Thanks to Crazy 8's, I've been butterfly obsessed. The thing that annoys me is not having insects show up UNLESS you have the collector thing. If you know where to go, you MIGHT get some of them. Obvious place in SV is the simhenge place. Got my first rainbow there, all the way to the rear. Here's a clue. If there are a lot of stones and other butterflies, chances are you will not see a rainbow. If there are only a few other butterflies, you might catch a rainbow OR you might get a Light Beetle in the same place.
I just checked France again today with a different sim, no collector, and very few butterflies anywhere, including the places where I know they spawn. Annoying, but proved my point.
Now, about that island. Errrhmm, how exactly do you get there? I'm thinking it's a secret tunnel in the house opposite it or a tunnel from the frog pond spot. Just a guess but rabid readers would like to know. (Can you tell Rita Skeeter was one of my favorite HP characters?)