If you live in the southern United States like I do, you probably know the red imported fire ant - especially if you've ever been stung by one! This little ant packs quite a sting. But, do you really KNOW the red imported fire ant?
If you are like me, you might not know that the red imported fire ant can accomplish the zombie-like feat of creating a living raft that literally floats a colony to safety if a flood comes along. That is one of the survival skills that makes the red imported fire ant so infamous, and invasively successful, in the southern US and elsewhere.
Rafting across the Rio Grande is the suspected mechanism for the imported fire ant range expansion into northern Mexico (Sánchez-Peña et al. 2005). - Raft Formation by the Red Imported Fire Ant, Solenopsis invicta
There are reports that living fire ant rafts can persist for weeks on the surface of a body of water. Yep, wish again - that flooding in your front yard isn't doing anything to get rid of your fire ant problem. Nor is pouring buckets of water over that fire ant mound.
But before the publication of a 2009 study in the Journal of Insect Science, scientists didn't know much, in fact, about how these living rafts form and why the imported red fire ant is so good at creating them. So a team of researchers at Louisiana State University set out to investigate raft formation among fire ants. One of the researchers is a friend of mine from LSU, Dr. Linda Hooper-Bui. (She also backed my Experiment.com project recently, and I'm thanking her by writing about this study!)
In order to establish base-line data for the formation, structure, and longevity of the raft behavior, several series of experiments were performed in which colonies were inundated in order to incite rafting. - Raft Formation by the Red Imported Fire Ant, Solenopsis invicta
The researchers collected red imported fire ant colonies from fields near Louisiana State University in Baton Rouge, Louisiana. They placed the ants in plastic containers coated on the inside with Teflon (think non-stick pans) and talcum powder (think baby powder) so that the ants could not escape. Genius ant-trapping methods, right?
Within a controlled environment, the researchers added water to samples of ants. Before adding the water, they took note of various conditions including the presence, amount and age of brood, or ant larvae and pupae.
Conglomerations of ants were considered rafts when multiple layers of workers were observed within the structure and the majority of individuals were free floating on the water with no attachment to solid substrate. During trials, colonies were not allowed to escape from the water. Any colony that successfully escaped the enclosure was removed from the data set. Rafts not maintained for ≥ 12h were considered a failed raft. - Raft Formation by the Red Imported Fire Ant, Solenopsis invicta
The researchers tested the movement, or cycling, of ants within the raft – by marking various groups of worker ants with modeling paint! They counted marked individuals every few hours, flipped the raft over, and continued to count. The researchers then froze the rafts and dissected them for microscope observation and photography. Through microscope observation, the researchers could examine for example how bubbles formed on the underwater worker ants and larvae, and how these bubbles might enhance the floating raft.
Finally, the researchers examined how long the rafts could remain intact, and how this longevity depended on the presence of various ant life stages (e.g. brood) in the raft.
So what did the researchers find out about raft formation in red imported fire ants?
First of all, at some point the collective of ants in the raft began to “pitch” extra ants overboard. The first to be pitched overboard to drown were the male “drones” or winged reproductive male ants. So much for being “king!” Wingless reproductive queen ants, however, were keep safe in the middle of the rafts. Brood was located on the bottom of the raft except for early stage larvae or eggs, which the workers keep in their mandibles.
The raft formation experiments in this study showed that the raft is a dynamic structure, with worker ants cycling from the bottom to the top of the raft, presumably to prevent drowning of individual ants. Red imported fire ants form the raft structure primarily by hooking and curling around each other’s hind legs in Velcro-like fashion. Score for teamwork!
The rafts could also remain floating for up to 12 days in undisturbed lab trials. The more brood in the raft, the longer it could stay afloat. In particular, curved bristles on the older larvae seemed to capture air bubbles and thus help the raft stay afloat on the surface of the water.
The bubbles accumulated and joined around the aggregations of brood and workers until buoyancy lifted the entire mass from the submerged substrate. The aggregation would then rise to the water's surface like an elevator, breaking the surface tension with workers in tow. - Raft Formation by the Red Imported Fire Ant, Solenopsis invicta
In summary, brood and bubbles appear to be key in helping red imported fire ant colonies create successful rafts in flooding conditions.
So what do we do with this new knowledge of how the red imported fire ant forms rafts? Particularly for this invasive species, knowing what helps the raft say afloat might help us control populations of these ants. As the researchers point out, fire ant “pest” control might be more successful if applied in conjunction with known flooding events such as during hurricane season in the United States – because colonies without high numbers of older larvae are less likely to successfully form rafts during these flooding events.And… soap! Soaps could help remove the bubbles on the hydrophobic bristles of the larvae holding up the raft!
Who knew there was so much to learn about how fire ants float?
Bonus: Take the red imported fire ant quiz!