In the media franchise, the bugs have a subspecies of Transport Bugs that are their primary means of space travel, as well as an even bigger subspecies called the Super Transport Bugs that can carry a Queen, several Transport Bugs, a full battery of Plasma Bugs, and a small army of Warrior bugs within it for 75 lightyears or so at the same speed as human ships.
There's no evidence that the Bugs themselves are capable of creating wormholes. It's more likely that if they use any, they're simply naturally occurring wormholes. Otherwise they rely on their own FTL capabilities (which the Transport Bugs have), or just launch spores into space at subluminal speeds.
And since the wormhole already exists, and they clearly have the ability to navigate interstellar distances, then it would just be a matter of maneuvering an asteroid through the wormhole in such a manner that it would strike Earth. They would probably use Super Transport Bugs and possibly the even larger Ice Bugs, which themselves are the size of large asteroids, to move the asteroids into the right trajectory.
Still, given that they haven't simply wiped out humanity with repeated bombardments or a single extinction-event-causing meteor strike, we can surmise that the Buenos Aires strike took considerable effort (and perhaps luck) on the part of the Bugs.
Also, I think it's important not to judge the capability of the Bugs in anthropocentric terms. It's not necessary for an alien species to have human-like technology/equipment or even human-like intelligence to be a threat.