We get a few queries about these at work. A couple of big hoverfly species are moving north in distribution: I got a real whopper in my garden this summer for the first time. I quote from the county hoverfly recorder:
Volucella zonaria and Volucella inanis: spectacular new additions to Gloucestershire’s hoverfly fauna.
Volucella zonaria and Volucella inanis are nationally notable hoverflies that were unknown in Gloucestershire before 2001. They have since gradually become established here, and are being seen frequently at this time of year.
Volucella zonaria (about 19 mm. in length ) is easily the largest of British hoverflies. Volucella inanis is smaller, but at about 16 mm. is still among the half dozen or so largest British species. Both are good mimics of social wasps, in particular the Hornet (Vespa crabro) – indeed Volucella zonaria is sometimes known as the Hornet Hoverfly. Many hoverflies are good mimics of bees and wasps, their mimicry giving them protection from predators such as birds. However these two Volucella species are associated with wasps in a more direct way: their females lay their eggs in the nests of hornets and other social wasps, and their larvae develop there as scavengers.
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While many scarce hoverflies can only be found in such habitats as ancient woodland or primary wetland, Volucella zonaria and Volucella inanis favour gardens and urban parks. Although they feed on nectar and pollen from a wide range of flowers, Volucella inanis is often at bramble and Volucella zonaria at Buddleia, while both species seem to have a special affinity with the Hebe cultivar “Great Orme”. The peak of their flight season is August.
Recent news suggests that Volucella zonaria, itself a relative newcomer to Britain, may be in danger of eventually losing its status as the country’s largest hoverfly. There have been two reports this year of sightings in the south-west of Milesia crabroniformis, a continental hoverfly, which, as its name implies, is another hornet mimic, and which is even larger, at nearly 25 mm., than Volucella zonaria.
There's loads more details, plus the usual excellent photos, on the Gloucestershire Naturalists' Society website: http://www.glosnats.org.uk/ - you'll have to scroll down to near the bottom of the page to get to the hoverfly section.
As you will gather, if you see a lot of these hoverflies then there's likely to be a wasp's nest somewhere in the vicinity. But then, we already knew that
