I also fear that some of the other, more established ant colonies around my place will get into my formacarium. I have small ventalation holes that smaller ants could find.
Unfortunately, it can happen. Even some indoor ants would not hesitate to raid whatever insects or foods you have indoor. What is worse is that this formicarium is outside and exposed to all types of ants, there are yellow ants which are even smaller than the black ants. I had to rescue the queen from the black ants last night. However, it appears that her tough exoskeleton has protected her very well. Nevertheless, she is paranoid now and I am no sure how well this colony would last.