I have search and destroy checked. I see the spidertrons going out and retreating but there is nothing to attack where they are going. Their patch changes between red, orange, and cyan, and they seem to do this over and over in mostly the same place. Im not sure why this is happening? Love the concept!

This issue seems to have gone away.

The spiders look for chunks that have at least one polluted neighbour in a 5x5 area. If one chunk switches pollution (e.g. by having a forest or nest nearby), it could be that the spiders go there and cancel because the pollution is gone. Next check, it's polluted again and they cancel.

This should only happen at the between the polluted and the unexplored area. If it annoys you too much, send a group manually to extend the explored area.