nartposting

Colonies have interesting inheritance rules so to speak, once bonded to a parasite user they can't really just go on to become someone else's colony easy peasy. Otherwise parasite users would just. Steal their opponents Colonies. (And, if you also hold the hc that losing their colony would kill them, even more reason to.) So, once a colony has become parasitic to a specific person it's just that person for the rest of their life.

Following this, the kikaichu (the "base" insects of the Aburame) occur frequently in the wild and are relatively mild to handle, which is why they're the standard. Other insects, like Yoji's mosquitos or Torune's beetles, can occur in the wild but do so much less frequently and require intentionally seeking them out.

When an Aburame is born, the newborns are essentially offered up to a selection of colonies to see which one takes. Most of the time, these would be wild caught, which is why (in addition to the skill required to handle them) every Aburame doesn't just use the most op bugs. The bugs is limited.

Theoretically if part of a colony was removed from the host, over many generations they could be far enough removed to be given to a different user. This would be a long and tedious process, as it is not exactly easy to maintain a colony outside of a host once they've become reliant on the parasitic relationship, making it an inefficient and undesirable method.

The last option is inheritance. While a parasite user is alive, others wouldn't be able to control their colony. Once they're dead though, it's free game. This is why Shino is able to breed and control Torune's beetles. I also believe that Torune himself inherited the colony from his father when he died. Of course rarer more specialized insects would be passed down rather than losing that power for the clan. Familial inheritance would also strengthen the chances of a user having the required toxin resistance.