The bee-fly is not harmless to solitary bees and wasps, however, because it is a parasite. It is a bee mimic so that it can get close to bees and their nests. The female bee-fly flicks her eggs into or near the nest of a solitary bee or solitary wasp and the larvae that hatch eat the food stored in the nest as well as the bee or wasp larvae.
When I saw hairy-footed flower bees in The Alnwick Garden the other day I noticed bee-flies were taking nectar from the same flowers at the same time, illustrating how good the mimicry is, to our eyes at least. This is a large bee-fly
And this is a male hairy-footed flower bee.
I presume that hairy-footed flower bees are unwitting hosts of bee-flies. They are also parasitised by the cuckoo bee Melecta albifrons but I didn't see any of those. Maybe they aren't found this far north (yet!).