It’s been roughly 80 years since the red imported fire ant (Solenopsis invicta) arrived from South America to Mobile, Alabama in soil used as ballast to weigh down boats. Needless to say, fire ants have adapted well in southern states like Texas, Louisiana and Florida, disrupting native wildlife and plants and causing problems for people ranging from shorting out street lights to stinging limbs.
But in the late 1990s and early 2000s, scientists brought over several strains of parasitoid flies from Argentina in an attempt to naturally eradicate the fire ants. A scientist recently found that one particular fly strain may be able to completely wipe out fire ants in northern Florida.
The U.S. Department of Agriculture plans to use the so-called decapitating fly as a self-sustaining biocontrol agent that could keep up with the spread of fire ants. These parasitoid flies rely on the ants for reproduction, using the ants’ heads for the fly pupa; so in theory, wherever the ants go, the flies will follow.
Female decapitating flies are attracted to pheromones released by fire ants when they become aggressive from, say, the destruction of their nest (this phenomenon was documented in studies from the early 2000s). Studies have also shown that the flies could also be attracted to the venomous alkaloids that the ants emit. The female fly follows the aggressive ant, waits for the right moment and oviposits a single egg into the thorax, or the mid-section, of the ant. Over the next couple of weeks, the egg hatches and the larva migrates to the ant’s head. The ant behaves normally for another two weeks as the larva matures, at which point it starts consuming the brain tissue of the ant.
In a 2007 study from Insectes Sociaux, researchers discovered that ants continued about their routines for approximately 8-10 hours during the final larval stage, despite the lack of brain tissue. In other words, the parasitoid fly controlled the ant in a zombie-like fashion to avoid arousing suspicion from other ants. Then, once the ant left the nest, enzymes separated the ant’s head from its thorax. For another two weeks, the pupa transforms into a fly within the brain cavity. This one egg, one ant process starts again with the birth of the new fly, which emerges just inches from a nest filled with fire ants.
But there are obstacles in eradicating U.S. fire ants with decapitating flies. While this process occurs naturally in places like Argentina, finding the right match of decapitating fly for the type of red fire ants rampant in the southern U.S. is a bit trickier. To ensure that the flies would want to use the red imported fire ant as a host, scientists reared two strains of decapitating flies in labs: Pseudacteon tricuspsis and Pseudacteon curvatus. The first reared and released fly species, P. tricuspsis, has taken hold in nine states and Puerto Rico since it was released through USDA programs in 1997.
As for P. curvatus, these flies were collected from the black fire ant Solenopsis richteri in 1997 and were reared on the imported red fire ant for three years in the lab before being released in the states. Tests showed that the flies would still prefer the native black fire ant when given a choice, but if only the imported red fire ant was present, they produced an equal number of offspring. These flies were released in seven sites in Florida, but they never took to the red fire ant in the wild. They were, however, successful in red-black ant hybrids in Alabama, Mississippi and Tennessee.
Flies collected in Formosa, Argentina (also P. curvatus) in 2001 were shown to be more host-specific after rearing. In other words, after being bred to favor the red imported fire ant over the native Argentinean ant, they chose the imported fire ant even when both options were present. In 2003, these flies were released in Gainesville, Florida and started taking out the red fire ant one-by-one.
Using simple traps made from sticky film, pizza box separators and Petri dishes, Sanford Porter from the Center for Medical, Agricultural and Veterinary Entomology at USDA-ARS tracked the spread of these flies in North Central Florida. In a study published this month in Florida Entomologist, Porter reported a 90% success rate of finding P. curvatus at sites estimated to have high red fire ant populations—including sites outside of the areas the flies were initially released. It seems, then, that not only have the flies been established at the release sites, they are rapidly spreading. As he described in his paper:
The decapitating fly P. curvatus is vigorously expanding outward and firmly established on red imported fire ant populations in North Central Florida. The survey data from this paper indicate that P. curvatus will eventually occupy virtually every site where its host is common.
Henne, D., & Johnson, S. (2007). Zombie fire ant workers: behavior controlled by decapitating fly parasitoids Insectes Sociaux, 54 (2), 150-153 DOI: 10.1007/s00040-007-0924-y
Porter, S. (2010). Distribution of the Formosa Strain of the Fire Ant Decapitating fly (Diptera: Phoridae) Three and a Half Years after Releases in North Florida Florida Entomologist, 93 (1), 107-112 DOI: 10.1653/024.093.0114