I have found similar larvae in my yard, but here they are only 3/4 the size of yours. I would check with your county agent or, since we have a land grant University here, we have an Urban Garden Center as well as a department of entomology.
I've never found out what mine are and I am a pesticide applicator in Turf and Ornamental issues. These have never been a problem for me, or my clients or the plant pathologists who send me work, so I never pursued it, but I do know where to go for info. Now you do too.
Your results may vary - Good luck
Carl