From Ender's Game:

Ender, look around you. Human beings didn't carve this place. We like taller ceilings, for one thing. This was the buggers' advance post in the First Invasion. They carved this place out before we even knew they were here. We're living in a bugger hive. But we already paid our rent. It cost the marines a thousand lives to clear them out of these honeycombs, room by room. The buggers fought for every meter of it."

From Ender's Shadow:

Bean knew at once that this place was not shaped by human hands. The tunnels were all too low -- and even then, the ceilings had obviously been raised after the initial construction, since the lower walls were smooth and only the top half-meter showed tool marks. The Buggers made this, probably when they were mounting the Second Invasion. What was once their forward base was now the center of the International Fleet.

Also, Eros does not appear anywhere in the First Formic War book series or comics that Card co-created with Aaron Johnson.

So then did the buggers use Eros during the first invasion or during the second invasion?