I have dozens of answers. None seem to uniquely require the fly and the tree, however, and so are unsatisfactory answers to a riddle.

Riddles are meant to be solved, and the creator knows they are meant to be solved -- this info is available to both riddler and riddlee. Hence, a riddle should contain elements that tell you you've found the right answer, not just "an" answer, and I can't find any answer that does that for this question. (Doesn't mean there isn't one.)