I have given up on desensitizing; whatever happens happens. "Why," you ask? Take last Sunday. I took Hamlet for a ride...we walked along the street that he usually gets a bit nervous on, except that day trash was put out for pick-up, and one house had a very large pile of trash that he didn't recall seeing there a few days before. I had a borderline bolt at that spot!
Ten minutes later, walking along the tree line of a field, we stirred up a snoozing deer not 20 feet away from us. It startled me, for crying out loud! Hamlet barely twitched a muscle in his flank and went on his merry way.
Oh, butterflies: disturbing a butterfly that he hadn't seen sitting there -- spook-worthy. Walking into a cloud of six or seven fluttering butterflies -- zero reaction.