This is an interesting question because there is a general belief - and observations by Brother Adam, the creator of the Buckfast strain - that F1 or F2 crosses are often hotter. Brother Adam was crossing between different bee subspecies and we are mostly crossing our selected line-bred strains with 'mongrels', so the situation not be exactly equivalent.

Although I've noticed the same thing, I've never been sure whether the crosses were especially bad tempered or just comparatively bad-tempered against the well-behaved pure bred bees. Is this just a regression towards the mean or is defensiveness in some way enhanced? 

I'm inclined to believe the former because I don't see a good mechanism for the latter, but a few very mean colonies have have had me wondering. We probably respond to these by seeking a plausible explanation, while ignoring the fact that some the other out-crossed colonies are not so bad.

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