[evan_au]

Is there some sort of sting index where you can compare the pain different insects inflict

Yes, it's called the Schmidt Sting Pain Index, after the heroic author, who stung himself with a wide variety of insect species.
- He put the Bullet Ant in the highest category.
- He described the sensation of each individual sting using evocative imagery.
See: https://en.wikipedia.org/wiki/Schmidt_sting_pain_index

[Colin2B]

So why I never saw bees attacking a hummingbird?

How often have you looked? 

Both bees and wasps are sensitive to rapid motion, but close in they use different senses - smell, heat, static.
A lot of their communications are based on pheromones so they are very sensitive to smell, mammal breath in particular. Lots of mammals predate bee nests but very few birds - those that do tend to go for occasional ones in flight, although woodpeckers will systematically attack a hive. So I suppose bees have not evolved a threat response to hummingbirds.

[Bored chemist]

I just wonder if President Trump has sponsored research in this field.