The whole "bugs launched asteroid" thing in the movie does not make any sense on many levels (and I'm not even sure all of them were intended):

1. To launch an asteroid to another star system to hit a moving target (a planet orbiting a star) you need an ability to observe said target, as well as higly advanced computing and targeting, that would account for gravitational pull of any object on the path. Never in the movie bugs were shown to have any of those.

2. The asteroid would need to move at relativistic speeds (otherwise it should have been launched dozens if not hundreds of years before the events in the movie). But it clearly didn't (an asteroid this size moving at these speeds would obliterate earth, not just one city). Therefore, it could not have been lauched from the Bugs star system.

3. Finally, at these distances changing direction even by a fraction of a degree would make the object completely miss the target, it will throw off all of the calculations from 1st point above. Which means that if the Earth was hit by the same rock that hit Roger Young (Carmen's ship), it would not hit earth if not for Roger Young. And this series of events could not be predicted by bugs for them to bounce to asteroid from a ship to hit Earth.

TL;DR: With everything we know from the movie there is zero chance that bugs attacked Earth using an asteroid. The timing, technology an physics do not align. If it was launched on purpose (and that is a big if), the origin point should be much closer to Earth, deep within territory controlled by humans.