
They certainly can become devastating to your rosebushes blooms if the are the grubs from Japanese Beetles. It is best to apply some control to get rid of them. I do not like to use insecticides unless I really have to, in the case of Japanese Beetles and their grubs control it is a must do.
Here are some links to articles on the grubs and damages the Japanese Beetles can do, along with a product to help control them:
https://www.gardeningknowhow.com/plant-problems/pests/lawn-grubs-how-to-get-rid-of-grub-worms.htm
https://www.gardeningknowhow.com/ornamental/flowers/roses/japanese-beetles-roses.htm
http://www.bayeradvanced.com/insects-pests/products/season-long-grub-control-plus-turf-revitalizer