I can relate. All of the spawns are on timers; otherwise the towns would get cluttered with uncollected goodies over time. Fishing spots also go through cycles where they are "great" places to fish for some period of time, and then become "decent" places to fish - until it's time for them to be great again.
I do think, though, that the insects aren't around as long as gems and metals. Having a rock disappear when my Sim was en route to snag it isn't something that has happened to me yet, but it has happened several times with insects.