A healthy diet for fussy eaters

By Athayde Tonhasca

Pollen, the fertilizing agent that carries the male gametes (reproductive cells) of flowering plants and grasses, is packed with protein, starch, sugars, fats, vitamins, and inorganic salts: carotenoids and flavonoids add the colouring. This rich resource wouldn’t go untapped by many insects and mites. Among them, bees are the ultimate palynivores (pollen eaters).

To us humans, one pollen grain is indistinguishable from the next: it’s that granular yellow stuff that may cause seasonal allergies such as hay fever. But pollen of different plant species comprises a smorgasbord of chemicals. Protein, by far the most important nutrient as the source of vital amino acids, ranges from 2 to 60% of pollen dry mass. The composition and amount of other essential nutrients vary as well. Some pollen contains secondary metabolites such as alkaloids and glycosides, which are harmful to some bees: buttercups and related species (Ranunculus spp.) for example are toxic to honey bees. Pollen grains of some plant families are coated with a sticky substance called pollenkitt, which probably helps pollination. But just as some people can’t digest lactose, some bees can’t digest pollenkitt.

Miscellaneous pollen grains © Dartmouth College Electron Microscope Facility, Wikipedia Creative Commons

Bees have adapted to the range of pollen quality by adopting diversified diets: most species are polylectic, that is, they collect pollen from various unrelated plants (as opposed to oligolectic species, which specialize on a few related plants). By taking pollen from many sources, bees get a balanced diet and reduce the relative intake of harmful chemicals. When polylectic bees are fed pollen from a single source, they often fail to reproduce or die. The need for nutritional diversity has deep implications for bee conservation. 

Agri-environment programmes throughout Europe have promoted the creation of flower-rich habitats to reduce the impact of agriculture intensification on pollinators. Field margins and other non-crop areas are planted with seed mixtures, and the practice has made a difference: bumble bee declines have slowed or sometimes reversed in recent decades. As a bonus, honey bees and butterflies have benefited as well. However, most solitary bees (which make up about 90% of the approximately 250 species of bees in UK) have been unintentionally left out.

Two of our solitary bees: a miner bee © Pauline Smith, and a leafcutter bee © Saxifraga – Pieter van Breugel

It turns out that seed mixtures comprise a high proportion of legumes (family Fabaceae) such as red clover, white clover and vetch. These plants are good for bumble bees, but are not the best or not suitable at all for many solitary bees. Most species get their pollen from plants such as smooth hawk’s-beard (Crepis capillaris)scentless mayweed(Tripleurospermum inodorum), field bindweed (Convolvulus arvensis), rough chervil (Chaerophyllum temulum), meadow crane’s-bill (Geranium pratense) and dandelions (Taraxacum agg.). Species from the families Asteraceae (daisies, marigold, snakeroot, tansy, thistles) and Apiaceae (cow parsley, wild carrot, ground elder) are also important. 

Weeds or food for pollinators? Smooth hawk’s-beard (L) © Michael Becker, Wikipedia Creative Commons, and wild mustard (R) © Hectonichus, Wikipedia Creative Commons.

These plants grow naturally in and around arable fields, but some of them are not welcomed by farmers because of their invasiveness. Wild mustard (Sinapsis arvensis) and wild rose (Rosa canina) for example are excellent sources of pollen for solitary bees, but the first is a serious weed of oilseed rape fields and other crops, and the latter is a climbing shrub, not suitable for field margin management. 

The inclusion of weeds in seed mixtures may not be an option, but a more tolerant attitude towards them would be beneficial and safe. A wild plant does not become a weed until it starts competing with crops, and this threshold may take a while – or it may never be reached. The same principle applies to our gardens: we don’t need to kill weeds willy-nilly for questionable aesthetic reasons.

As in so many areas of conservation, the answer lies in finding a middle ground. We need to cultivate an appreciation for wildness over manicured fields and gardens because just as a varied diet is best for human health, a diversified flora represents an essential buffet for bees and other pollinators.