Ye olde bee log

A couple of years ago (how time flies...) I wrote about the diversity of bees in our garden, including an image of Osmia rufa (the red mason bee) using a log that I had attached to our garden fence. Since then, I've made a rather more sophisticated insect hotel including new bee-blocks and have been following the development of bee diversity in our garden as the various features mature and are added to. In the meantime, the old log had become very rotten with little activity noted around it last summer, and so I decided it was time to take it down and see what was inside.

Old bee-log showing extensive fungal decay and woodworm activity.

The outsides had been heavily arracked by woodworm and many holes and crevices had suffered fungal decay with tufts of white mould clearly visible. Onto the deadwood pile with those bits... Splitting the core open provided more interesting features however.

Empty O. rufa cocoons, one in situ in the hole drilled when making the bee-log, the other removed to show the thin papery texture.

The remains of the walls/plus separating cells which are arranged in sequence along a hole.

A view through two O. rufa cocoons - because they are arranged along a hole, the outermost one must leave first, the next then passing through its cocoon to emerge.

The remains of a cocoon including frass (insect faeces), presuambly from the insect before the cocoon was formed.

These are simple enough observations but rarely seen as logs need to be split open to view them - this is of course destructive and is only being done here because the log was past its 'sell by' date given the amount of decay and lack of activity last year. The life cycle of O. rufa is well documented (e.g. here) so rather than repeat this, I simply want to show which aspects of it are readily visible with a little careful effort. For example, although not photographed, a number of holes contained mouldy yellowish masses - pollen stores created by females for their young, but which were uneaten and subsequently began to decay, presumably as the young did not develop for whatever reason (it was a cold, wet summer in 2012 which can not have helped). Others showed mere traces of pollen storage, suggesting successful emergence. Also, as holes in logs are useful shelter for other species, it wasn't too surprising to find evidence of use by insects other than O. rufa.

Smaller pupal cases inside the bee log

One of these non-Osmia rufa pupal cases removed and magnified, approx 6mm long. The red line follows the split where the insect (whatever it may be) emerged; the green arrows indicate a pair of short posterior processes (breathing tubes) typical of many fly pupae.

Beyond determining that it is a fly, I can tell no more about this species - it might be parasitic, it might simply be using a handy hole for pupation with no other link to O. rufa. However, among the various debris and evidence of O. rufa long since left, I did find one intact bee cocoon.

The one remaining O. rufa cocoon, approx 11mm long blocked in by a number of soil and wood particles.

A small female O. rufa emerging from the cocoon.

The small 'prongs' or 'tampers' (one indicated by a red arrow) on the face of the female O. rufa, used to push mud into place when sealing a hole, although they are under-developed in this specimen.

I must say I was surprised to find an intact cocoon and initially thought it was one that had failed to emerge last year. However, in the warmth of my study I noticed some movement and a bee began to emerge. Its movements were rather feeble so a helping hand (well, pin) was used to pull off part of the cocoon. The female that emerged did not move strongly and was very small with (to me) under-developed facial prongs - possibly a poorly developed specimen from last year's poor summer which had managed to hibernate successfully. She has now been released on the bee-hotel - hopefully there will be a emergence of plenty more from the many plugged holes in the newer logs.