this was probably going to happen. those japanese hornets are nasty. not to mention the european bees are not native creatures being introduced into a locale that has the hornets that already attack a native bee for their larvae.
if someone were farming honey with european bees in japan, the hornets are going to attack and eat their larvae.
the japanese bees have developed an interesting defense against the hornets.
click around on some of the related videos for some more knowledge...