Communal mining bees

A few years ago, a crack in the concrete path at the front of our house became busy with bees coming in and out. We assumed they were honeybees that had settled their hive there, but on closer inspection they turned out to be mining bees instead. Their cleptoparasite bees, Nomada, hung around the nest entrance awaiting an opportunity to get in and lay their eggs. There are many similar species of Andrena mining bees in the UK and several of them looked very similar to mine so I finally gave up with obtaining an ID and I didn't find much more about my front path bee. The bees are back again in the same spot, and a couple of days ago, thanks to tweep @DayMoonRoseDawn I finally knew the identity of these curious bees: they are Andrena carantonica, a large, furry mining bee. Although mining bees are solitary, a handful of species in the UK often nest communally. Communal bees have a simple form of social structure: they share a nest, but individual females will build their own cells, lay eggs and provision their own offspring: they do not have the complex social organisation with workers and division of labour of honeybees. Nesting aggregations of A. carantonica consist of hundreds of females (up to 600) females using the same nest entrance. In my street, cracks on concrete in front garden paths are favoured. Adults emerge in April, with the smaller males doing so just before females and awaiting them inside the nest. In a series of studies on this species, Robert Paxton and collaborators revealed fascinating details of the social structure of this species. In one of these studies they installed nets on nest entrances to be able to capture recently emerged bees: 75% of females were already inseminated when they left the nest for the first time, indicating that copulation happens most of the time inside the communal nests. Although the females in the nest are not highly related, there is a large amount of inbreeding in this species: genetic analysis suggests that 44% of females mate with their full brothers. Emerging females are much more numerous in a given nest (about 1 male to 3 females), although, as Rose Dawn pointed out, it is likely that many males never leave the nest and die inside after having mated, so emergence nets do not capture them. Females must by necessity swap nests after emergence so that the relatedness levels is still low within a given nest, and the bees show a high degree of tolerance to conspecifics, regardless of their nest of origin. If a high level of relatedness is not responsible for the evolution of communality, why do these bees nest communally? A potential benefit is improved nest defence. There are always bees going in or out, and in my own observations, a few bees often sit by the nest entrance (like as in the photo at the top), therefore opportunities for parasitism by cleptoparasites might be reduced. There are still many questions to answer, which probably will have to await to observations being carried out inside nests, but the study of these and other communal species might offer clues as to how and why complex social systems like those of honeybees evolved.

A male Andrena carantonica near the nest entrance

Female Andrena carantonica

A cleptoparasite bee Nomada flava by mining bee nest entrance.

Thanks to Rose Dawn for providing an ID and sharing lots of information on this bee, including results from her own research

More information

Paxton, R., Thoren, P., Tengo, J., Estoup, A., & Pamilo, P. (1996). Mating structure and nestmate relatedness in a communal bee, Andrena jacobi (Hymenoptera, Andrenidae), using microsatellites Molecular Ecology, 5 (4), 511-519 DOI: 10.1046/j.1365-294X.1996.00117.x

PAXTON, R., THOREN, P.A., & GYLLENSTRAND, N. (2000). Microsatellite DNA analysis reveals low diploid male production in a communal bee with inbreeding Biological Journal of the Linnean Society, 69 (4), 483-502 DOI: 10.1006/bijl.1999.0371

Nomada bees

A female Nomada cuckoo bee

Something that never fails to surprise me, is that once I have a name to some beast or plant, I start to come across it more often: Learning allows you to see. This happened to me recently with Nomada bees. Before I had an ID I thought they were some sort of wasp. I had seen them often in my front garden. A few days ago I came across three of them, loitering around a crack in the pavement on a south facing garden in my street and got a few shots. Why wouldn't I identify them as wasps? They are almost hairless, slender, with a thin wasp-like waist and neat black and yellow striped abdomens. Again Wild About Britain, my favourite wildlife forum, proved me wrong. The wasps were in fact, cuckoo bees of the genus Nomada. This is a very large genus, worldwide over 800 species (around 30 British sp.) which mainly cleptoparasitise bees of the genus Andrena,  particular Nomada species tend to be host-specific, that is, they specialize in parasitizing a single Andrena species. Note I wrote 'cleptoparasites', what means is that they actually lay their eggs in nests of bee species, on the pollen supply intended for the developent of the bee larva. Nomada males and females are often found around their host nests. Males waiting for females to emerge and then mate with them. The females lay their eggs in cells being provisioned by the host. Their larva kills the host larva and eats the pollen provision during its development.
 Research into Nomada chemical communication has revealed a complex, puzzling system in which some of the hosts chemicals are also produced by the parasite. Nomada males produce chemicals identical in composition to secretions of the female Andrena species they parasitize which they use to line their nest cells. Why is the male and not the female which produces this chemicals is not know. It could be that upon copulation the male parasite transfers the secretion to the female and thus she can enter the nests with a chemical 'camouflage'? Is this to show the female the right species' nest? Both explanations seem feeble, as it would seem simpler that both sexes would learn the odor of the nests they grew in, and that females could in theory produce the smell themselves if that renders them less likely to be attacked in the host's nest.
 The particular Nomada species in our front garden seems to be N. marshamella, which parasitises several species of Andrena (A. carantonica, A. scotica, A. trimmerana, A. nigroaena). Some of these nest communally in hole or cracks in pavements. So, all this fascinating chemical communication and tales of robbery and killing might be happening, literally, just outside your front door.

Two Nomada bees waiting at the entrance of an Andrena nest.

This one is about to enter the nest

Andrena bee resting (possibly A. carantonica). This genus is not only parasitized from Nomada cuckoo bees, but males are also deceived by orchids!