Killing the smaller mantid works mostly as a distraction. It forces them to replenish their numbers before they consider any serious offensive.
The big ones, though - we kill a few of those, and we really disrupt operations on the island. That buys ourselves a substantial amount of time. With mantid, it's like their size is directly proportional to their importance in the empire.
So kill some of the big ones for me, while you're out there.