The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: I planted a whole bunch of marigolds in my backyard a couple of months ago and they were doing so well until recently I noticed a whole bunch of bugs that I don't know the technical name for, but my mom used to call them "Roley Poleys." They're little gray bugs that roll up into a little hard ball when you bother them. My marigolds are getting very brittle and a lot of them are breaking at the stem. There is also a few large slugs here and there. Please help me save my marigolds. But, I do have three cats that play in the backyard and I don't want to use any pesticides that will hurt them. Also, could you please tell me the technical name for these Roley Poleys?
Answer: Those Roley poley's are actually sowbugs (or pillbugs). They are not known to cause major damage to marigolds. So what is causing the damage? It is possible that the slugs you mentioned may be the culprits.
Slug Control: Slugs can often be controlled simply by eliminating their hiding places. Remove rotting boards and debris left on the ground. Pay special attention to keeping the crawl space under the house free of trash. Keep gardens weeded and surrounding grass cut short.
Also, small pans, can lids, or dishes at least 1/2 inch deep sunk into the ground at 3 to 4 foot intervals and filled with beer may attract and drown the slugs. The containers must be refilled after every rain.
Slugs can also be controlled merely by hand-collecting them. Do this after 9 or 10 at night.
-Corey Gerber, Insect Diagnostician