By Athayde Tonhasca
There is nothing visibly remarkable about the mining bee Andrena florea. This bee, one of the 67 Andrena species in Britain, is found in open scrubby areas, grassland and woodland edges of south-east England. But one thing makes this bee unusual; it only takes pollen from white bryony (Bryonia dioica).
Andrena florea, which is commonly and unsurprisingly called the white bryony mining bee, is a rare British example of a bee that forages on a single plant species. This dietary restriction is circumstantial, because white bryony is the only plant of this group occurring in Britain. In continental Europe, A. florea has other Bryonia species available. So in a wider geographical context, this bee is oligolectic (or an oligolege) that is, it collects pollen from a few related plant species (from the Greek oligo: few, scant; and lect: chosen, picked).
Pollen specialisation can be a considerable drawback for a bee because food may be scarce even in a landscape full of flowers, and this may limit populations of some species. For example, until recently the white bryony mining bee was rare and threatened in Poland. This has changed with the spread of white bryony into the country’s urban areas. And yet, a considerable number of species are pollen specialists; in some habitats, they make up the majority of the bee fauna. So pollen specialisation must have its advantages, for example by allowing more efficient flower visitation and pollination rates, which benefits bees and plants.
Polylectic bees are at the other end of the spectrum: they collect pollen from various unrelated kinds of flowers. The advantages of being a pollen generalist seem evident: there is more food to choose from and it’s available for longer, as flowers blossom at different times. But these bees must also have an array of physiological adaptations to overcome a variety of chemical and physical barriers to different types of pollen. This could be too costly for a bee’s metabolism.
Pollen is a rich source of protein, lipids, vitamins and minerals. But it also contains secondary compounds that may be noxious to some bees, and pollen grains are often protected by indigestible coating. These barriers explain why few insect taxa rely on pollen alone for food, and could also explain why most polyleges (polylectic bees) exhibit a degree of pollen specialisation: for example, heather (family Ericaceae) and legumes (family Fabaceae) make up over 70% of the pollen collected by British bumble bees, despite local abundance of other pollen sources.
Experiments with the closely related red mason bee (Osmia bicornis) and horned mason bee (Osmia cornuta) show the effects of different types of pollen. Red mason bee larvae develop well on buttercup pollen (genus Ranunculus), but fail to do so on pollen from viper’s bugloss and related plants (genus Echium); the reverse happens for the horned mason bee. Both bees do well on field mustard pollen (genus Sinapis), while neither develop on pollen from tansies and related species (genus Tanacetum). But the story is a bit more complex: neither bee shows any negative effect as long as they are not restricted to ‘bad’ pollen. In fact, unsuitable pollen is part of the bees’ natural diet. Other bee species show similar patterns.
So what can we conclude from all this?
Oligolecty and polylecty are both successful evolutionary strategies. Some bees depend on a few plants, others have diversified pollen diets. The range of hosts can be narrow or wide, depending on the species, but setting aside a handful of exceptions, bees need pollen from different plants to complement nutritional imbalances or to mitigate the effects of harmful secondary metabolites. But even pollen of low nutritional quality or digestibility is taken, as long as it’s a portion of a balanced diet.
These aspects have important consequences for the conservation of bees. They need a diversity of flowers, and plenty of them. Habitats such as semi-natural grassland, hedgerows, field borders, cover crops, brown sites, road verges, wild gardens and weedy parks are all suitable. Planting is helpful, but except for the honey bee and some bumble bees, we know little about what plant species to use. The safest action is to let our wild plants go wild, so that we have bigger, and more diverse flower-rich habitats. That’s not much or too difficult a task to assure the future of our most important pollinators.