I found this VERY interesting that spinach was mentioned. About a month ago I gut loaded my king mealworms with a mixed salad of spinach, watercress and rocket. Usually I just buy a bag of watercress but on this occasion they were sold out so I thought I'd give the mixed bag a try. 48 hours later 70% of the king mealworms were dead. I was so baffled I mentioned it to other people on another forum to find out if anyone else had experienced this. The answer was no. I have used king mealworms as feeders for over 5 years now and rarely lose one let alone about 20 in one go!! I think what I have read here about the spinach is evidence enough to me that it was the spinach that killed them. By the way, my other feeder insects were unaffected by the spinach.