Post world war 3 trilogy with slugs?

[emilyelizabeth]

I read a trilogy when I was younger that I loved and would like to read them again. I vaguely remember it so bear with me. I remember it being after world war 3, or maybe post apocalyptic. There is a human colony on another planet that is being taken over by strange mutant slug things. The books are centered around the people that are trying to secure the colony. I remember the cover of one of the books having a guy in his space suit that is reminiscent of the current Halo costumes. Sorry that this is so nondescript, it's all I've got. Thanks