I appreciate that the correct name for a group of bees is 'swarm', which is accurate whether the bees are clustered together or not.
With that in mind, how correct is it to refer to the same group of bees as a bunch, as opposed to a swarm?
The NOAD reports that bunch means:
Swarm (except for the meaning #3) is understood as a group made of a large number of people/insects/things, which is similar to the meaning of bunch in #3.
Looking at the Corpus of Contemporary American I obtain the following data (I added a swarm, which includes all the phrases where a swarm is not followed by of bees, just as comparison):
Swarm is a technical term relating to bees. It doesn't mean any collection of them (as herd or flock might mean.) It's a really specific thing that bees do, and it happens pretty rarely - many bees will live their whole lives without being in a swarm (especially if they live in a well managed hive) and I doubt any bee has ever been in two. A hive of bees refers to not just the physical box they live in, but the mass of (generally all one family) bees that live there.
If you're out and about and you just meet a large quantity of bees for some reason, and they really are bees and not wasps, I think you can use whatever word you like that means "a lot of".
"A bunch" can informally be used to mean "many", "a lot", or "a large group" in many contexts. In this case, I'd say it's OK. I think it's a subtle distinction; where "swarm" refers to the actual group of bees, but "a bunch" emphasizes the number and not the group. That is, "There's a bunch of bees in that tree!" emphasizes that there are many bees, while "There's a swarm of bees in that tree." is simply stating that there's a group. Though "swarm" indicates a large group of insects, a group capable of swarming, it not emphasize the number.