I need to add that these are GREEN June beetles that fly by day, not the kind that go to the lights at night. Apparently they're two different bugs.

Your mom is right. They don't eat plants. So that means they don't eat your tansy.

They're around the compost pile because that's where they're laying their eggs.

So when you mix the compost, likely you'll see some *really* large grubs. The grubs don't eat live plants either. Instead they specialize in the already dead stuff.

As far as buzzing you, consider that as a huge nuisance. They won't bite or otherwise *attack.*

The adult June Beetles do not do much of anything except mating so the females can lay eggs that will become the grubs, those "C" shaped creamy white with orange heads thingys in your soil, that will become next years June beetles. The life span of the June Beetles is pretty short and I would be really surprised if you had any around now.
Other that being a nuisance, on rare occasssion I have had one latch onto me with those pinchers, they do no harm, do provide a food source (both as grubs and as adults) for many other critters.
Around here these June Beetles were few and far between, possibly because it was cooler than average here.

Those things may not be eating the garden, but I loathe them all the same.

They're uncoordinated and rude as hell. I mean honestly, how rude is it to go buzzing noisily around with a fat, hard body and absolutely no intelligence? They're like noisy flying pebbles, only not as smart. I can't count the number of times I've seen one fail to dodge an entire building.

Whack, the June bug hits the ground and I can imagine it think, "woah, duuuuude... did you see that house jump out in front of me?"

Hate. Them.

Plus, one put a dent in my truck a few years back when I was driving long distance on the highway. I've had a special hatred for them ever since.

I had many around here each June-July, and they're at the end of their cycle now. But I can't say I know what they eat, as I don't get any real plant damage. I know they eat compost, and spend time in my pile through the winter. Sometimes I turn the pile, and a mole-rat will pop out. Apparently they like to eat those grubs. The Japanese Beetle was another thing as far as eating plant leaves. The last couple of years I had more than ever. I was emptying my traps at least once a week. This year, I have hardly noticed them. The bag for my trap has been hanging for almost a month, and I don't need to bother emptying it until I take it down soon, for the season.