My L. niger colony started hunting at size of 7 workers. I have moss in my formicarium and a lot of very small critters live there. Most of them are smaller than worker's head, so they aren't any serious meal, I think ants hunt them for fun
There are some alternatives of insects:
- raw meat, especially things like liver etc. Most ants sometimes eat vertebrates meat when they meet any carrion. Very small pieces of dog or cat food can be quite good protein source.
- egg yolk mixed with honey - very good universal meal.
- dried blood (used for fishing, so you can buy it in fishing shops), melted in water. Beware, however, it STINKS. Fresh blood is also good if you have some to spare.
I use all of the above, AND also insects - this is most natural. But most ants are very versatile and can eat many things. So their food should be a bit diversed, I wouldn't like to eat only one species of insects every time if I were the ant.