Bee-flies, tawny mining bees and blackcurrant bushes

There seemed to be dark-edged bee-flies Bombylius major all over the garden during most of April, either sunning themselves on stones or collecting nectar from lesser celandines, aubretia and primroses. 

These parasites drop their eggs near the nest holes of mining bees, where their larvae take over the unfortunate host's pollen store and complete their development at the expense of the bee's own offspring. The season for this activity in spring is quite short so the parasite and host's flight periods need to synchronise and the parasite needs to be able to find those egg-laying sites, which can be quite inconspicuous.

Bee-flies are restless, highly active insects that hover while the feed, and this makes them fairly easy to photograph, but they also spend a lot of time simply hovering in one spot, high above the ground, without feeding.

In our garden they almost always choose the area around the blackcurrant bushes to hang motionless in the air, sometimes for many minutes, and I've often wondered why they indulge in this energetically expensive behaviour, where they burn fuel in much the same way as a helicopter, simply to stay in one place. And what it is about the blackcurrant bushes that attracts them? It could be that  they congregate here to find mates, but they can easily do that at the floral feeding sites. 

I think the answer to why they congregate in the air above the blackcurrants is that one of the common pollinators of blackcurrant flowers is the tawny mining bee Andrena fulva, whose nest holes they parasitize. 

So could it be that this is where the bee-flies loiter to  identify their hosts, and then follow their pollen-laden victims back to their nest holes? If so, do they identify the mining bees visually - that orange fur does make them conspicuous - or is there something more subtle that gives away the host bees' presence - scent, maybe?

No easy way to answer that last question, but next spring I'll be watching again, to see if the hovering bee-flies do follow the mining bees when they leave the blackcurrant flowers and head for their burrows.