From careful experimentation (i.e. randomly shooting bugs) I've discovered that the aliens become hostile if you attack them. However, I can't quite tell if this is per-unit (i.e. that unit becomes hostile), per-nest (all units from the nest) or global (all aliens) become hostile. They certainly don't all attack at once the same as if I declared war on another civilization. I've also seen them attack first if I stray too near their nest.
I also noticed an upgrade (can't find it now) that claims to cause their hostility to go back to normal twice as fast, which suggests they have some internal aggression counter that gets increased when I attack and decreases naturally at some rate?
So, does anyone have a good summary of how alien hostility works and what I should watch out for?
Best Answer
Aliens become more hostile if you attack them or if you step within one tile of an alien nest.
Aliens become less hostile over time, particularly with one rank of Harmony,
which causes aliens to return to neutral twice as fast;
they also become less hostile if an alien nest actually exists within the borders of your city, eventually becoming friendly.
(Note that allowing a unit to walk over the nest will still destroy it, causing all friendship gains to disappear.)
Neutral aliens, particularly siege worms, seem to attack infrequently for no discernable reason.
I've read rumors that hostility broadly affects alien relations with EVERY civilization, but I don't know yet how much truth there is to this, much less how this interacts with any particular civ's unique situation.