Chris, Robin, and John Phips appear to be under the impression that the threat comes from this hornet, Vespa velutina nigrithorax, entering the hive. It would be an easy problem to solve if it were true, but in fact these insects hunt in groups, once a hive has been located, and a dozen of them can destroy a hive in a very short time. They seize the bees from behind and decapitate them.
There is a good piece of documentary film on you tube  at:
                    http://www.youtube.com/watch?v=L54exo8JTUs
The commentary claims 30 hornets kill 30,000 bees in 5 hours.
The map showing the spread of these hornets is based on sightings in 2010 and last summer I had reports from the fire service of sightings only 120 km south of Lyon, so they would seem to be moving up the Rhone valley. I expect to have to face them this season or next and am not looking forward to it.
Bil Harley (France) 

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