They told us in 2002 when our trip was ruined affected by the "friendly flies" in June that it was usually a ten-year cycle.  But it was my impression that the tent caterpillar infestation came the year before, and then the next year was the worst for the flies.

At any rate, we did not plan a spring trip this year.  We are going in September.  You folks who go in late spring can tell us how it went for you, and whether we need to think about "friendly flies" (ugh!) for next year!   :P

I completely agree:  they were NOT our friends.