Thanks for the tips, 'catters. I think Janie might have the answer. I'd assumed the few hollowed out roots were anomalies of dehydration, but I'll bet iris borers are the culprit.

Pdq, thanks for defending the moles: I knew they were insectivores for the most part, but wondered if they might chew roots to get them out of their way when tunneling.

I really try to strike a balance with the wildlife, including the insects. If a plant is getting eaten by insects, I usually don't replant that species for a couple years. If it's aphids, I wash 'em off. Grubs get thrown to the birds if I find them, and I'm happy when there's a skunk around to eat them - less happy when the occasional bear comes through and literally rolls up the lawn to get grubs.