Apocalypse Swarm (Pathfinder)

[Benthesquid]

So, I'm considering having my party go up against a Blight Druid who's been incubating an Apocalypse (http://www.d20pfsrd.com/bestiary/monster-listings/templates/apocalypse-swarm-cr-3)Wasp Swarm (http://www.d20pfsrd.com/bestiary/monster-listings/vermin/bees-and-wasps/wasp/wasp-swarm) in his body, which will be released upon his death. A couple questions-

1) Since these will be subsequent encounters (IE, the Swarm won't appear until the Druid is no longer a threat), but one will probably immediately follow the other (taking prisoners has not been a major feature of the party's strategy thus far), should CR be calculated as one or two encounters?

2) Given the very real possibility of the Wasp Swarm multiplying, (it splits into two identical swarms, with half the HP, each time more than ten damage is dealt with a single attack) and the fact that they'll probably be fighting them in an enclosed space, is CR 6 really a reasonable number for an Apocalypse Wasp Swarm? I don't have a problem putting my party in danger (that was the deal when they got a 25 point point buy), but I'd prefer it not to be too overwhelming.

[Keneth]

Reasonable CR is any oxymoron. Combining two CRs into a single CR makes even more of a mess of everything. The question whether the party can overcome two CR 6 creatures is entirely different from whether the party can overcome a CR 8 encounter, even though they supposedly amount to the same thing.

So what you need to do is ask yourself is not what the CR of the encounter is supposed to be, but whether the party will be able to defeat the swarm after defeating the druid. You know the party best, if you expect the druid to die in one, maybe two rounds, then the answer is probably yes. If you expect the fight to last a minute and the party expending most of their resources, including any potential way of defeating the swarm (since it's immune to weapon damage), then you might have to figure out a way for the party to escape or deal with the swarm in one or two different ways.