Wasp vs. Hoverfly: no contest

Here's something I've never seen before - a common wasp killing a hoverfly. They both landed in a rose flower at the same moment and immediately there was a fierce tussle, as the wasp grabbed its prey. 

Then the wasp bit off the hoverfly's head by chewing through its joint with the thorax. Strange behaviour because at this time of year there will no longer be any grubs for the wasp to feed in the nest, and when the nest begins to break up wasps tend to focus on sweet liquid food, like nectar and rotting fruit, rather than on live animal prey.