Head-hunter flies decapitate ants

Decapitating fly attacks fire ant (S.D. Porter, USDA-ARS) A decapitating fly takes aim

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It's a story worthy of a Halloween film.

A plague of alien ants sweeps across a country, attacking wildlife and people, and killing some.

In a bid to halt their advance, scientists turn to another, even more insidious creature: a type of fly that literally "head-hunts" ants, decapitating them.

The fly hunts down the ant and injects an egg into it. The egg hatches into a maggot, which then migrates into the ant's head.

Here the maggot releases a chemical that dissolves membranes, causing the ant's head to fall off.

The maggot proceeds to eat the ant from the inside out, eventually making its home in the empty severed body part.

Finally the maggot becomes a fly - ready to join a new generation of head-hunters.

Ant in 3D: BBC Nature

But this isn't science fiction; instead it's a description of the latest efforts being undertaken to halt the spread of an invasive ant species that has been imported into, and is spreading across, the US.

These fire ants can cause havoc, damaging agriculture, homes and even costing some people their life.

Red fire ants (Solenopsis invicta) were accidentally introduced into the US from the Formosa province of north-east Argentina during the 1930s.

They have colonised agricultural areas, deserts and coastal habitats as well as towns and cities, becoming a pervasive problem, spreading across the south of the country. They have also spread to other countries such as China and New Zealand.

The ants are small, just 2-6mm long, but they are aggressive and occur in high densities.

They breed and spread rapidly and, if disturbed, can relocate quickly to ensure survival of the colony.

Tiny killers

Fire ants can devastate native ant populations.

Research published last month demonstrated for the first time that not only do introduced fire ants compete with native ants for food, they also exclude other ants from creating mutually beneficial relationships with other animals. Ants often protect aphids, for example, in exchange for feeding on a sugary honey dew produced by the smaller insects.

Fire ants eat a variety of foods, and use stings to subdue and kill their prey, deterring other larger animals from where they are living.

Head-hunting know-how

Decapitating fly (S.D. Porter, USDA-ARS)
  • The flies (above) are attracted to fire ant alarm pheromones
  • When they find an ant, a female fly injects an egg into the ant's thorax
  • The maggot hatches and moves into the ant's head where it lives for several weeks
  • The maggot then makes its ant host leave its nest to find a place to die
  • The maggot releases a chemical that dissolves membranes that hold the ant's body together
  • The maggot eats everything in the head and finally pupates inside the empty head capsule, which it uses as a pupal case

They can kill frogs, lizards or small mammals and they can invade swimming pools, houses and even institutions.

They can also kill people.

Their stings may cause an allergic reaction in humans and medical literature records tens of individuals having died after suffering anaphylactic shock from fire ant bites. Last month, researchers published research attempting to analyse the chemical constituents of red fire ant venom, to better understand how to counteract it.

This ant invasion costs individual states billions of dollars each year, so scientists are exploring ways to control them.

In their native range the ants are controlled by several dozen natural enemies, including viruses, nematode worms, a parasitic ant and a wasp.

But these enemies are absent in the US, allowing fire ant populations to grow to numbers five to 10 times greater than in their native homelands.

So scientists are turning to various species of tiny, head-hunting flies, which help to control ant numbers in their native Argentina.

Researchers have been examining whether, and how, these flies might be introduced into the US to challenge the invasive ants.

Six species of fly have been released and two are now spreading with the ants, says Dr Sanford Porter of the US Department of Agriculture (USDA), based in Gainesville, Florida, who has been working on the biocontrol of fire ants for 15 years.

Fire ant (S.D. Porter, USDA-ARS) Fire ants like to sting

Several more species have been released more recently and scientists are now trying to measure their impact.

The latest battle ground is north-central Florida.

There Dr Porter and colleague Dr Luis Calcaterra, who works for the Foundation for the Study of Invasive Species and the USDA in Buenos Aires, Argentina, have conducted a study into which species of head-hunting fly are most effective against the ants.

The idea is not eradicate to fire ants in the US, but to create an ecological balance between fire ant and native ant populations.

In the journal Biological Control, they report on the success of releasing three different species of fly in the same territory, to see whether they would do better collectively.

The experiment was a success, with the third species already competing well with its head-hunting rivals and expanding its range.

"More is better because they attack different sizes of fire ants at different times of day and in more habitats," says Dr Porter.

However successful, "decapitating flies are part of a larger biocontrol effort, which includes pathogens and other parasites," he explains.

Other invaders

Dr Calcaterra is evaluating other potential biological control agents present in Argentina to fight against the fire ants in the US.

"Several promising organisms, such as the fungus Vairimorpha invictae or the parasitic ant Solenopsis daguerrei are being further investigated for their eventual field release in the near future," he says.

These species have their own, gruesome ways of killing fire ants.

Queens of the parasitic ant con their way into fire ant nests, for example. There, they latch onto fire ant queens, stealing and eating all their food, slowly starving the queen fire ants to death.

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