You will need an efficient ant collection device.



An aspirator (pictured below) will suck up the ants from the ground and collect them into a container. You can buy an aspirator from a science supplier, but it is just as easy to make one yourself. Below is one I use built from a cordless hand vacuum. The essential components are the collection chamber (plastic food storage container) and the mesh screening over the source of suction.



An aspirator will allow you to collect large numbers of army ants when you encounter them (see step 2). Collection of the evacuating ants also necessitates an aspirator, as they will be evacuating their nests rapidly (see step 4).



