What type of fly larvae? Off the top of my head, I can't think of any fly larvae that would be harmful. Most make delicious and nutritious box turtle food!
I have never looked into the avocado thing, mostly because they are just so expensive. Here's a link where the people have researched the risks...it seems that nobody really knows.
http://kgkat.tripod.com/avocado.html
It sounds more like an allergy in relation to the fruit, and toxicity in other areas of the plant.
All things considered, it really just doesn't seem worth it. Avocado is pretty high in fat (although it is "plant fat"
, they are expensive, and the risk is not clearly defined.
Hope this helps. Good luck.