As far as I know, flowers play a major role in getting five stars on your island. In my case, I had many, many outdoor furniture items around my island, several trees with fruit, no weeds or trash, et cetera; and I would only get four stars because of a lack of flower variety. In order to boost my rating back to five stars, I had to mix up some of the flowers and breed a few hybrid ones.
If you have many of the same flower, chances are you probably won’t get five stars. Off the top of my head, the game does a calculation of what you have on your island, and anything that’s considered an outdoor furniture item provides a small bonus towards the rating, and there are minimum requirements which include flowers. It will help boost the rating greatly if you plant several flowers all around, and maybe experiment with breeding a little bit.
There was a very helpful guide somewhere, but I forgot where it was. Google it, and it’ll most likely come up.
In regards to shrubs, they don’t possess special bug spawning powers, unfortunately (except the snail).