Invasion of the Crazy Ants

A war is being waged across the southern US. Over a decade ago, the invaders established a beachhead in Texas and now the battle lines have spread all the way to Florida. In some areas, the invaders have become conquerors, killing millions in a campaign to exterminate their enemies. They came well-prepared, uniquely able to neutralize the heavily-armed defenders' weapons and tactics. Across the southern US, the red imported fire ant, Solenopsis invicta, is falling back in the face on a merciless onslaught by Nylanderia fulva, the tawny crazy ant.

The red fire ant is an invader in its own right, having made landfall in the port of Mobile, Alabama in the 1930s and spread from there; today, their nest mounds are found throughout the southern states. These aggressive ants cause hundreds of millions of dollars in damage to electrical systems and agricultural assets every year. They're known for their painful "sting", a potent insecticide two or three times as strong as DDT which they release from their gaster. With this weapon, they've reshaped ecosystems, out-competing other ants and preying on everything from arthropods to small vertebrates. Unfortunately for them, it doesn't work on N. fulva.

N. fulva colonies were first reported in the US by Tom Rasberry, an exterminator who spotted colonies around Houston in 2002. Individual workers scramble around seemingly randomly and at high speeds, earning the "crazy ant" moniker for the species. Last year, Edward LeBrun of the Brackenridge Field Laboratory at the University of Texas discovered that N. fulva is out-competing S. invicta where the two overlap. The tawny crazy ants have also proven more difficult to control than the red fire ant, since they don't consume the poison baits used against fire ants and have a more resilient supercolony structure. To make matters worse, the crazy ants go everywhere, even to the extent of invading people's homes. "There are videos on YouTube of people sweeping out dustpans full of these ants from their bathroom," said LeBrun. "You have to call pest control operators every three or four months just to keep the infestation under control. It's very expensive."

The crazy ants don't have a particularly powerful sting, so how are they managing to overcome the fire ants and drive them out? LeBrun's initial insight came from watching the two battle species over a dead cricket at a field site in Texas. The fire ants had found the corpse and come out in large numbers to claim it, a formidable presence that usually keeps other ants away. The crazy ants, however, were undeterred. "The crazy ants charged into the fire ants, spraying venom," said LeBrun. "When the crazy ants were dabbed with fire ant venom, they would go off and do this odd behavior where they would curl up their gaster and touch their mouths."

LeBrun thought this behaviour might be the secret to the crazy ants' success. If the N. fulva workers had a way of detoxifying the fire ants' venom, that would explain their remarkable ability to defeat S. invicta nine times out of ten in conflicts over resources. Together with labmates Nathan Jones and Lawrence Gilbert, LeBrun tested this hypothesis by staging fights between the two in the lab. The team thought N. fulva was using secretions from its gaster to detoxify the venom, so they sealed the crazy ants' gasters with nail polish to see what would happen. Without access to their secretion, half of the crazy ants died in a fight with S. invicta; by contrast, there was a 98% survival rate in the control group, which had been dabbed with nail polish without obstructing the gaster. Since N. fulva's venom is mainly made up of concentrated formic acid, the team hypothesized that this might be the detoxifying agent. To find out, they prepared a formic acid solution of the same pH and tested how well it protected a third species, the Argentine ant (Linepithema humile), from S. invicta venom. While less than 20% of the untreated Argentine ants survived a fight with S. invicta, 95% of the treated ants made it through, demonstrating the efficacy of formic acid, though the researchers still don't know how it nullifies the fire ants' venom.

S. invicta and N. fulva are old foes, clashing in their native South America long before they reached the US. A range of interactions with other species keeps the two in check in their home range, stopping N. fulva from overwhelming S. invicta and preventing either from completely dominating the ecosystem. Without those interactions, the story takes a different turn, as we're witnessing with the imported populations in the US. Except when transported by humans, the crazy ants expand their territory slowly, and we still don't know what environmental factors will limit their range. So far, they've stayed near the coast, so fire ants may be able to persist in the drier, colder inland climates, but we don't know how it will play out in the long run. "The whole system has changed around fire ants," said LeBrun. "Things that can't tolerate fire ants are gone. Many that can have flourished. New things have come in. Now we are going to go through and whack the fire ants and put something in its place that has a very different biology. There are going to be a lot of changes that come from that."

References
LeBrun, EG, Jones, NT, and Gilbert, LE. Chemical Warfare Among Invaders: A Detoxification Interaction Facilitates an Ant Invasion. Science 343:1014-1017. (2014) doi: 10.1126/science.1245833
Press releases from UT about the domination and detoxification by N. fulva