B4 is a very harsh and unforgiving world. The atmosphere is toxic, and human troopers without highly sophisticated breathing equipment will die within minutes from inhaling even a small amount of the air on this planet. All of the lakes are poisonous. Despite this hostile terrain, the Bugs have no problem operating on the planet.
Season 8 Alternate Timeline
In the Middle Kingdom alternate timeline in Season 8, the Bugs were driven off B4, humans colonized the planet, renamed it Julian, and it became the capital of the Western Reserve faction. This was mostly backstory, though, so details like whether the planet was terraformed of if the colonists lived in habitation domes instead was never established.