Acromyrmex heyeri

Acromyrmex heyeri

Acromyrmex heyeri workers grow to about 7mm and show a reddish-brown coloration with darker abdomens ^[1][2]. These ants are immediately recognizable in the field by their distinctive dome-shaped thatch mounds constructed from dried grass fragments and plant material ^[3][4]. Unlike their tree-foraging relatives, they specialize in cutting narrow monocot leaves and grasses, earning them the nickname "narrow leaf cutter" ^[5].

Native to the temperate grasslands of southern Brazil, Argentina, Uruguay and Paraguay, they represent the southernmost distribution of the Acromyrmex genus ^[3]. Their biology revolves around maintaining a fungus garden of Leucoagaricus gongylophorus at precisely 25°C, insulated by their thatch nest which keeps the garden 5°C warmer than surrounding soil ^[3]. Interestingly, while most colonies have a single queen, approximately 20% of wild nests contain two queens cooperating together ^[6][4].

Quick Summary

• Difficulty: Expert
• Origin & Habitat: Temperate grasslands (pampas) of southern Brazil, Argentina, Uruguay and Paraguay ^[3][2]
• Colony Type: Facultatively polygynous, colonies usually have one queen but can contain up to two queens ^[6][4]
  • Colony: Optionally polygyne
• Size & Growth:
  • Queen: 6-7 mm (estimated from morphological data) ^[4]
  • Worker: 7 mm ^[1]
  • Colony: Up to several thousand workers (estimated based on nest dimensions and related species)
  • Growth: Moderate (estimated)
  • Development: 8-12 weeks at 25°C (estimated based on related Acromyrmex species) (Development is temperature-dependent, fungus garden must remain at 24-25°C for optimal growth ^[3])
• Antkeeping:
  • Temperature: Maintain fungus garden at 24-25°C, ambient air can vary but avoid fluctuations ^[3]
  • Humidity: High humidity required, fungus garden needs moist but not waterlogged conditions ^[3]
  • Diapause: Yes, being a temperate species, they require winter rest with reduced temperatures ^[3]
  • Nesting: Large setup required with fungus chamber space, thatch material or equivalent insulation helps maintain temperature ^[3][7]
• Behavior: Specialized grass-foragers that build thatch mounds, moderate aggression, workers forage along trunk trails ^[8]. They regulate nest openings based on humidity ^[9].
• Common Issues: fungus garden crashes if temperature drops below 20°C or exceeds 30°C for extended periods ^[3]., thatch or nest material can develop mold if kept too wet without adequate ventilation., colonies require constant supply of fresh grass, dried leaves are not sufficient., winter diapause must be managed carefully, too cold kills the fungus, too warm depletes reserves., susceptible to Escovopsis parasitic fungus infection ^[9].

Thatch Nest Architecture and Building Behavior

In nature, Acromyrmex heyeri builds distinctive dome-shaped mounds called thatch nests. Workers collect plant fragments and arrange them to form a thatch covering 8-10 cm thick over a central chamber located about 15 cm below ground ^[3]. This thatch acts as insulation, keeping the fungus garden at approximately 25°C even when soil temperatures drop to 15°C in winter ^[3]. The garden itself sits on a layer of exhausted grass fragments rather than directly on soil ^[3].

In captivity, you should provide a large nest box with space for a fungus chamber and room to add insulating material. While you don't need to build a full thatch dome, placing a layer of dried grass or coconut fiber over the nest chamber helps buffer temperature swings. Ensure the covering material is dry to prevent mold, but maintain high humidity inside the fungus chamber itself.

Fungus Garden Thermoregulation

The symbiotic fungus Leucoagaricus gongylophorus grows best at 25°C and requires high humidity ^[3]. Workers actively thermoregulate by positioning brood and fungus in zones that match this ideal temperature, preferring areas around 24.1°C ^[3]. The thatch nest prevents heat loss at night and avoids overheating during the day ^[3].

You must provide a stable heat source. Use a heating cable or mat placed on one side of the nest to create a gradient, but ensure the fungus chamber itself stays near 25°C. Avoid placing heat sources directly on top of the nest as this can dry out the garden. Monitor temperature constantly, if the fungus drops below 20°C or exceeds 30°C for long periods, it will die and the colony will starve ^[3].

Diet and Foraging Ecology

Unlike many leaf-cutters that harvest tree leaves, Acromyrmex heyeri specializes in narrow monocot leaves and grasses ^[5][2]. They cut fragments of entire leaves without breaking them into tiny pieces like some other species ^[5]. In Paraguay, they commonly cut the grass Paspalum notatum ^[10], and they also attack wheat and pine seedlings ^[11][12].

Feed your colony fresh grass blades, narrow leaves, or oat grass. The material must be fresh and clean, avoid plants treated with pesticides. You can offer small amounts of fruit or vegetables occasionally, but grass should form the bulk of their diet. Remove old, dried-out plant material daily to prevent mold. They do not accept seeds or sugars as these provide no nutrition for the fungus.

Social Structure and Reproduction

These ants show flexible social organization. While 80% of colonies have a single queen (monogyne), about 20% contain two queens (polygyne) working together ^[6][4]. Queens mate with multiple males (polyandry), with some queens mating up to 12 times ^[6]. Mating flights occur during the southern hemisphere spring, from October to December ^[4].

If you collect a founding queen, expect her to raise the first workers alone. While founding behavior hasn't been directly documented for this species, related Acromyrmex queens seal themselves in with a fungus pellet and live off stored fat until workers hatch. Do not disturb the founding chamber during this vulnerable period.

Seasonal Care and Winter Diapause

As a temperate species from the southern pampas, Acromyrmex heyeri experiences distinct seasons. Soil temperatures in their native range average 15°C in winter,17.9°C in spring, and 22.7°C in summer ^[3]. Colonies slow down or stop activity during winter, effectively hibernating while maintaining the fungus garden at survivable temperatures.

In captivity, you must replicate this winter rest. Gradually lower temperatures to 15-18°C for 2-3 months during winter, while ensuring the fungus doesn't die from cold or dry conditions. Reduce feeding during this period. In spring, gradually warm them back to 24-25°C to trigger resumed growth and mating flights if you have alates.

Frequently Asked Questions

References