Someone else asked about this a month or so ago. Yes. JB's eat just about everything--in my garden at least. However, they do not seem to really destroy the dahlia folliage to the extent they have decimated my hardy hybiscus folliage.

Do the rose trick. Every morning or afternoon take your pan of soapy water and knock the JB's into it. Or if your not the timid type, squish the little devils with your fingers.