I had the same question while pulling all the grass out of the overgrown veg garden yesterday. Bought the house last fall and the prior owners didn't garden last year. Dozens of Japanese beetle larvae. Won't have the chicks for 2 weeks and I guess they would have to be a week or 2 old before throwing them something that size.
I may have missed the garden prep "fresh treats for chicks" period this year. I'm pondering freezing some but I'm not sure the family would care to find a container of grubs in the freezer. You can be sure those girls will be out there with me next spring, though there might not be too many JB grubs without all the grass.