There are happily a lot of them about now Kay, which is lovely to see, but I understand your concern. I think much depends on the cats themselves.
Both Ross and Tinky dont bother the bumblebees much in the garden - occasionally they take a half hearted swipe, but the bees are a bit too erratic for them, flightwise. Ross had one land on him the other night, while he was taking his ease by the back door. He just stared at it, half interested and half unconcerned. It flew off before he'd decided what, if anything, he should be doing about it. Paddy and Mac never got stung either, and didnt bother about the bees. The boys do try to catch the butterflies, but as I've never heard of anything being butterflyed to death, it doesn't worry me.
When we had Flynn, he was stung twice, but then he was a very curious and persistent little lad, bless his heart. His paw swelled up a fair bit each time, but an application of bicarb after we got the sting out settled it down, and you can give cats anti-histamines in the same way you do people.