Grapes are one of the foods that do hold a larger concentration of pesticide, as compared to other fruit. I try to buy organic grapes when possible. Still better for you than processed foods.
And I know this is rather unrelated as I'm talking about people rather than pets but I thought I'd throw it out there in case there was anyone else like me, for whom pesticides are a consideration. (I have cancer in my medical history so I exercise a little extra caution in things like this.)