I have successfully kept Leptothorax colonies in small formicariums (microscope slide nest type) for a few years now. Keeping them clean and changing food regularly has kept them free from mould, diseases etc.
But suddenly prepupae are dying in large numbers! Worker ants seem to be "aware" of the problem, since they carry the affected prepupae outside the nest. To my eyes the prepupae carried out look normal, but they collapse in a day or two, so obviously they are not. :-(
I have not experienced this kind of disease(?) before. Any advice for saving the remaining brood?