Natural drone culling

At the hive entrance

At the hive entrance. Click for close-up

Bees are clean creatures, and the bodies of those that die in the hive are removed by “undertaker bees”. I noticed a worker struggling to fly off while carrying a white pupa away the other day. Then there was a day of heavy rain, which prevented flying with loads, at the end of which there was a collection of bodies at the hive entrance.

Close-up view, different angle. Click for larger image

Close-up view, different angle. Click for larger image

Let’s have a look close up at two of those bodies. You can see the big eyes on one, compared to the standing worker in the other picture – it’s a drone. Also note the rounded bottoms (no stings) and large size relative to the standing worker in the first image, again this indicates these are drones. Note also the way the nearest body’s legs are curled up: they’ve never unfolded, this drone has been pulled straight out of a cell near the end of pupation.

So is this a sign of a problem? Not exactly. It could be an indicator of a number of things:

It could be an example of the colony changing its mind about swarming, because our weather was lovely for it a couple of weeks ago but has turned colder, with intermittent wet and wind since then. As someone said, it’s as if the seasons have run backwards for a while.

It could be an example of “hygienic behaviour” – the hive senses there is something wrong with these pupae and is expelling them from the hive before the problem spreads to others.

This is one way that colonies can control the varroa parasite. Varroa mites preferentially lay their eggs in drone cells, because drone pupae take longer to mature, giving the varroa more time to reproduce in those sealed cocoons. So colonies use drones as bait, deliberately sacrificing them. However, we see no wing damage on these bodies (an indicator of varroa), and I don’t think they would leave drones right to the end of maturation for that.

There are no other signs of disease in the hive – no bad smells, odd behaviour. The workers seem very happy foraging and are not defensive.

It may also be due to the drones being damaged by being chilled. Drones seem to be considered more expendable, and their cells are always near the entrance / outside of the nest so raiding wasps etc get them first. Chilling can kill pupae, and if they don’t die their lifespan is significantly reduced (by weeks – researchers have measured this in impressive experiments involving tagging and monitoring many bees). This would explain why it only happened with one hive, which is really a relatively poorly insulated nucleus box which I couldn’t get them out of. It’s currently sitting on top of a Warré as a “growdown”, i.e. “start building comb down in that nice warm box, you idiots!”

So my conclusion is that it is probably chilled brood because I don’t see this in the Warrés; possibly a change of heart about swarming; but not disease or varroa. The change-of-heart theory is also supported by the observation that there are almost no drones around at any of our hives, and swarm season has been delayed throughout Oxfordshire – previously it has started in April, but everyone I talk to in the area (conventional and natural beeks) are remarking about how their hives aren’t building up as fast as usual this year with the weather, and swarming only just seems to be kicking off now.

On a delightfully gruesome end note: I kept some of the bodies in the photo for a day while I checked with Gareth if this was a sign of disease, or just the drone culling I’d read about. A day later, I noticed one of the more normal looking drones was wiggling its legs feebly. The workers don’t care if they’re still alive when they are thrown out.

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