Loitering with intent. Parasitic fly at my Bee Hotel.

18th May 2012
Within a few days of reporting that my Bee Hotel was getting its first Red Mason Bee tenants, I have some other less welcome but fascinating visitors.

I was checking on activity at the box, when I noticed a very small fly (about 3mm long) loitering by one of the holes. A Red Mason Bee female arrived and went into the hole but the fly stayed put. An entomologist friend of mine told me to always take note of flies associated with bee colonies as they may be potential parasites. This was good advice and I’ve managed to photograph several parasites flies recently. I’ve noticed that generally, they tend to be quite “bold” and untroubled by bee (and photographer) activity. I assume that this may well assist in the parasitic lifestyle.

I got a couple of good shots; the fly allowing the lens to approach within a couple of centimetres - necessary for detailed images of such a small fly. I posted one of the images on the Dipterists Forum and was advised that it looked like Cacoxenus indagator; a drosophilid fly and known parasite of solitary bees.

Reading up about it, I found this to be a most fascinating creature. They particularly favour nests of the Red Mason Bee - Osmia bicornis (=Osmia rufa). Like many other solitary bees, the Mason Bee provisions her nest hole with pollen and nectar stores, contained in individual brood cells. Each cell is separated by a wall of mud and a single egg is laid on each food store. Cacoxenus indagator females use these brood cells to lay around eight eggs. The fly larvae consume the food store, depriving the bee larva of the food. The following Spring when the adult bees and flies start to emerge, some of the flies have a problem. The larger bees can break through the mud walls easily and this may make an exit hole for the flies too. Where this hasn’t happened, the flies break the barrier by pushing their heads into crevices in the mud and inflating them. This is done by pumping haemopymph, the insect equivalent of blood, into their head. Even though other flies can use a similar mechanism for escaping their pupae, in Cacoxenus indagator, this is a much more powerful process. Fascinating. Nature magazine nicknamed it the Houdini Fly!

See another BLOG post about flies that parasitise bees.


Photo comment By Paul: For people like myself who try and increase mason bee populations - I also raise Osmia cornuta more prevalent in Europe - these flies are one of the toughest creatures that we have to deal with. It's good to get some more information on them, so I've posted your source blog post (on Miner bees) on our Campaign for Solitary Bees FB fan page. Thanks.

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