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Watch out: Fire ants' sting significantly more painful when they're rafting.
September 6, 2017

ASU researcher's 2006 paper on mechanisms of these disquieting sights getting interest thanks to Hurricane Harvey insect upheaval

“Life likes to live,” Kevin Haight said after viewing a photo of reddish-brown swirls in a floodwater eddy in southeast Texas.

Haight, a researcher in the School of Life Sciences’ Social Insect Research Group at Arizona State University, was referring to tens of thousands of refugees uprooted by Hurricane Harvey.

Just not the ones you think.

The nation (or at least the media, both social and traditional) has been transfixed by the spectacle of floating rafts of fire ants drifting through Houston’s floodwaters.

“Nightmarish.” “Creepy.” “Stuff of the Book of Revelations.”

There is a complex and fascinating story behind this survival behavior, however, one that evokes more sympathy than revulsion. You still won’t want to bump into a fire-ant raft after you hear it, but you will stare in fascination if you’re lucky enough to see one.

Fire ants sting. There is a scale called the Schmidt Pain Index that attempts to systematize pain from insect venom. Created by an Arizona entomologist, it goes from pain level 1 (“a lover just bit your earlobe a little too hard”) to pain level 4 (“blinding, fierce [and] shockingly electric”).

A single fire-ant sting rates a 1.2, with a honeybee sting rating a 2.

“However, that is for just for a single sting, which is not common to experience with (fire ants),” Haight said. “Multiple stings from multiple workers are much more usual.

“Furthermore, (fire-ant) venom not only causes pain, which is felt almost immediately, but also causes tissue damage in the form of little pustules, which become evident hours to days after being stung and can lead to secondary infection if opened.”

Here’s the kicker: Their stings become significantly more painful and venomous when they’re rafting. How much more painful? Eighty-seven percent, according to Haight. He published a paper in 2006 describing his discovery, which to his amusement is now getting a great deal of attention.

Why are their stings more painful? It’s simple: They are at their most defenseless, and the survival of the colony is on the line.

Here’s what goes on:

As the water table rises in the nest, the ants concentrate as dry space dwindles. As they do that, they gather the queen and brood (the eggs, larvae, pupae, workers). They’re becoming what people would refer to as “stressed.”

“The close proximity of so many workers during rafting may result in a concomitantly higher concentration of alarm pheromones, and thus the increase in defensiveness,” Haight wrote in his paper.

The water continues to rise. Now they float in the water above their nest.

They have some food with them. Their crop contains sugary liquids they can share with larvae and each other. (Fire ants do stash “ant jerky” in their nests — dried protein flakes — but they don’t carry it with them in a flood. As Haight pointed out, “If your house is on fire, what are you going to do — empty the fridge or grab your kids?”)

They’re floating, and they hold on to each other. They cycle from the bottom of the raft to the top in a convection pattern. The rafts are more buoyant if they have larvae, research has shown.

They are completely at the mercy of the current.

“They are a rudderless boat,” Haight said. “They have no control over where they end up. Typically it’s downstream.”

Now the ants are at their most dangerous. Large concentrations of workers are exposed, available for defense, and loaded with more potent venom. They’re also on high alert to grab on to something, anything, and get out of the water.

“If they bump against a tree branch or something in the water, they’ll do that,” Haight said. “It’s one of the things that makes them dangerous in a flood situation when you have people wading or people in boats. If the ants bump up against a boat and they’re able to climb up the sides, then they’re in the boat with you now.”

When they hit dry ground, they will begin digging a new nest. They are incredibly efficient diggers. Depending on how hard the ground is, they can dig a new nest and get the whole colony underground in five to six hours, Haight said.

“They’re never more vulnerable than when they’re rafting,” he said. “They have no place to go. They can’t go deeper in their nest because they don’t have a nest.”

Animals or people trudging through water could break the colony up, or predators like fish. The colony could perish. Without a big workforce, survival becomes dicey. Increasingly painful stings increase the likelihood of survival.

“I would argue that yes, it is an adaptive survival mechanism,” Haight said. “Higher venom doses mean more pain delivered to a threat in a shorter amount of time, which translates to higher repellency.  This would be especially important to a rafting colony since quick repulsion would reduce the chances of worker loss, which would be beneficial since colonies with more workers will have a higher chance of reestablishing a successful nest and territory after the flood recedes.”


Top photo: A "raft" of red imported fire ants in North Carolina floats over land that normally forms the bank of a pond. The land had become submerged due to excessive rain and resultant flooding, which inundated the nest. The raft is anchored to some blades of grass extending above the water's surface. Photo by TheCoz (own work) [CC BY-SA 4.0 (], via Wikimedia Commons

Scott Seckel

Reporter , ASU Now


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September 7, 2017

In the wake of Harvey and Irma, ASU's Charles Redman talks about how to make cities more able to weather calamities

The flooding in Houston was exacerbated by how the city was built. Like many cities, Houston basically paved over the existing landscape, a grassy plain that evolved to handle large rainfalls by acting like a sponge.

New ideas on how to build more resilient cities focus on working with nature, rather than trying to master it, says Arizona State University Professor Charles Redman, the founding director of ASU’s School of Sustainability and a distinguished scientist in ASU’s Julie Ann Wrigley Global Institute of Sustainability.

What this means, Redman says, is building infrastructure systems that are safe-to-fail, rather than fail-safe, and recognizing that the city should be able to take advantage of natural features of the land rather than to solidify it with concrete.

Redman leads a group of researchers from 15 institutions in a National Science Foundation-sponsored project called the Urban Resilience to Extreme Weather Related Events Sustainability Research Network (UREx SRN), which focuses on ways to make cities more resilient to natural calamities.

Here, he talks about how cities can be better prepared to withstand natural calamities.

Question: What can cities do to be better prepared for events like Harvey and Irma?

Answer: There are two areas of improvement, the first being altering the nature of the “hard” infrastructure, and second to enrich and make more effective the “soft” infrastructure of organization, cooperation, information flow, etc.

The overarching problem with cities like Houston is that they have built over the natural landscape with impervious surfaces, and with impediments to the natural flow of surface runoff. A more effective approach may be to implement infrastructure systems that work with the land to facilitate runoff rather than try to control it, but acknowledge and plan that if a specific threshold is exceeded and the system “fails” in some sense there are backup plans in place that minimize the adverse impacts.

Q: How does a safe-to-fail system operate vs. a fail-safe system?

A: This is about managing risk in an increasingly uncertain world. The fail-safe system assumes you know what is coming and that it can be handled if we build a big enough dam, levee, pipe, sea wall, etc., for protection. The problem is that many extreme events such as Katrina, Fukushima, Sandy and now Harvey go beyond the expected and result in disaster. A fail-safe approach seeks to totally prevent harm related to the weather event; however, if the event does exceed the design of the infrastructure there is little back up protection and disaster ensues.

In each case, much of the disaster might have been avoided or at least minimized through more effective awareness of the threats, better planning and well-thought-out responses. This is at the heart of the safe-to-fail approach that accepts the possibility that the hard infrastructure might be insufficient and has multiple back-up plans to ameliorate the impact. Using green infrastructure with secondary benefits, relying on multiple approaches, and moving people out of harm’s way are among the approaches to minimize negative impacts.

Q: What are some of the “soft” infrastructure approaches to becoming more resilient?

A: Short of changing the physical structure of the city, “safe-to-fail” strategies would focus on how the city prepares and responds to weather-related extreme events. The first is to map the relative vulnerability of people in different locations of the city, and as the event approaches the rescue teams don’t have to wait for 911 calls but can focus on the most likely areas to flood before the disaster develops. 

The second is to develop a system of refuge centers that are appropriate to the scale of the potential dislocation and pre-plan strategies for getting people there. In Houston planned shelters were a fraction of the size needed. A separate issue is that two of the Houston’s reservoirs threatened to overtop their embankments. It is a common mismanagement problem that reservoirs are kept too full in order to maximize available water when they should be kept lower during hurricane season.

Q: What are some of the tools being developed in the UREx SRN program?

A: We are focusing on a variety of tools, and more, to build a method of bringing together the diverse elements of city government with relevant citizen groups to plan for future hard and soft infrastructure. Primary among the tools are a variety of green infrastructure constructions that take advantage of natural ecosystem services to ameliorate the impact of extreme weather events and then, in between events, to provide desired amenities like greenbelts and recreation fields. We are also constructing maps of populations most vulnerable to various types of extreme events and systems to minimize the impact on these populations.

Q: How does the program make cities more resilient?

A: We believe there are basic steps in gathering information, planning for the scale and intensity of events that were not common in the past, but are expected more today and in the future. We want to create planning templates that will ensure that cities are more “resilient-ready” to floods, heatwaves, droughts and other weather-related extreme events.

Another category of action that will enhance the capacity of a city to handle extreme events is to build the cooperative attitude of helping each other and working together to face the challenge rather than relying totally on outside interventions. Community members working together is the best line of defense to extreme events and can leverage and fill in with the outside interventions.


Top photo: Soldiers with the Texas Army National Guard move through flooded Houston streets as floodwaters from Hurricane Harvey continued to rise Aug. 28. More than 12,000 members of the Texas National Guard were been called out to support local authorities in response to the storm. Photo by U.S. Army photo by 1st Lt. Zachary West [Public domain], via Wikimedia Commons

Associate Director , Media Relations & Strategic Communications