It seems like a weird past-time, magnetizing ants, but it has some practical purposes. At his blog, media engineer Andrew Quitmeyer explains how he mixed magnetic powder into insect-safe enamel paint, and what he was able to do with it.
The big benefit to something like this is that it could allow scientists to easily alter the populations of social insect groups. Each colony of ants functions, in many ways, like a single organism. So what happens to that hive mind if you remove all the ants doing one particular type of task? Instead of painstakingly picking out each worker with a pair of tweezers every time you want to try this, you could create a colony in which all the workers have had magnetic paint daubed onto their abdomens. Then, you could quickly and easily collect some of them, or all of them, using a magnet. Hunting ants with a tweezer once > hunting ants with a tweezer over and over and over.
Another, possibly less legitimate, use of the paint is demonstrated by Quitmeyer in this video. (Quitmeyer, for the record, is not a social insects researcher.) Using single painted ants in a population of unpainted ants, he plays around with the way colonies remove unhealthy members of their own community. When a magnetized ant starts flopping around erratically in response to a nearby magnet, nearby ants quickly react.
As Quitmeyer says in the video, this demonstration quickly passes from science into mad science (or, at least, YouTube science).
Thanks to Leah Shaffer!