Dark-edged bee-fly

 

Saw the first dark-edged bee-flies Bombylius major in the garden at the beginning of this week, feeding on lesser celandine nectar. These cheerful little spring flowers have spread all over the garden and I gave up trying to control them long ago, as their nectar supplies quite a few early-emerging insects, including bees, butterflies and hoverflies. 

The agility of hyperactive bee-flies is wonderful to watch. They rarely settle for more than a minute or two. The image below shows a curious feature of the rigid proboscis, which is three-pronged at the tip - I imagine there is a protective outer sheath surrounding the long inner tongue.

Bee-flies parasitize the nesting tunnels of mining bees, laying their eggs in flight in the mouth of the tunnel. When they hatch they feed on the grub of the bee host. Unfortunate for the host, but the bee-flies presence must be a sign of a healthy mining bee population in the garden.

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.

Bee-fly - every mining bee's worst nightmare?

I photographed this bee-fly Bombylius major on a sunny bank neat Thorsgill wood in Teesdale last week.

These furry little flies that mimic bees are parasites of mining bees and several of them seemed to have just emerged from their underground nursery in bees' nests.

Their first actions on emergence are to feed on nectar, darting from blossom to blossom with incredible speed, rarely stopping to settle and usually just hovering in front of a flower, sucking up nectar with that long and deadly-looking proboscis.Violets and primroses, both flowering at this site, are popular nectar sources. 

Few flies look so menacing but they are totally harmless - unless you happen to be a mining bee.

When mining bees dig their new tunnels and provision them with pollen for their developing larvae a bee-fly will hover close to the ground near the entrance,flicking eggs into the tunnel entrance with its tail. When the eggs hatch the larvae will eat the mining bee host's own larva.

You can find more information, a bee-fly ID guide and a video of the insect laying eggs at http://www.brc.ac.uk/soldierflies-and-allies/bee-flies