Land mine
Land mines are the only passive defense system. The placement of landmines is not bound to the "square" raster, they can be placed "free". A land mine does enough damage to kill 1 Small Biter.
Being the passive defense that it is; if the aliens are not already aggro-ed to the player, the player can kill every single biter without them noticing a thing, provided the player has enough land mines and stays out of the way of the explosions.