Army Ants: Behavior, Habitat, and Facts

According to the University of Florida Entomology Department, army ants are among the most dramatic insects on Earth. Unlike most ant species that build permanent nests, army ants are nomadic predators that march through forests in massive columns, overwhelming and consuming nearly everything in their path. Their cooperative hunting strategies, living nests, and constant movement make them a unique branch of the ant family.

For a comprehensive overview, see our Complete Guide to Ants.

What Are Army Ants?

Sign or symptom	Likely cause	Risk level	What to do next
Fresh activity related to Army Ants	ants are active nearby or recently passed through the area.	High if signs repeat or appear in multiple rooms.	Inspect the surrounding cracks, seams, food sources, and travel paths.
Old or isolated evidence	A past problem, accidental introduction, or inactive nesting site.	Moderate until you confirm whether activity is current.	Clean and mark the area, then recheck in 24 to 48 hours.
Multiple signs together	A developing infestation rather than a one-off sighting.	High because populations can spread before they are obvious.	Start control steps immediately and consider professional inspection.

The term "army ant" refers to over 200 ant species across several genera that share a set of distinctive behaviors:

• Nomadic lifestyle: Colonies do not maintain permanent nests. They alternate between stationary (bivouac) phases and nomadic (raiding) phases.
• Group predation: Workers hunt cooperatively in massive swarm raids, overwhelming prey through sheer numbers.
• Wingless queens: Army ant queens are permanently wingless, blind, and never leave the colony. New colonies form by colony fission rather than nuptial flights.

The most well-known army ants include:

• Eciton burchellii (New World army ants) — the most studied species, found in Central and South America.
• Dorylus species (African driver ants) — form the largest colonies of any ant, with up to 20 million workers.

The Raid

Swarm Raids

Army ant raids are awe-inspiring events. At dawn, the colony sends out a massive swarm of workers — hundreds of thousands strong — that fans out across the forest floor in a broad front up to 60 feet wide.

The swarm overwhelms and consumes:

• Other ant colonies
• Spiders and scorpions
• Cockroaches and beetles
• Small reptiles and amphibians
• Bird nestlings (occasionally)
• Any small creature too slow to escape

The column of ants behind the leading swarm front processes and transports prey back to the bivouac. Workers form living bridges, highways, and tunnels with their bodies to facilitate efficient transport.

Column Raids

Some army ant species use column raids rather than swarm raids. Instead of a broad fan, they send narrow columns of workers along specific routes. Column raiders tend to specialize in raiding other ant colonies.

The Bivouac: A Living Nest

Army ants do not build traditional nests. Instead, workers link their bodies together — legs interlocking — to form a living structure called a bivouac. This mass of interlinked ants protects the queen and brood at its center.

A bivouac can contain 100,000 to 700,000 ants and measures roughly the size of a basketball. Workers on the outside form a protective shell, while those inside maintain stable temperature and humidity for the queen and developing brood.

Bivouacs are typically formed in sheltered locations — under fallen logs, in hollow trees, or in dense vegetation.

Nomadic and Stationary Phases

Army ant colonies alternate between two behavioral phases:

Nomadic Phase (2–3 weeks)

During the nomadic phase:

• The colony raids daily.
• The bivouac relocates each evening to a new site, following the direction of that day's raid.
• Workers carry the queen and brood during migrations.
• This phase coincides with larval development — the growing larvae need constant food input.

Stationary Phase (2–3 weeks)

During the stationary phase:

• The colony remains at a single bivouac site for 2–3 weeks.
• Raids are less frequent and smaller.
• The queen's abdomen swells enormously as she produces 100,000–300,000 eggs.
• Existing larvae pupate during this time.
• When the new workers emerge from their pupae, the colony transitions back to the nomadic phase.

Colony Reproduction

Army ants do not reproduce through nuptial flights like most ant species. Instead, colonies reproduce by fission:

1. A mature colony grows large enough to divide.
2. A new queen is raised within the colony.
3. The colony splits — half the workers follow the old queen, half follow the new queen.
4. Each group becomes an independent colony.

Males are winged and fly to other colonies to mate. They are large, wasp-like insects that look nothing like the workers.

Army Ants and Humans

Not Household Pests

Army ants are not a household pest concern in North America, Europe, or most temperate regions. They are tropical species that do not establish in urban environments.

Encounters in the Tropics

In tropical regions, army ant raids occasionally pass through villages or structures. While the ants do not specifically target humans, their bites are painful, and walking through a raid column results in many simultaneous bites. Residents of affected areas typically vacate buildings temporarily and let the ants pass — the ants actually provide a service by clearing the structure of cockroaches, spiders, and other pests.

Ecological Role

The National Pest Management Association notes that army ants are keystone predators in tropical ecosystems. Their raids:

• Regulate insect and arthropod populations
• Create feeding opportunities for ant-following birds (species that specialize in catching prey flushed by army ant raids)
• Drive nutrient cycling through their consumption and waste deposition

Fascinating Army Ant Facts

• Colony size: Research cited by Purdue Extension Entomology documents that African driver ant colonies can contain up to 20 million workers — the largest known ant colonies.
• Speed: Raid columns advance at roughly 20 meters per hour.
• Bridges: Workers build living bridges across gaps, streams, and obstacles using their own bodies.
• Temperature regulation: Bivouac workers adjust their body density to maintain the internal temperature within a degree or two.
• Blindness: Many army ant workers are functionally blind, navigating entirely by touch and chemical signals.

During a research trip to Costa Rica early in my entomology career, I observed an Eciton burchellii swarm raid firsthand. The leading edge was roughly 50 feet wide, and I watched as the column efficiently dismantled a spider's web and carried the spider back within minutes — one of the most impressive displays of cooperative behavior I have ever witnessed.

Army ants represent one of nature's most impressive examples of collective behavior — thousands of nearly blind individuals coordinating to perform complex tasks that no individual ant could accomplish alone.

How to Identify

Army ants are identified by their nomadic behavior and raiding columns rather than by fixed nests. New World army ant species form massive columns of hundreds of thousands of workers moving through leaf litter and sweeping the forest floor. A raiding column looks like a living river of ants, typically 1-2 cm wide and potentially hundreds of meters long. Workers are polymorphic: smaller media workers form the main column, while large soldiers with curved, sickle-shaped mandibles defend the column's edges. In the United States, the most commonly encountered species (Neivamyrmex) are smaller than tropical species, but their raiding columns are still unmistakable as fast-moving, organized streams of ants crossing a wide swath of ground.

Risk and Severity

Army ants in the United States rarely enter or threaten structures under normal conditions. In tropical regions, army ant swarms entering homes present a genuine hazard as the raiding column moves through any accessible space and can overwhelm small, confined animals. The primary North American risk comes from accidental contact with a raiding column, which can result in painful bites from the large soldier caste. In the southwestern United States, desert army ant species occasionally enter homes during wet weather but do not establish permanent indoor colonies. Their populations are native and ecologically important, rarely requiring active control measures.

Prevention

Army ants in North America rarely require active prevention measures for homeowners, since established indoor infestations do not occur. If Neivamyrmex species are raiding through your property during wet weather, temporarily close low-level gaps under doors and ground-level windows until the column passes: army ant raids are brief and move on quickly. In tropical regions where large Eciton species are present, maintain door sweeps, seal ground-level gaps, and clear vegetation from against the foundation. Some residents in tropical areas welcome army ant raids as temporary pest control, since the columns clear cockroaches and other insects from the structure as they pass through.

Main Causes

Indoor ants activity typically traces to outdoor colonies in mulch beds, lawn soil, decking voids, or wall cavities near the foundation. Scouts enter through gaps under doors, foundation cracks, utility penetrations, and damaged weatherstripping when food residue, water from leaks, or warmth from heating runs is available inside. Pheromone trails reinforce within hours of a successful foraging trip, drawing dozens to hundreds of workers along the same route. Heavy rain, drought, or disturbance to an outdoor nest pushes whole colonies inside in pulses. Sweet residue on counters, unsealed pantry items, pet food bowls left out overnight, and leaking pipes are the most common triggers, and the closer an outdoor colony sits to the structure, the harder the pressure becomes to manage.

Solutions and Actions

Effective ant control combines bait, perimeter exclusion, and sanitation rather than relying on contact sprays. Identify the species first because bait selection depends on the colony's current dietary preference — sweet baits for odorous house ants and Argentine ants, protein-based or grease baits for thief ants, multi-bait stations for opportunistic species. Place bait stations directly on active trails, not in random locations, and allow workers to carry the slow-acting active ingredient back to the colony untouched — avoid spraying anywhere near bait. Treat outdoor satellite nests within twenty feet of the structure with a non-repellent residual. Seal entry points only after bait has had time to reach the colony, otherwise foragers seal their access while the colony continues producing replacements.

Frequently Asked Questions

Are army ants found in the United States?

Army ants are primarily tropical and not a household pest concern in the continental U.S. A few small species exist in the southern states but lack the dramatic raiding behavior of tropical species.

Can army ants kill humans?

Healthy, mobile adults are not at risk. In rare cases, immobile individuals in tropical regions have been overwhelmed by driver ant raids. The primary danger is painful bites, not venom.

How do army ants form bridges with their bodies?

Workers link legs and bodies to span gaps, creating living bridges. These structures form and dissolve dynamically based on traffic flow, requiring no central coordination.

Are army ants a typical household infestation in the United States?

No. True army ants are not the typical ants invading kitchens or bathrooms in most U.S. homes. They are best understood for their nomadic hunting behavior, while household problems are usually caused by species such as odorous house ants, pavement ants, or pharaoh ants.