Can you try the local dendro dept in the uni or college?

Well, I called the extension office. No one is there, of course, but they say they'll get back to me.

Updating this post as of possible interest to growers in the Chicago region and north.

I contacted the IL state entomologist, who says they don't monitor for OFM in this part of the state, on account of they aren't supposed to be here. He looked at the specimens of larvae I sent and declared them not to be OFM but some sort of weevil, like plum curculio.

He also says we don't get a 2nd generation of PC here in this part of the state, which leaves me as baffled as when I began.

There weren't any egg-laying scars such as PC make on the infested fruits.

I've had this damage before in my Stanley plums, never before in the Japanese - which haven't had a proper fruiting season before this year.