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Having read a bit more about Achanthiptera, I think the puparia are too dark for this species - but there are many other flies that inhabit hornet nests. If you have a sample of larvae and puparia, then examination of the mouthparts and spiracles should enable a family determination.
Often larvae will leave their food source to find somewhere to pupariate, but the density of these larvae suggests that there may have been a food source under the PVC.