The large gland reserve appropriated for this purpose makes eating adults a distasteful experience. Even an ant with a full load of sweet honeydew in its crop tastes extremely acidic. The pupae on the other hand, do not have this acid flavor and are, if I must admit it, quite tasty. Having dug into so many mounds to document the colony cycle of Formica, I learned that the brood cycle is very predictable for a given species in any one region. Regular checking of a single mound will quickly reveal when the pupae can be harvested from all the mounds.
Ant workers take meticulous care of their young.
The smallest larvae are kept in moist areas of the mound. The pupae however need dry and warm conditions and are kept separate from the rest of the brood. The mound-builders make it particularly easy for pupae collection because the workers move them to the highest reaches of the mound where the sun can warm them. Formica adults will even remove the paper-like cocoon from the pupae several weeks before they have sclerotized, sort of like shelling peanuts. The best time to go pupae collecting is one hour after the sun has hit the mound in the morning.