Something is eating the rose bushes!

I haven't blogged about this before, but for a few weeks now - almost since it began been growing this spring - I have noticed something was eating the leaves of the rose bushes (not the newly planted one, thankfully). The leaves look like this:

Leaf damage to rose bush.

The new rose bush planted earlier this year appears to be ok and is growing quite nicely now:

The Crescendo rose bush has many more leaves now and
is thankfully is not being eaten by the rose slugs.

I wasn't sure what was causing it - until today. I happened to look under one of the leaves and found this little guy:

Bristly Rose Slug on rose bush leaf

It looks and sounds like Bristly Rose Slugs to me judging by my completely unscientific internet search. From my little research, possible remedies include:

1. picking them off by hand (if there are only a few)
2. spraying with insecticidal soap
3. sprinkling flour on the bush for 24 hours, then rinsing it off (?)

I don't know who came up with the 3rd one or why that would work, but apparently it has for some people. I may give #1 a try since it doesn't seem like there are very many, but I'm very intrigued by #3.

One thing's for sure: I've need to act quickly because it's getting worse and worse every day.