One of my hives that successfully over-wintered here in NorCal has been absolutely booming but had also turned very aggressive after being quite calm last year. Last week I split it by simply putting half the boxes on a new bottom board 10 feet from the original location, leaving the rest where it was. I didn't bother to check where the queen was. The new location hive has two deeps, the original has one deep and two mediums.
Today I went back and took a look. My first observation was that the original hive had lots of activity and almost none on the new hive - makes sense given that the foragers all would still be going to the original hive. I opened the hive at the new location - a good population of bees who were very calm. Pretty quickly I found the queen as well as saw lots of capped and uncapped brood. OK, so this hive is fine.
I then opened the hive at the original location. Youch! Very quickly I was attacked by hordes of very aggressive bees. Smoke didn't make a difference. I managed a quick look at a few frames before giving up and did see some capped queen cells, so they're well on their way to making a new queen. I ended up with at least 8 stings thru the suit and on my ankles, and was chased by bees back to my car. VERY hot hive.
Obviously the two hives have the same genetics, and the existing queen is the source of the now-aggressive hive. However, the hive she's in now is quite calm (though it has a relatively low population and few foragers). I'm wondering if i should re-queen one or both of these with queens from a breeder. I can get queens easily and now would be the time to re-queen the hot hive at least. Is this a case of genetics or environment?