Is Australia’s scorched earth baiting program actually paving the way for fire ant invasion?

Right now, Australia is undertaking the world’s largest removal of invasive ants. The goal: eradicate fire ants (Solenopsis invicta).

These aggressive South American ants are named for the burning sensation of their sting. They pose risks to many native species – and to human health.

Fire ants have made it to Australia nine times, arriving in cargo ships. Eight times, authorities were able to stamp them out early. But an infestation detected in Brisbane suburb in 2001 has now spread across more than a million hectares of South East Queensland.

Authorities have used broadcast baiting to tackle fire ants, releasing pesticides over massive stretches of land since 2001. This approach works for small outbreaks. But my recent research suggests it may actually be making it easier for fire ants to spread.

When large areas are baited, the result is an ecological vacuum. Competitor species are wiped out and hardy fire ant survivors can press forward.

Fire ants are a “weedy” species. They love environments heavily modified by human behaviour, such as roadsides, industrial areas and paddocks. When baits are laid on the edge of their infestation, competitors and predators are also decimated – and advancing fire ants find it much easier to survive.

What’s behind the current strategy?

Decades ago, researchers found a weak spot for fire ants. The biggest larvae act like a distributed stomach for the colony. They take solid food, digest it and transfer it as liquid to adult ants to eat.

Queensland authorities use two insect growth regulators (Pyriproxyfen and S-methoprene) to target this stage. These chemicals are infused into tasty corn grit and soybean oil. Once taken back to the nest by workers, these delicious treats are fed to the larvae, who spread the toxins by liquid feeding. Over a few weeks, the fire ant colony collapses.

To date, eradication using this method has succeeded only in areas under 10,000 hectares. Authorities have to treat the entire area multiple times to ensure no nest is missed.

Fire ants have predators and competitors

Evidence from the United States – where control efforts have been underway since the 1950s – suggests fire ants are not actually a superior competitor.

Instead, they thrive where native ants and invertebrates are found in lower abundance and diversity. They find it much harder to penetrate undisturbed forests with thick leaf litter, where competitors and predators can repel them or keep them in check.

To spread, new queens must leave the nest, mate mid-air and land in a vacant area to start a fresh colony. This is when fire ants are most vulnerable.

In suburbs and rural areas, new queens have to run the gauntlet of invertebrate defenders. Native species such as meat ants (Iridomyrmex species) and green-headed ants (Rhytidoponera metallica) are aggressive defenders of territory. Even the invasive coastal brown ant (Pheidole megacephala) is a fierce competitor. Spiders, lacewings, earwigs, birds and predatory beetles all find a slow-moving fire ant queen to be an energy-rich meal.

These defenders should be our key allies in the fight against fire ants. Unfortunately, the chemical baits are indiscriminate. Many other invertebrates eat the baits – including rival ant species and predators.

The problem of scale

At over 1 million hectares, South East Queensland’s infestation is 100 times larger than any area ever successfully eradicated. Covering this entire area perfectly and doing so multiple times is effectively impossible.

In southern US states, authorities tried broadcast baiting for decades before giving up. In Georgia, a massive baiting program at first seemed to have succeeded. But within 14 months, fire ants had returned, moving faster and at higher densities than native ants. In Florida, new infestations are almost always found in disturbed areas where competitors were removed.

Fire ant queens can survive baiting

While newly mated fire ant queens are vulnerable to predators, they are not vulnerable to baiting.

This is due to a biological quirk. After the mating flight, a newly mated red fire ant queen digs a hole and seals the entrance for up to four weeks.

During this time, the queen lives off her fat reserves while she raises her first batch of workers. If authorities drop baits during this time, the new nest won’t be affected.