Smorgasbord or Spartan: the consequences of pollen diets

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).

A white bryony mining bee and its pollen source, white bryony © Aiwok (L) and H. Zell (R), Wikipedia Creative Commons

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.

Viper’s bugloss (1), creeping buttercup (2), field mustard (3) and tansy (4): nutritious/poisonous food for the right/wrong bee. © Wikipedia Creative Commons

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.  

The life and troubles of the orange-tailed mining bee

The season is picking up and the first pollinators are already out, scouting for nesting sites and food, writes Athayde Tonhasca. It’s a harsh start, as cold spells may be deadly and flowers are still scarce. The orange-tailed mining bee, also known as the early mining bee (Andrena haemorrhoa) is one of the first bees to emerge in the spring. The female bee has bright, rusty red hairs covering her thorax – hence the inspiration for the species name, which derived for the Greek haima (blood) and rheo (flow); the same roots as ‘haemorrhage’. There is a tuft of red hairs at the tip of the abdomen as well, which helps distinguish this bee from others of similar colour.

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Andrena haemorrhoa © Pauline Smith.

The orange-tailed mining bee in found in a variety of habitats, and like other mining bees, it nests underground. This bee is not picky at all: it will excavate its nest in parks, playing fields, gardens lawns, paths, roadsides – any open, dry spot with light soil will do, especially on south-facing slopes and banks. A little pile of spoil around a hole in the ground is a tell-tale sign of a nest. Several bees may nest next to each other, but that’s because they are all making use of a suitable nesting site: each bee will build its own nest and ignore the others.

 

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A mining bee nest entrance.

At this time of the year you may find orange-tailed mining bees visiting the few spring-flowering plants such as dandelions, blackthorn, willows and gorse, but its diet will expand as the season progresses. In fact, this bee is one of the most frequently recorded species on flowering crops grown in the UK. It is also one of the pollinators of the Lady’s slipper orchid (Cypripedium calceolus), which is critically endangered in many countries.

But it’s not all rosy for this pollinator: a fork-jawed nomad bee (Nomada ruficornis) may be lurking, flying close to the ground in search of an orange-tailed mining bee nest. Once a nest is found, the nomad bee uses its sense of smell to tell whether the nest is stocked with pollen and its owner is nearby. If conditions are right, the nomad bee will invade the nest, sometimes kill the host’s offspring, lay her eggs and leave. The invading bee larvae will eat the host’s larvae if their mom has not already done so, then will eat each other until only one is left alive. This larva will then feed on the pollen and nectar stored by the host bee. This type of parasitism is known as kleptoparasitism (parasitism by theft) or brood parasitism, and these parasites are known as cuckoo bees.

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Nomada ruficornis © Janet Graham, Wikipedia Creative Commons.

Over 3,700 parasitic bee species have been described (at least 850 of them from the genus Nomada). Most of them are host-specific, and some are threatened. If you are wondering why we should protect a bee-killing species, think about other loved, charismatic creatures that are no less deadly to their prey such as dragonflies, owls, eagles and wildcats. Cuckoo bees are marvellous examples of natural selection and evolution, and they do no less than their hosts to be successful and survive.