By Gavin Ramsay
Three years ago we introduced you to the genus Andrena, the mining bee group with 68 species in the UK, via a commentary on one widespread species that often finds its way into homes later in spring, the chocolate mining bee or Andrena scotica. In fact, this genus is one of the largest of any animal internationally with over 1,500 species. With spring unfolding, the season for mining bees is already getting underway across the UK. Some of the early spring species out now are specialists on willow. Andrena clarkella (Clarke’s mining bee) and Andrena ruficrus (Northern mining bee) are likely present (though not always recorded) across much of Scotland including the Hebrides and Orkney too, at least Andrena clarkella. There are additional species that use willow exclusively in England, but the northward expansion of these species has not reached us yet. These two species seem cold-hardy and will nest in the hills as well as in lowland spots timing their emergence for the first flowers of sallow which happening now.
The Clarke’s mining bee is a striking and attractive species, a little larger than a honey bee and hairier than other species with dense black hair on the head and abdomen plus a foxy pile across the back of the thorax. The pollen collecting brushes on the hind legs are also composed of dense foxy hairs, giving a strong contrast to the mostly blackish abdomen.
Once the females have fed and mating has been accomplished, they select some bare ground or thin grass to excavate their burrows, leaving small volcanoes of loose soil as a give-away. This species seems to cope well with compacted soil and can make its burrows in bare soil in small groups beside paths and in parks, in my experience favouring flat but often slightly elevated spots away from the likelihood of waterlogging. The burrows of Andrena can go deep, in some species over one metre, with side chambers for pollen stores and a single egg. Nesting sites are not necessarily close to the food plant, but they will be in the same general area. The first time I saw this bee was when the SNHBS speaker in 2019, Dylan Elen, dived on one and caught it in his hands while on a walk up Dunsinane Hill of Macbeth fame at the end of March, over a hundred metres from the nearest willow plant.
This is one species that takes the time to cover over its burrow entrance when heading out on missions to collect more pollen. Here is a low-quality video of this behaviour on the banks of the Tay near Dunkeld.
Do they do this to try to distract their nest parasite, Nomada leucophthalma? If so, it doesn’t seem to work. This handsome parasitic bee can often be seen searching for and entering burrows of this species, aided by the geranyl octanoate scent trail from the secretion from the Dufour gland of the Clarke’s mining bee. This nomad bee, like the other nomad bees, dives into the loose soil and effectively swims down to the chambers where the host has laid eggs.
Is it any wonder these Andrena bees tend to have rather large heads to accommodate the strong mandibles used to break up the soil and dig those tunnels?