I'm starting a new playthrough, determined to use powers that I haven't tried yet. Do the rats summoned by Devouring Swarm trigger any alertness of Corvo (going for a Ghost playthrough, but not a No Kill one)? I want to try using this power heavily, both to kill and to distract.

I don't know how powerful it will be, but it sure looks fun.

Hmm...I can't say for sure since I didn't try to Ghost yet, but I don't think it counts as the guards detecting you. As far as I could tell on my high chaos playthrough, anyway.

Guards have attacked swarms of rats in the streets during my play-through's before, without any notice of me. I'd assume no, summoning a Devouring Swarm of Rats to distract/attack/kill Guards doesn't immediately alert them to your presence.

I used DS quite a lot, and can tell you that it triggers a heavy alert in the area. If you use it, you need to clear the area quickly.

For a Ghost playthrough, I think there are better powers to invest in.