The only significant die-offs I've seen are from overheating. I'm sure there are several things that could have caused the problem but your theory seems like a good starting point. I assume that vegetables are washed during processing, before they get to the stores, and if you're getting them directly from the farmer they might have enough pesticides on them to be toxic to the larvae. I wish I could be more help, but truthfully, your guess is as good as mine in this case.
I normally sell BSF starter kits, but I shut down that operation a few weeks ago due to several family issues I'm dealing with. I'm traveling so much that I can't maintain a large number of larvae.
A starter kit isn't necessary in most states and in Florida you should be able to attract wild BSF easily. Fermented corn has been working great for me and I've also had good results with rotten cabbage. I'm hesitant to talk about creating sour smells to attract BSF because people usually assume a BSF unit needs to be stinky. The smell of fermenting/rotting grains and vegetables is great for attracting BSF but strong smells are not a necessary part of operating a BSF unit. Once the colony is established the subtle scent of the larvae is the best attractant. Any strong smells after that point should be a warning sign that the colony is out of balance. Did you read my post about using corn? - http://blacksoldierflyblog.com/2010/04/28/attracting-black-soldier-flies-with-corn/ If I were you I would first get a few cabbages, seal them in plastic bags and leave them in the heat until they start to rot. Open it once it gets smelly and it won't be long before the female BSF find it.
