Q: My thornless blackberry died suddenly and I found larva in the cane. Can you ID these critters?

A: Looks like red-necked cane borer. The larvae grow in the blackberry stem, causing it to enlarge, about a foot from ground level.

Scout your plants in winter, looking for the tell-tale knots. Remove infected canes from your property.

see
Red-necked Cane Borer