Phoenix rising from the sand

By Athayde Tonhasca

As the waters subside in Germany and the country recovers from July’s catastrophic floods, naturalists may soon be able to evaluate the damage to one species caught in the deluge: the grey-backed mining bee (Andrena vaga). This bee is at home on river flood plains, grasslands, meadows, coastal areas, anywhere with alluvial soils – soil derived from sand and earth deposited by running water – and plenty of willows (Salix spp.) nearby.

A grey-backed mining bee © Ocrdu, Wikipedia Creative Commons

Female grey-backed mining bees dig their nests on spots of firm, sandy soil with sparse vegetation. The species is solitary, although females tend to nest close to each other in aggregations that can be thousands strong. Willows are their only source of pollen, but nectar is taken from a variety of flowers.

A grey-backed mining bee nesting aggregation © Mohra et al. 2004. Solitary Bees: conservation, rearing and management for pollination

Calamitous floods aside, to build a home on flood plains seems like a disaster waiting to happen. Water levels rise and fall, waterways change courses, river banks are washed away: riparian habitats are fragile and ephemeral. But none of this is the end of the world for grey-backed mining bees. Although floods may destroy large numbers of nests or even wipe out whole populations, these bees are well-adapted to disperse and colonise new places. In fact, fragmented populations dispersed over large areas are genetically similar, which suggests free and frequent interconnections between them.

Seven grey-backed mining bee nesting aggregations (red dots) in Germany © Mohra et al. 2004. Solitary Bees: conservation, rearing and management for pollination

Moreover, finding a new neighbourhood has a health benefit. Local populations of grey-backed mining bees grow steadily over the years, with more and more females sharing a nice nesting spot. These agglomerations do not go unnoticed by predators and parasites such as the nomad bee Nomada lathburiana. This parasite invades mining bee nests and lays an egg in the host’s brood cell; the invader’s larva emerges, kills the host’s egg or larva, then eats its provisions. A grey-backed mining bee aggregation targeted by parasites may contract by 50% in four years. But these population crashes are not all caused by natural enemies; some females just up sticks to build new nests on parasite-free sites.

The parasitic bee Nomada lathburiana © James K. Lindsey, Wikipedia Creative Commons

The grey-backed mining bee has been recorded intermittently in Britain since the 1930s, although its identification has not been confirmed until 2014. Currently this bee is confined to a few colonies is southern Britain: its populations may expand or be eliminated if nest aggregations are to be hard hit by rising waters. But even in this doomsday scenario, the grey-backed mining bee is not likely to be gone for long: wandering females in continental Europe should have no problem in crossing the English Channel and making themselves at home in Britain. This ‘here today, gone tomorrow’ lifestyle helps explain the difficulty in tracking the grey-backed mining bee and assessing its conservation status: it was labelled ‘endangered’ in 1987, ‘believed extinct’ in 1991, and ‘data deficient’ in 2020. As a species fine-tuned to transitory and unstable habitats, and highly adept at dispersing and colonising new territories, this unassuming bee takes natural disasters in stride; they are just facts of life.