I don't know what june bug larvae look like, but it's common to get Soldier Fly larvae in compost. They are big and segmented. Google up a pic and see if that's what you have. Soldier flies are harmless and will fly away rather than hanging around making more flies like house flies do.

It usually means too much moisture and/or nitrogen. Sawdust or other browns are actually a cure for that rather than the cause.

If the pile is gooshy and smells bad, you need more sawdust or other browns to balance it out.

Probably Japanese Beetle grubs - very common problem in compost piles. I just go through the compost and squish them. I can't think of a time since I've been composting, about 30 years or more, in four different states and even more locations, that I didn't have grubs in my piles. Of course, my piles are hot and crazy in the beginning, but then I have them just sit for six months to a year, maybe with only one turning in the middle. I keep trying to convince my kids that they should work to adjust their thinking, or at least their kid's(someday) thinking to include these grubs in their diet. A great protein source - breaded and fried, just like shrimp!! A nice stir fry. With the amount of composting I do, I could probably meet all the protein needs of a family in grubs. With world population growth, demands on food infrastructure, definitely something to consider - eating insects. I know - off topic!

As a rule the larva of the Scarab beetles, Japanese, June, about 30,000 members of this family, tend to live in the soil or other moist sources of food and seldom would be on the surface while the larva of the various flies will be digesting the organic matter on the surface of a compost pile if that material is moist enough.
As a rule the presence of maggots on compost is an indication the compost is too wet.

Just grass clippings and sawdust? My vote it is just a fluke and whatever it is is just looking for a favorable environment, not food.

I have no training to base this opinion on, it's just a WAG.

Lloyd

I get some of the huge larva in my compost too, maybe one a yer or less. I just get rid of it.

Japanese beetles, both adults & larvae, eat live plants. The larvae (grubs) eat roots, the adult, leaves and flowers.

If you truly have grubs, they're the kind that eat dead stuff.

If you have far more grass clippings than wood sawdust, you likely have soldier fly larvae.

We need a picture to tell you what you have.