Fire ants possess an extraordinary survival strategy. As floodwaters rise, entire colonies can abandon their nests and assemble themselves into floating rafts. Thousands of ants link their bodies together to form a living platform that carries workers, queens, larvae, and other colony members across the water. These rafts can remain afloat for extended periods, allowing colonies to survive conditions that would overwhelm many other insects.

This behaviour has attracted attention for decades because it seems to offer an ideal mechanism for long-distance dispersal. Images of floating ant rafts drifting downstream naturally create the impression that floods could dramatically expand fire ant infestations. Yet evidence from Australia suggests that reality is far more nuanced.

Researchers working within Australia’s National Fire Ant Eradication Program examined the impacts of major flooding events in southeast Queensland, where the country’s largest fire ant infestation is located. The program has been operating since 2001 and represents one of the most extensive invasive ant eradication efforts ever undertaken. Over more than two decades, it has accumulated a vast database of fire ant detections, providing a rare opportunity to study the relationship between floods and ant movement on a large scale.

The researchers’ work helped shed light on what actually happens when floodwaters interact with fire ant populations. The findings challenge the assumption that flooding is a major driver of fire ant expansion. While floods certainly move ants around, the research found little evidence that they have pushed infestations beyond their known boundaries in southeast Queensland. Instead, flooding appears to contribute mainly to localised spread within areas that are already infested.

This distinction is important. Moving a colony a short distance downstream is very different from establishing entirely new populations far from existing infestations. One reason lies in the nature of the floodwaters themselves. Although ant rafts are impressive, they are not indestructible. Fast-moving, turbulent water can tear them apart. Colonies that survive initial flooding may still struggle to establish themselves in suitable locations afterward. Geography also plays a role. In southeast Queensland, many rivers flow through areas where fire ants are already present before eventually reaching the ocean. Saltwater poses a serious challenge to the ants, reducing the likelihood that rafts drifting out to sea will survive long enough to establish new colonies.

The researchers examined two major flood events, one in 2011 and another in 2017. Despite extensive flooding, neither event produced evidence that fire ants had expanded beyond the known infestation zone. Surveys conducted after the floods found no sign of successful establishment along nearby coastlines or in previously uninfested regions.

However, flooding was not without consequences. One particularly revealing case involved a property south-west of Brisbane in Queensland. Before flooding occurred, the site contained a dense infestation of a specific type of fire ant colony known as a polygyne colony. Unlike standard colonies, which contain a single queen, polygyne colonies contain multiple queens. This difference significantly affects how colonies respond to flooding.

When floodwaters break apart a polygyne colony, different queens may end up in separate raft fragments. Each fragment has the potential to become a new colony. In effect, a single flooded colony can multiply into several colonies after the waters recede.

At the fieldsite, researchers observed striking examples of this process. Ants gathered in clumps of grass, on tree trunks, and within piles of woody debris left behind by the flood. When scientists returned to survey the area, they found substantially more mounds than had existed before the flood. Although some colonies had died, the overall number of mounds had increased dramatically, suggesting that colony splitting and relocation had occurred.

This finding highlights one of the key lessons from the research. Not all fire ant colonies pose the same flood-related risk. Polygyne colonies appear far more capable of exploiting flooding events than colonies containing only a single queen.

The study also revealed just how resilient fire ants can be. Some colonies survived being submerged for days. Others appeared to take advantage of natural refuges such as elevated ground, fallen logs, and debris piles. In some cases, live ants and brood were found in nests that had remained underwater for surprisingly long periods.

These observations underline the remarkable adaptations that have helped fire ants thrive in flood-prone environments throughout their evolutionary history. Yet survival is only part of the story. Floods can also be deadly.

The Queensland research found evidence that many colonies are lost during major flood events, particularly when waters rise rapidly or remain high for extended periods. Colonies trapped by sudden inundation may not have time to evacuate. Others may succumb after prolonged submersion or when their rafts are destroyed by turbulent currents. This dual nature of flooding makes it both a threat and an opportunity for fire ants. Some colonies survive and relocate, while others disappear entirely.

Understanding these dynamics has practical implications for Australia’s eradication efforts. The researchers developed a framework to help assess the risk posed by flooding. Factors such as the presence of polygyne colonies, the speed at which waters rise, the turbulence of floodwaters, previous treatment history, and river flow patterns can all influence the likelihood of fire ant spread.

This approach allows authorities to focus surveillance and control efforts where risks are greatest after major flood events. The work of Ross Wylie and his colleagues demonstrates the value of combining long-term monitoring with detailed field observations. Rather than relying on assumptions about how fire ants might behave, the researchers used real-world evidence gathered across years of flooding and management activities.

Their findings offer an encouraging message for those working to eliminate fire ants from Australia. Despite the insects’ remarkable rafting abilities, floods have not become the unstoppable dispersal mechanism many feared. While they can contribute to local movement and create challenges for eradication programs, they have not fundamentally altered the boundaries of infestation in southeast Queensland.

At the same time, the study serves as a reminder of nature’s ingenuity. Fire ants have evolved an extraordinary toolkit for surviving environmental extremes. Living rafts, waterproof nests, colony splitting, and prolonged endurance underwater all illustrate the remarkable adaptability of social insects.

As climate change increases the frequency and intensity of extreme weather events in many parts of the world, understanding these survival strategies will become even more important. The story of Australia’s fire ants is about resilience, adaptation, and the complex ways in which animals respond to a changing environment.