized basidiomycete fungus (genus Leucoagaricus, often L. gongylophorus) as their sole food source [2]. Leaf fragments are cut, chewed, and brought into fungus chambers where the fungus breaks down complex plant polymers. The fungus produces swollen nutrient-rich hyphal bodies called gongylidia, which serve as the primary food for the ants [2][6]. The relationship is vertically transmitted: a founding queen carries a piece of the maternal fungus to start the new garden [3]. They maintain hygienic gardens and harbor symbiotic bacteria (genus Streptomyces) on their exoskeletons that produce antibiotics to inhibit Escovopsis fungal parasites [3]. If garden disease is detected, workers can abandon contaminated fungus chambers. This symbiosis (ant–fungus–bacteria) is highly coevolved and essential to colony survival [2][3].

Sources: [6] Leal-Dutra, C.A., Yuen, L.M., Guedes, B.A.M. et al. Evidence that the domesticated fungus Leucoagaricus gongylophorus recycles its cytoplasmic contents as nutritional rewards to feed its leafcutter ant farmers. IMA Fungus 14, 19 (2023). https://doi.org/10.1186/s43008-023-00126-5

Conservation Status and Threats

Atta sexdens is common across its range and currently faces no conservation threats. It is not listed on the IUCN Red List, CITES, or any national endangered-species list [3]. Indeed, the species thrives in many human-altered landscapes, and is often expanding its range under current climatic conditions [3][7]. Habitat loss is not a major concern for A. sexdens. The main “threats” to the ant itself are pest-control efforts. Overall, A. sexdens is classified as “no conservation concern” [3].

Sources: [7] Sarnat E. (2012). Atta sexdens https://antkey.myspecies.info/en/content/atta-sexdens-1?citethispage=79142 last used on 12.06.2025

Geographical Distribution

Atta sexdens is native to the Neotropics. Its range extends throughout Central and South America, from southern Mexico and southeastern USA, Texas, southward through Central America (Panama, Costa Rica, etc.) into the amazon region and eastern South America. It has been recorded in Brazil, Guyana, Suriname, Venezuela, Colombia, Ecuador, Peru, Bolivia, Paraguay, and as far south as northern Argentina [7]. It also occurs in Trinidad and Tobago [7].

References

[1] Ramalho, M. d. O., Martins, C., Morini, M. S. C., & Bueno, O. C. (2020). What Can the Bacterial Community of Atta sexdens (Linnaeus, 1758) Tell Us about the Habitats in Which This Ant Species Evolves? Insects, 11(6), 332. https://doi.org/10.3390/insects11060332

[2] Maurício Bacci, Scott E. Solomon, Ulrich G. Mueller, Vanderlei G. Martins, Alfredo O.R. Carvalho, Luiz G.E. Vieira, Ana Carla O. Silva-Pinhati,

Phylogeny of leafcutter ants in the genus Atta Fabricius (Formicidae: Attini) based on mitochondrial and nuclear DNA sequences,

Molecular Phylogenetics and Evolution,

Volume 51, Issue 3,

2009,

Pages 427-437,

ISSN 1055-7903,

https://doi.org/10.1016/j.ympev.2008.11.005.

[3] Byrne, A. (2002). Atta sexdens. Animal Diversity Web. [University of Michigan Museum of Zoology]. animaldiversity.org animaldiversity.org last used on 11.06.2025

[4] Helanterä H, Ratnieks FL. Geometry explains the benefits of division of labour in a leafcutter ant. Proc Biol Sci. 2008 Jun 7;275(1640):1255-60. doi: 10.1098/rspb.2008.0024. PMID: 18319212; PMCID: PMC2602677.

[5] Swanson AC, Schwendenmann L, Allen MF, Aronson EL, on A-L, Dierick D, Fernandez-Bou AS, Harmon TC, Murillo-Cruz C, Oberbauer SF, et al. Welcome to the Atta world: A framework for understanding the effects of leaf-cutter ants on ecosystem functions.. Functional Ecology. 2019;(0(0). doi:https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2435.13319

[6] Leal-Dutra, C.A., Yuen, L.M., Guedes, B.A.M. et al. Evidence that the domesticated fungus Leucoagaricus gongylophorus recycles its cytoplasmic contents as nutritional rewards to feed its leafcutter ant farmers. IMA Fungus 14, 19 (2023). https://doi.org/10.1186/s43008-023-00126-5

[7] Sarnat E. (2012). Atta sexdens https://antkey.myspecies.info/en/content/atta-sexdens-1?citethispage=79142 last used on 12.06.2025