Fascinating life of the ‘sweat bee’ (Lassioglossum spp.)
Even though my attention is mainly focused on honey bees I am fascinated by the amazing array of wild bees that quietly or not so quietly go about their less observed lives. Walking up the compacted gravel path I have been noticing an increasing number of perfectly formed holes in the hard clay ground, its mid-April and I am in central Italy. I realise that they are the work of solitary bees so I have been observing the behaviour around them for almost two weeks now. The bee in question is Lassioglossum species; I think it may be Lassioglossum malachurum but I cannot be certain. It’s one of the mining bees, as they mine the tunnels in the ground and are sometimes called sweat bees due to their supposed liking for human perspiration! (Must be seeking salts). Easily recognised, despite its small size, they are characterised by their capacity to carry huge amount of pollen on their hind legs; but unlike honey bees they do not possess the pollen baskets (corbicula), instead they have long hairs (scopae) that the pollen adheres too.
Over a period of few days the number of holes rose up to about 35 in an area of 1m2. Each hole is a nest to a single female bee that tirelessly (for at least a portion of the day) flies in and out bringing loads of pollen to pack and provision her eggs. This tendency for solitary bees to aggregate is a wonderful example of an intermediate evolutionary step between solitary and social bees. These are called primitively eusocial bees, as they are in essence solitary yet choose the vicinity of the others of the same kind. I did find them mostly on sow thistle and dandelion, both from Taraxacum family but a little bit of research shows these bees are polylectic, meaning it feeds on a vast array of floral sources. Thankfully this species is not considered threatened, and can raise up to 4 generations in one season. A couple of days into my observations I notice another similar sized bee hanging around the holes but without the obvious large yellow pollen loads. It’s quite flighty as I approach but after a few attempts I manage to get a picture which on examination is identified as Sphecodes spp. This is a so called cuckoo bee, a kleptoparasite that lays its eggs in nests of other bees like the Lassioglossum. Its larvae hatch and eat the pollen provisions and sometimes the larvae of Lassioglossum. You can see in the picture that Sphecodes looks rather smooth, its lack of hairs is due to it not collecting pollen; it just feeds on nectar itself. Interestingly, around mid-day the holes are closed up. First I thought my children have walked all over them despite my warnings but I soon realise that the plug is freshly ground soil. Also I noticed that the flower they visited most frequently, the saw thistle (Sonchus arvensis) is also closed. (The leaves are edible and collected by locals). Closing the hole requires energy but it must pay off in terms of nest protection, whether for environmental regulation in the nest or protection from parasite.
Sandra Evans, 10 May 2018