I've heard of cases of some kind of unknown substance can kill an entire room of ant farms. I remember reading over on Antdude's forum...not sure what it came out to be, maybe unknown. This guy had almost all his colonies drop dead within days of each other, when they were healthy. I see this does, like you saw, happen with Solenopsis A LOT. Maybe they were sick to begin with. He ended up removing all the sick colonies, cleaning all his ant farms and I don't really remember what came of it.
Could be fungal (like mold or something which seems logical. Especially if it was black mold.), bacteria of some kind (which in a artificial environment may make sense)...or maybe if pesticides have been sprayed outside (they also do sometimes fly over and spray in some areas, mostly farms though) and they somehow get in, could do it too.
I'd in any case, if you haven't already, move the healthy Solenopsis colony and any other ant away from them and clean things out. Personally, I wouldn't risk all my other ants. Just my suggestion though. Maybe its just something about Solenopsis or something less apocalyptic.
Edited by Vendayn, February 7 2015 - 7:41 PM.