(per) When entomologists talk about wild bees, many people think of honey bees. However, there are 600 species of bees in Switzerland, most of them unnoticed because they are small and inconspicuous and do not form colonies. Nevertheless these wild bees are important pollinators for many plant species.

Quite a number of bee species are now seriously endangered. Evidently they lack a sufficient supply of flowers in the cleared, intensively utilised landscape. This is shown by a study by ETH researchers from Andreas Müller’s Applied Entomology Group (Institute for Plant Sciences). (1) The researchers’ investigation demonstrates how many flowers 41 wild bee species, most of them rare or threatened, need to raise a single larva. For six of the species they compared the pollen content of brood cells directly with the content in the flowers of the corresponding food plants. For the remaining species the scientists used a model with which they can calculate the pollen requirement – assuming that the bees can make full use of all the pollen from a flower.

More than 1100 flower visits are needed

The researchers were astonished by the large consumption of pollen. The Hairless or Bald-bee abeille, Megachile parietina, is a large consumer and needs to visit at least 1100 flowers of her food plant, the Sainfoin, to enable it to supply a single brood cell with food. 85 percent of the species studied needed at least 30 flowers. The Harebell carpenter bee, Chelostoma campanularum, has a slightly easier job, it is satisfied with seven flowers of the Round-leaved harebell.

Even these values are less than the actual minimum subsistence level. Field studies revealed that the whole of a flower’s pollen is not available to a single bee, as assumed in the model, but only about two fifths of it. Therefore, taking into account the competition for flower pollen, the researchers reckon with the fact that about two and a half times more flowers are needed to supply one brood cell with nourishment . Thus for the Bald-bee abeille that would be 2700. And because a female can construct 10 to 30 brood cells in one year, the supply of flowers must be correspondingly much larger in order to provide for a population of wild bees.

No longer a blossoming country

Andreas Müller says that “This large demand for pollen by wild bees is probably partly responsible for the dramatic decline in numerous species.“ He says that in many places intensive agriculture has driven out species-rich meadows and vegetation margins. As a result plant biodiversity was greatly reduced. However, this took away the basic nutritional basis of wild bees, some of which have specialised on one or a few food plants.