Benign delinquents

By Athayde Tonhasca

Christian Konrad Sprengel (1750-1816) is not widely known nowadays, but the German teacher, naturalist and theologian was a pioneer in recognising flowers as lures to insects. Sprengel made significant contributions to our understanding of the role played by insects in plant fertilization, although his writings, published in German, were mostly ignored outside Germany (which is a common fate in the Anglo-centric scientific world). Even still, Sprengel’s discoveries were acknowledged by Darwin in his own work with plants.

A page of Sprengel’s Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen (‘The secret of nature discovered in the structure and pollination of flowers’), 1793 © Uwe Thobae, Wikimedia Commons.

Among many novel contributions, Sprengel recorded the ‘outrage against a flower’ played by some bumble bees; they perforate the base of a flower to get access to its nectar, bypassing its opening. From the plant’s perspective, this is cheating. A bee that avoids the flower’s reproductive parts may not pollinate it: the metabolically expensive nectar could be for nothing. This behaviour is known as nectar robbery, a term that reflects a sympathetic bias towards plants; after all, bees – and other insects and some birds as well – are just getting a resource that would be inaccessible otherwise. Most robbed flowers have tubular corollas or nectar spurs (hollow extensions that contains nectar-producing organs) which are out of reach for many visitors, especially bees with short tongues. You can watch them in the act here.

Nectar spurs on Aquilegia formosa; not reachable by traditional means © Daniel Schwen, Wikimedia Commons.

It has been long assumed, reasonably, that primary nectar robbers (those that perforate the flower to access nectar) and secondary nectar robbers (species that take advantage of existing perforations), are bad: ‘all plants must suffer in some degree when bees obtain their nectar in a felonious manner by biting holes through the corolla’ (Darwin, 1872). Indeed, robbers may reduce the availability of nectar to conventional flower visitors, therefore affecting plants’ reproductive success. Robbers may also destroy floral structures while in the act of breaking in.

A buff-tailed bumble bee (Bombus terrestris) pilfering nectar © Alvesgaspar, Wikimedia Commons.

In Brazil’s Atlantic Forest, the understory shrub Besleria longimucronata is pollinated by the reddish hermit (Phaethornis ruber) and violet-capped woodnymph (Thalurania glaucopis) hummingbirds – that is, if the stingless bee Trigona spinipes is not around. Despite lacking a sting, this bee is quite aggressive, pursuing and biting intruders with its sharp teeth, so that it can perforate the flowers and take their nectar at leisure. Those hummingbirds that are not driven away avoid the nectar-depleted flowers; even worse for the plant, some hummingbird individuals slip into a criminal life themselves and become secondary robbers, taking advantage of the holes created by the bees. As a consequence of the robber’s direct and indirect actions, the shrub suffers a reduction in seed production (Bergamo & Sazima, 2018). Trigona spp. are notorious nectar rustlers throughout the Neotropical region, damaging many wild plants and crops in varying degrees. 

A Besleria sp. shrub (art by Louis van Houtte), its violet-capped woodnymph pollinator (©
Dario Sanches), and the nectar robber T. spinipes (© José Reynaldo da Fonseca), Wikimedia Commons.

But as is invariably the case in biology, things are more nuanced. Bees tend to stick around patches of rewarding flowers to save energy and forage more efficiently. But if flowers are low in nectar because of robbing, bees are forced to fly longer distances to get what they need. Also, they often spend less time in a given flower and visit more flowers per unit of time to compensate for lower nectar volume. All this shuffling about has a positive outcome for plants: more flowers are visited, more pollen is deposited on stigmas, and outcrossing (mating of unrelated individuals) is more frequent: the end result is increased reproduction and fitness. 

Some of these effects were elegantly demonstrated by Mayer et al. (2014) in experiments with potted aconite or monkshood (Aconitum napellus lusitanicum). This endangered herb is pollinated by the common carder bee (Bombus pascuorum), and often robbed by the European honey bee (Apis mellifera). The researchers simulated nectar robbing by removing nectaries from some flowers and estimated pollen dispersal by dabbing anthers with fluorescent dye, a pollen surrogate, which was subsequently detected in stigmas collected from plants placed at some distance from the source. The results: bumble bees visited fewer flowers per plant and spent less time per flower. Also, fluorescent dye from patches with robbed flowers was dispersed over larger distances when compared to dye from control plants that had not been artificially robbed. 

A bumble bee making way among the petals of an aconite to get access to its nectar © Franz van Duns, Wikimedia Commons.

And robbers often do more than rob. In northwest Spain, the hairy-footed flower bee (Anthophora plumipes) is the main pollinator of kidney vetch (Anthyllis vulneraria vulgaris), but muggers interfere in this relationship: the buff-tailed (B. terrestris) and the heath (B. jonellus) bumble bees may purloin over 3/4 of all kidney vetch flowers. Despite this rampage, robbed flowers have a higher probability of setting fruit than intact flowers. It turns out that robbers are forced to trample all over the plant’s capitulum (an inflorescence of closely packed flowers), touching anthers and stigmas during the act of thievery, pollinating the flowers (Navarro, 2000).  

The heath bumble bee, a nectar robber, stomps around a kidney vetch capitulum, pollinating the flowers © Arnstein Staverløkk and Ivar Leidus, respectively. Wikimedia Commons.

Other studies have confirmed the pollination role of nectar robbers, such as the case of the fuzzy-horned (B. mixtus) and frigid (B. frigidus) bumble bees when visiting tall bluebells (Mertensia paniculata) in Alaska. These two bees pollinate flowers during their early stages of development, when pollen is plentiful, but shift to nectar robbing when nectar becomes abundant later on. But this is not only about a change of diet preferences: older flowers to be robbed of their nectar attract pollinators to young flowers nearby, which means that nectar pilfering aids the pollination of tall bluebells (Morris, 1996).

Tall bluebell flowers are pollinated then robbed, with a positive outcome for the plant © Walter Siegmund, Wikimedia Commons.

Sprengel labelled nectar robbing an ‘outrage against a flower’ and Darwin considered it ‘a felony’, but there’s more to it than meets the eye. Thorough investigations have shown that in some cases flower larceny reduces plant reproduction and fitness, but there are many instances of no ill effects on plants, or even beneficial outcomes. It all depends on flower and robber morphologies, insect behaviour, flower density, how much nectar is available, how much of it is taken away, and so on. 

‘Robbery’ sounds like a wrench thrown in the mutualistic relationship between plants and pollinators, but the phenomenon is way too common and widespread to be considered an anomaly. And like many other natural events, first impressions can be deceiving: the sight of a flower damaged by a rough visitor is not necessarily a harbinger of harm. 

Flowers with punctured corollas, indicating nectar robbing. This could be bad, neutral or good for the plant © Raju Kasambe, Wikimedia Commons.

Just the ticket

In 2018 it was reckoned that transport accounted for 25% of greenhouse gas emissions across the European Union. That’s a big environmental footprint, and explains why developments such as electrification of rail systems gather momentum. Rail travel remains one of the few areas of travel that governments seem confident in encouraging. It is, after all, the least impactful mass travel mode out with walking and cycling. 

Nicole Tyson, Sustainability Manager at ScotRail, is increasingly on track when it comes to identifying opportunities to help nature – especially pollinators – around our rail network. She oversees the roll out of a suite of projects across Scotland intended to boost nature whilst making travelling by train a more pleasant experience. 

“We have impressive projects at Gartcosh, Kilpatrick and Dalry, for example, where station adopters have developed wildflower areas adjacent to the stations,” she notes. “We also have ongoing projects, with our partners The Conservation Volunteers, at our Yoker and Shields Depots, where we have reduced mowing and introduced wildflower areas, installed bug hotels, and created heritage variety orchards.” 

We recently met with Nicole to discuss Scotland’s Pollinator Strategy and to identify areas where we could collaborate further to effectively deliver positive results for pollinators. One outcome is that we will look to enhance their work with station adopters on a practical level by providing pollinator seed packs and information signs to help transform pockets of land next to stations.  

Nicole is particularly fulsome in her praise of ScotRail’s long-term partners – The Conservation Volunteers. “It’s great to see our partnership biodiversity improvement programme continue to flourish,” she explains. “These projects help more than just the environment as by volunteering outdoors, it also enhances individual mental wellbeing through increased contact with nature and connecting with other people. There are clear social benefits through group activity, which helps people contribute something positive to their community. 

“We recognise the part we have to play meeting Scotland’s ambition to be a world leader on tackling climate change, and we’re fully committed to creating a sustainable railway and which contributes to an environmentally aware Scotland.” 

The train operator currently has more than 250 stations and over 1,000 volunteers across the country enrolled in its impressive Adopt-a-Station volunteering programme. From Dyce to Dalry volunteer groups work to improve the physical environment of their local station which includes activities such as installing planters, station gardens, and information boards.  

The successful work around the depots at Yoker and Shields has been replicated to transform Bathgate and Haymarket depots. Again a mix of tactics have been employed, from wildflower spaces, orchards, and bug hotels to increasingly popular reduced mowing regimes. These changes can have surprising impacts, such as field voles exploiting the relaxed mowing in one location by quickly inhabiting several embankments. 

Recently ScotRail has invested £40,000 annually in their biodiversity improvement programme, supporting more than 50 projects. In enhancing habitats for foraging insects and creating nine wildflower meadows they have made a telling impact for pollinators.  

It isn’t just the insects which have benefited from ScotRail’s confident approach. It has introduced almost 190 volunteers in local communities to valuable new environmental skills. Staff too increasingly relish the benefits of having improved access to wildlife-friendly spaces on their doorstep at depots.  

Nicole sums things up nicely when she says that “We believe that there is a place for nature in all our towns, cities, and local communities. This includes in and around the railway too. Our stations and depots are all key places that can support biodiversity efforts. That’s why you’ll find floral displays at stations across the country to encourage wildlife, and wildflower meadows, ponds and orchards planted across our depots.” 

We sometimes have a blind spot when it comes to considering transport corridors and their value to pollinators. Thank goodness that people like Nicole have rail network opportunities firmly in their sights. 

Further reading

NatureScot have an online leaflet detailing some of the opportunities our transport corridors offer for pollinators.

Undesirables in your garden

By Athayde Tonhasca

The Greek philosopher Theophrastus (c. 371 – c. 287 BC) dabbled expertly in sundry areas, from biology to physics, ethics, metaphysics, mineralogy, and languages. But he’s best known today for his contributions to botany: thanks to his two surviving botanical treatises, Theophrastus was titled ‘the father of botany’ by Carl Linnaeus. 

Among many observations about the plant world, Theophrastus recognised something unusual about some roses (Rosa spp.): their flowers had an anomalous number of petals. Theophrastus had no way to know that these atypical flowers are the result of mutations; cells that were supposed to become reproductive parts develop instead into extra petals. In the botanical world, these flowers with additional petals are known as double-flowered.

A wild R. rubiginosa and a double-flowered cultivar of R. chinensis © Stan Shebs and Sakurai Midori, respectively. Wikimedia Commons.

Since ancient times, the double-flowers’ showy, intricate blooms with densely packed petals have sparked the interest of plant lovers and plant breeders. They have been keeping and propagating spontaneous double-flowered forms of roses and other garden plants such as camellias (Camellia spp.) and carnation (Dianthus caryophyllus). Double-flowered varieties, which are identified by their scientific names followed by the abbreviation fl. pl. (from the Latin flore pleno, meaning ‘full flower’), have inundated the market of ornamental plants. Nowadays most cultivated rose varieties are double-flowered, and some of them have up to 40 petals. Because of their aesthetic properties, double-flowers are widely used in hanging baskets, urban flower beds and public gardens.

Cultivated roses originated from two main regions of domestication: Europe/Middle-East and China. Double-flowers were selected independently in the European and Chinese lineages © Dubois et al., 2010.

Plant breeding is a lucrative and sophisticated activity. Flower producers can attend to an array of consumers’ demands such as plant size, uniformity, flower durability, shapes, colours, and the all-time favourite, flamboyant flowers. The artificial selection of characteristics for our convenience and aesthetic preferences may please some gardeners and urban planners, but they do not please one interest group: pollinators. Because all or a large portion of their reproductive structures have been turned into petals, many double-flowered plants have little or no pollen – a favourable aspect in one regard: they don’t trigger allergies.  Carpels (the female parts of a flower) and anthers (the pollen-bearing structures) are found underneath the petals of some varieties, but they are few and hard to get at. So some double-flowered plants are fertile, but produce a small number of seeds; some can’t reproduce at all, and have to be propagated through cuttings. And there are more bad marks for double-flowers. Their extra petals prevent or obstruct access to the nectaries, which usually are at the base of the petals. So insects don’t bother with these plants, or even worse: they may bother. Many double-flowers produce scents just like their wild counterparts, attracting visitors that go away empty-handed after wasting time and energy.

A bee with full access to a wild rose © Debivort, Wikimedia Commons.

If you want to do your bit for pollinators in your garden or allotment, you don’t need to ban double-flowered varieties altogether, but do give preference to the old-fashioned single-flowered geraniums (Geranium spp.), petunias (Petunia spp.) and fuchsias (Fuchsia spp.), and other pollinators’ favourites such as snapdragons (Antirrhinum spp.), borage (Borago officinalis), crocuses (Crocus spp.) and hollyhocks (Alcea spp.). There are many options of colours, growing habits, sizes, time of flowering, etc., to make a garden beautiful, diverse and attractive to wildlife. 

You may be willing to accommodate plants that will not help you win a gardening prize, and may even provoke a raised eyebrow from a neighbour. Plants like the smooth hawk’s beard (Crepis capillaris) and wild mustard (Sinapis arvensis) are unassuming and often undesirable, but they are very good in attracting solitary bees (Nichols et al., 2019). If there’s a hedge or wall in your garden, you couldn’t have a better choice than the dog rose (Rosa canina), a climbing plant that is another excellent source of pollen for mason (Osmia spp.) and leafcutter (Megachile spp.) bees (Gresty et al., 2018).

Smooth hawk’s beard (L) and wild mustard, good sources of pollen © Michael Becker and Hectonichus, respectively. Wikimedia Commons.

And as a bonus to laidback gardeners, some plants like the lesser calamint (Clinopodium nepeta), and the autumn sneezeweed (Helenium autumnale) attract a wide range of insects and require little or no maintenance: just let them grow. A similar attitude towards unwanted species such as bramble (Rubus fruticosus) buttercups (Ranunculus spp.) and dandelion (Taraxacum officinale agg.) would be another boost to pollinators, as these plants are also outstanding providers of pollen or nectar (Lowe et al., 2022). You don’t need to let them take over, just show some tolerance to these humble garden companions.  

A dog rose (© H. Zell), a great source of pollen for the red mason bee (O. bicornis), top right (© bemma) and the patchwork leafcutter bee (M. centuncularis) (© James K. Lindsey), Wikimedia Commons. 

Close to 7% of the UK land area is classified as urban, home to about 80% of the population (Davies et al., 2009). Around 87% of UK households have access to a domestic garden (Gibbons et al., 2014). If all these city people have pollinators in their minds when designing their gardens and choosing their plants, the benefits to nature, pollinators and ultimately to ourselves would be huge. Avoiding gaudy, artificial-looking flowers would be a good start.

Beauty and food for pollinators from simple flowers © Alvesgaspar, Wikimedia Commons.

A flag and a sword

The Flag Iris is a hard plant to ignore. Tall, imposing and colourful. References to it abound in Scottish literature, one of the simplest, and perhaps my favourite, belongs to Norman Collie, the famous climber, who noted in a 1932 Cairngorm Club Journal, with more than a hint of admiration, that the ’The tall iris nods slowly in the wind’.

It’s a hard plant to overlook.  Flowering may be short-lived, and falls between late spring and early summer, but the plant structure remains visible in right through until October. The Flag Iris, Iris pseudacorus, is often found in damp, boggy locations and then frequently in clumps. It’s a popular plant with pollinators, and as the flowers are often at eye level a good choice for some pollinator watching.

The bold yellow flower is certainly distinctive. Large and flimsy it gives the plant its common ‘flag’ name. It was a staple of the Scottish clothing industry in the 18th century – used as a dye. The roots were used in the colouring of tartan and tweed, mixed with other elements to achieve dark colours, whilst the sword-shaped leaves helped create green dyes.  In days gone by in parts of Tiree the broad leaves of the Iris were often used in roof thatch.

It still helps offer a home today, to the elusive corncrake no less, which is quite happy to skulk in the deep cover it provides.

Of course you will expect this common plant to have a medicinal use too, and you wouldn’t be disappointed. The roots were dried and crushed before being used to tackle a range of ailments including toothache, headaches and colds. In some places the roots were also employed as a laxative. That is unlikely to have been any more pleasant than the anecdote a colleague shared with me about how the rhizomes were crushed and the resulting liquid poured or inhaled through each nostril as a treatment for toothache.

Delve deeper and you find that the Yellow Flag was once a heraldic symbol. The fleur-de-lis, adorned the symbols of the French monarchy and in Scotland was included in the Royal Arms of Scotland. Going further back and into the world of Greek myths Iris was revered as a messenger of the gods, and credited with steering the departed souls of women to the land of Eternal Peace. Hence Iris were, in some parts, planted near graves.

The sword-like leaves have, not surprisingly, been behind several of the ancient names for the Flag Iris.  Water Skegg, Jacob’s Sword, Swordgrass, and Daggers all reflected the shape of the leaves. Yellow Flag and Sword Flag derived their name from the fluttering of the yellow petals. Surprisingly, the bulbous green seedpods of our native Iris don’t appear to have spawned any peculiar names.

There has been a British Iris Society since 1922, devoted diligently to the cultivation of this striking plant. Indeed there are Iris societies across the globe. They will confirm that the element of the name pseudacorus literally means ‘false acorus’. The pseudo element stems from the Greek word meaning ‘false’, and acorus, indicates that the plant it isn’t is the Sweet Flag plant; all of which I suspect tells us that this plant was once labelled as an imposter.

When it comes to Flag Iris pollinators – there are many. It is easy to observe both flies and bees, especially bumblebees, visiting this flower. Most pollinators land on the lower outer petals and thereafter they quickly become dusted in pollen as they slip past the three smaller inner petals and dip into the flower head. Suitably dusted, the thorax will brush against the stigma in the next flower visitation and the job is done.