If it is the Lily Weevil, the chart shows when the overwintering larvae are inactive in chart A. Eggs laid around the end of April to beginning of May the following year, the article states it was 1st May for them. Newly hatched larvae active from around mid May or a little later which would be the grazing on the stems but it's a little early, I would expect some extension either end.
The first part of chart A gives overwintering larvae as inactive until mid July, I find that odd as you would think they would feed until pupating when the lily is in growth. I think they should have added activity within that period and if so then it would fit that the larvae has woken up and started to fatten up before pupating, that's what they do.
OR, it's the moth.
