Digging up fire ant colonies (Solenopsis invicta) in the field.
Although the mounds of fire ants are the most visible part of their home, this structure is used only for regulating the temperature of ants and brood during certain seasons and parts of the day. A large portion of the nest is hidden from view underground. The subterranean nesting habits of ant colonies can make the transition from field to lab studies tricky– how does one go about separating thousand and thousands of ants and their delicate brood from a heap of dirt?
In fire ants, the drip floatation method takes advantage of a flood-survival adaptation of the ants to accomplish this task (Banks et al, 1981).
Baby powder lined buckets with fire ant colonies.
The first step is, of course, to obtain field colonies. This is best done early in the morning, when the queens and brood are likely to be high in the nest, warming up from the cold night. I also recommend gloves, long pants, and boots. Fire ant colonies can be dug up from the field and stored in a container lined with baby powder, which will prevent the ants from climbing the sides and escaping. Colonies should be allowed to settle into the container for at least 24 hours. This allows the ants time to locate brood and re-establish tunnels to the surface.
A lab set up for the drip floatation method for separating ant colonies from nest dirt.
Water is then slowly dripped into the container, drop by drop. Again, this process should be slow enough to allow the ants time to rescue brood and move to the surface. I find this takes somewhere between 4-8 hours depending on the amount of dirt and the size of the container.
In our lab we have a PVC pipe arrangement allowing multiple colonies to be dripped at once, but this set up could be as simple as a bucket under a slowly dripping faucet, or hose.
Fire ants undergoing the drip floatation method.
The water should drip straight down into the center of the bucket, to avoid washing away the baby powder. (Keep an eye out for splashing!)
As the water fills the bucket, the worker ants swarm to the surface, carrying brood and chivvying along queens and alates. These swarming ants will quickly explore every avenue for escape, so make sure surfaces are too slick with baby powder to climb, and any grass stems and debris have previously been removed.
Fire ants rescuing brood from rising water in the drip floatation method.
Large numbers of fire ants can make quite a pile, and each damp fire ant can wash a little of the baby powder away. The fire ants should be checked on frequently, and more baby powder added as needed, or you may enter the room to find a mostly empty bucket, and a swarm of agitated escaped fire ants roaming your floor!
Fire ants escaping during the drip floatation method.
As the water level rises, and the worker ants will form a living raft with their bodies, sacrificing the ants on the bottom to save the workers, brood, queens and alates which pile on top.
Fire ants form a raft on the rising water after the drip floatation method is employed.
This behavior allows fire ants in the wild to survive flooded conditions, floating to safety on a raft of their drowning nestmates. When they strike land (or another dry surface) the ants swarm out and seek a new location for their nest. These ‘rafts’ of concentrated fire ants can be dangerous to people who encounter them unawares during a flooded, as they will swarm over the person if disturbed.
Straining a fire ant raft from the water in the drip floatation method.
At this point in the drip floatation method, the ants can be strained from the surface of the water and moved to another container. For my research, I then divide field colonies into standardized experimental colonies with a set number of workers, queens, and brood.
A fire ant colony after being separated from the soil via the drip floatation method.
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Back to the techniques section.
6legs2many 
Excellent write-up and fascinating! I don’t suppose you notice if their mites come with them?
Do you know if this behaviour generally distributed among ants?
I’ve never specifically noticed their mites, but I do know that other symbiotes (silverfish and scarab beetles) often tag along.
From what I know this behavior is not found in all ants, and I’ve only ever heard the term ‘rafting’ applied to fire ants. I have heard of driver and army ants forming living bridges across water, and I wouldn’t be surprised if some other species were capable of rafting behaviors as well.
Fascinating. I’ll have to try this next time I go to SC. How deep would you have to do to get a nest?
Thanks,
Jody
Fairly deep if you’re trying to get the whole colony, at least if it’s a large one, but if you’re just trying to get a good chunk of the ants time of day is more important. Early in the morning the brood and queens tend to be high in the mound warming up.
Fasinating! I remember growing up in South Georgia – our perpetual battle with fire ants herding colonies to the next door neighbors yards. Question – what camera and lenses do you use to capture your images?
Enjoyed your website very much! Love Francee
what the hell is the purpose of this you are insane!
Obviously the purpose is to obtain highly concentrated masses of fire ants. 😀
But actually, I study them and I use this technique to move them into controlled laboratory colonies for observation.