Young colonies should be kept in test tubes until they have AT LEAST 50+ workers (which won't take long for Lasius niger to achieve).
Also Lasius niger have a reputation in that they don't like to move until absolutely necessary, so don't worry if they stay in the tube for quite some time, they WILL move if they must (but probably not before).
I recommend putting the test tube into an outworld and giving them some sand to work with. My Lasius niger stuck garbage to the cotton which created some mold and they used sand to very effectively suppress that mold.
DO NOT TRY TO FEED THEM INSIDE THE TUBE. Lasius niger is REALLY fast and ants will inevitably escape if you try that.
Also make sure they can't get out (check you barrier and put a lid with mesh on top of the outworld), Lasius niger are kinda escape artists.
Edited by Serafine, December 14 2017 - 9:49 PM.