Alone in a crowded house

By Athayde Tonhasca

A hole in a stone wall on a busy street does not sound like a great housing choice, but it is a haven for the chocolate mining bee (Andrena scotica). You can find this bee in streets, gardens, parks and countryside across Britain at this time of year, even during this never-ending wintery spring. Like approximately 70% of the 20,000 or so known bees in the world, this species is fossorial (from the Latin fossor for ‘digger’), that is, it excavates a nest in the soil and spends most of its life underground. And like about 80% of all bees, it is solitary: each female builds her own nest chambers in which she stocks pollen and lays her eggs.

Although solitary, chocolate mining bees tend to nest near each other. Many other solitary bees do the same, sometimes forming nest aggregations hundreds strong. As far as we know, they don’t interact with each other. But chocolate mining bees have one quirk:  they often share an entrance to their burrows. Behind a communal gate and out of sight, each of several female bees constructs and provisions her nest. 

Chocolate mining bees’ communal nest entrance along a pavement © Athayde Tonhasca

These concentrations of bees frighten some people. Beekeepers and council officers have been summoned to deal with ‘swarms,’ only to witness the comings and goings of chocolate mining bees. Worse, people afraid of being stung or concerned about damage to structures have wiped out whole colonies with insecticides. But these citizens are mistaken on both counts. Like any other solitary bee, the chocolate mining bee is docile and never goes out of its way to attack people or animals; besides, her sting is too weak to penetrate human skin. And their nesting galleries are temporary and too small to cause any damage to a wall.

A group of chocolate mining bees hanging around a communal nest entrance © Athayde Tonhasca

We don’t know why some bees chose to gather together for nesting. It’s a risky strategy because these concentrations may lure parasites and predators, and this seems to be the case for the chocolate mining bee. Bees may aggregate out of necessity; a sturdy, long-lasting nest requires a specific set of conditions (soil texture, moisture level, temperature, inclination), so bees may have to share suitable but limited real estate. Aggregations may be a manifestation of natal philopatry, which is an animal’s tendency to return to its place of birth. Or it could be that bees are attracted to each other out of self-interest: by building on a good spot already discovered by a trail blazer, a bee saves time and energy that would be spent searching for nesting sites on her own.

Whatever the reasons for its communal living, the chocolate mining bee should be welcomed. This species is highly polylectic – it collects pollen from a variety of unrelated plants – so it helps pollinate a wide range of spring-flowering shrubs, trees and wild flowers. They are not put off by nosy humans, so if you find them going in and out of a crack in an old wall or behind a stone in your local park, stop and watch for a moment. Soon you may spot a bee sprinkled with pollen grains, which is a sure sign she has been hard at work providing for her next generation and assuring the reproduction of many of our plants.  

A chocolate mining bee with her ‘pantaloons’ (pollen brushes) covered with pollen © Dave Smith

Creating a buzz for pollinators

By Annie Robinson

X-Polli:Nation is an actionable citizen science project that cross pollinates ideas, methods and technologies for pollinator citizen science. X-Polli:Nation aims to create a ‘buzz’ for pollinators by supporting:

 ‘people’ to learn and protect them,

‘practice’ to improve citizen science tools and of course,

‘pollinators’ by creating more habitat and supporting campaigns for their protection. 

Whether you are gardener, teacher, online recorder, or someone new to the field, we welcome everyone to get involved.  Our online tools and downloadable resources provide a programme of activities to support learning, recording, planting, and communicating the importance of pollinators. There are also complementary worksheets and teacher guides and we have a teacher training workshop on the 26th May.  This short video gives an overview of the project. Please do explore our website or join the conversation online (@XpolliProject #xpolli) to find out more about the wonderful world of pollinators. 

We are currently working across the UK and Italy. To give you a taste of the activities, our new citizen science survey is based on the Pollinator Monitoring Scheme (PoMS) FitCount methodology and combined with our new AI species identification tool for bumblebees, collects valuable data on pollinators and their foodplants. Citizen science data feeds into our Planting for Pollinators tool to provide species-specific planting advice for certain species of bumblebee. Our Pollinator Promise campaign can be used to encourage your local community to protect pollinators.  Whether you have 5 minutes or 5 hours to delve into the world of pollinator citizen science, want to be desk based or out in nature, we have a full programme of activities around the citizen science cycle below:

Learning about Pollinators 

XPolli:Nation has lots of resources for learning (& teaching) all about pollinators. 

Bumblebee and Butterfly Training tool

You can discover how to identify pollinator species through our interactive bumblebee and butterfly training tools. You will see a series of photos that you can practise identifying to species level using our interactive feature keys, with help from artificial intelligence technologies, including automated image recognition and automatically generated feedback on your attempts. There are varying difficulty levels so its suits whatever level you are at. It’s great fun testing your identification skills. 

Pollinator Open Learn Create course

Or if you are just after a course giving an overview of what pollinators are, why they are important, why they are under threat and what we can do to help we have created an Open University course that is free and accessible to all. 

Recording Pollinators

We would be delighted if you take part in a citizen science survey and contribute valuable new data about vital insect populations and their habitats.

There are two steps:

Citizen Science Survey: We have teamed up with the Pollinator Monitoring Scheme (PoMS) – which provides data for UK Biodiversity Indicators – to create a survey where you can count the number of insects in different groups (e.g. bumblebees, beetles, butterflies etc.) visiting a patch of target flowers for 10 minutes.

Photo-Record Submission: While taking part in the survey or at any time, you can take photos of bumblebees and submit to our dedicated recording scheme Planting for Pollinators, where you will be guided by technology to help identify the species (e.g. Red Tailed or Buff-Tailed Bumblebee etc.).

Planting for Pollinators

Ever wondered what you can do in your own garden or local green spaces to help pollinators? Explore which plants support different species of pollinators throughout the season using our Planting for Pollinators tool. 

Communicating the Importance of Pollinators

I imagine lots of you do this already but inspiring your friends, family and others in your community to protect pollinators can make a real difference. 

There are two parts to the Communicate activities: 

1. Making a Pollinator Promise: When you sign up to the Pollinator Promise, you pledge to plant a pot or put aside a 1 x 1 metre area in your garden, school grounds or business to grow pollinator-friendly plants. The Pollinator Promise was started by pupils at St. Alban’s Primary Church of England Primary School. They asked their parents, friends, local businesses and community groups to sign up.  You can watch a video here

2046 Polli-promises have been made so far to create 4820 square metres of pollinator-friendly habitat. The pupils at St Alban’s hope to spread the Pollinator Promise campaign throughout the UK, and they are counting on your help. Wouldn’t it be brilliant if we could get more dots on the map in Scotland!  Sign up here

2. Designing your own Campaign for Pollinators: We also encourage you to come up with your own creative ideas about how you can spread the message to protect pollinators and share these on our social media account (@XpolliProject #xpolli #pollipromise)

Please do explore our website or join the conversation online (@XpolliProject #xpolli) to find out more about the wonderful world of pollinators.  If you are a teacher join us at the teacher training event on the 26th May

Thick-headed undertakers

By Athayde Tonhasca

If you watched the film Alien and jumped out of your seat when the creature burst from the unfortunate astronauts’ chest, an entomologist in the audience may have nodded knowingly: ‘Ah, a human parasitoid!’ Indeed, the screenwriters acknowledged entomological inspirations for the alien’s life cycle. 

Here on Earth, a parasitoid is an insect that is free-living as an adult, but completes its larval development inside the body of a host (usually another insect), eventually killing it. A female parasitoid lays her eggs on or inside the host; the eggs hatch and the larvae consume the host. This type of life history lies between a predator’s and a parasite’s: a predator such as a dragonfly takes several prey and kills them outright, while parasites such as lice, fleas and ticks live off hosts without killing them.

Most parasitoid species are wasps, followed by flies. Among the latter, there are about 800 species (24 in Britain) of thick-headed flies, also known as bee-grabbers or conopids (family Conopidae). Many conopid species look more like wasps, bees or perhaps hoverflies. The size of their heads is another noticeable feature, which explains their common name.

A conopid fly © Fir0002, Wikipedia Creative Commons

Thick-headed flies hang around flowers, mostly between June and August. Sometimes they are looking for a sip of nectar, but if a female is lurking, she may be waiting for an opportunity to lay her eggs. Which is bad news for a visiting bee or wasp, and in some cases for crickets or grasshoppers. 

It goes like this: an unsuspecting bumble bee approaches a flower. A female conopid closes in and grabs the bee in mid-air. Still afloat, she pries open the bumble bee’s abdominal segments with her theca, which is a pad-like, hardened structure at the end of her abdomen. Sometimes attacker and victim fall to the ground, but the outcome is the same; the female fly lays a single egg inside the bumble bee and lets it go.

A female conopid with her menacing theca clearly visible © Hectonichus, Wikipedia Creative Commons

The drama is over within seconds, and both insects fly away. The fly will wait for another opportunity to attack. But the bumble bee is done for.

The egg hatches and the larva develops inside the bumble bee, consuming its innards. But the larva does not penetrate the host’s thorax, thus keeping her flight muscles intact. The bee carries on with her life, feeding and taking nectar back to her nest, although less and less efficiently as the parasitoid grows. Within 10 to 12 days, her abdomen is completely taken up by the larva, which has nothing more to eat. The bee dies and falls to the ground (if you find a dead bumble bee with a swollen abdomen, conopid parasitism could be the causa mortis). The larva pupates and overwinters inside the bee’s body, and the adult emerges in the following year.

A conopid cocoon lodged inside a bumble bee’s abdomen © Abdalla et al. 2014. Revista Brasileira de Entomologia 58: 343-348

Conopids are among many species of parasites and parasitoids capable of changing hosts’ behaviour for their own benefit. There are examples of wasps that turn ladybirds into inert bodyguards over parasitoid eggs, and fungi that make ants climb up plants so they can release spores. Perhaps the most notorious case is the effect of toxoplasmosis cells on rats and mice. Infected rodents become attracted to cat’s urine and are less likely to hide. This altered behaviour is a death wish: they became easy prey for cats, in which toxoplasmosis cells complete their development. See other examples here.  

Some conopids increase the chances of their pupae making it through the winter with a trick that may seem macabre to human eyes: they induce their bumble bee hosts to dig their own graves. In America, bumble bees parasitized by the conopid Physocephala tibialis bury themselves in the soil just before death. This grave-digging behaviour does not make a difference for the doomed bee, but it shelters the parasitoid pupa from cold and dehydration during winter months, and reduces its exposure to pathogens and its own parasites. Hibernation in the soil also leads to larger and healthier adult flies.  

The grave-digging inducer Physocephala tibialis © Beatriz Moisset, Wikipedia Creative Commons

How common are conopid attacks against bumble bees? Accounts differ: in Britain, parasitism rates vary from 13 to 30%; in America, local figures can reach 80%. But bees don’t take it lying down. When parasitism pressure becomes high, some bumble bees shift their reproduction cycle to later in the year to avoid peaks of conopid populations. And bees have an immune system against invasive agents. Some bumble bees – like many other insects – secrete melanin, which encapsulates and suffocates the parasitoid larva. Some studies have shown that melanisation kills up to 30% of invading conopid larvae. 

We don’t know the consequences of conopids for bumble bee populations, but parasitism is a fact of life for every insect. About 10% of all known insect species are parasitoids, although specialists believe this figure is a huge underestimation. 

Parasitism seems gruesome and cruel. Even Darwin was dismayed by it, as he expressed in one of his letters: ‘I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidæ [ a group of parasitic wasps] with the express intention of their feeding within the living bodies of caterpillars.’ But such anthropomorphism (attribution of human values to natural phenomena) is misguided and biased. Parasitoids, like predators and parasites, are important regulators of the natural world: they prevent excessive population growth, including of agricultural pests and disease vectors. Parasitism helps shape biodiversity and ecosystems, so it is not intrinsically bad or good. It is a characteristic of life on our planet.

Making wildlife connections at Dance Connect

One of the biggest challenges facing emerging pollinators each spring is finding food. What we plant in our gardens, parks and around our workplaces can be a huge help for foraging insects. So take a bow Dance Connect in Kinross who have skilfully transformed the area around their dance and fitness studio into a pollinator-friendly hot spot.

Inspired by the work of the energetic Kinross-shire Raingardens Challenge, Dance Connect have combined choreographing a pollinator banquet with improving the environmental and social value of the area on their doorstep.

There are certainly multiple benefits in what Dance Connect have done.  First and foremost their actions are great news for local nature, however their approach is also creating an attractive environment for people to admire, which acknowledges the recognised links between health and well-being and the natural world. It’s what PR gurus at one time labelled a ‘win-win’.

So how did this project to transform the area around the studio come about?

When Dance Connect purchased their site from Perth and Kinross Council the swales and much of the willow planting, which fringe their car park, were already in place. However, there was an opportunity to make more of the site, to better protect the swales, and extend the planting.

Swales should not be undervalued. Nowadays, we often experience water in our towns and villages as a problem: flooding property or roads, bubbling up from overloaded sewers in wet weather. 

One of the attractive characteristics of a swale is channelling and making use of heavy rainfall. Well-designed swales take the peak flow of rainwater and only slowly release it; in that way they help manage flood risks. The water seeps into the ground in a more controlled fashion, and it is worth adding that beyond potentially reducing the instances of flash floods, a swale can also help plants survive during dry spells. 

However, the swales at Dance Connect were likely in time to be damaged by cars, the soil was susceptible to being compacted, and surrounding plants in danger of being crushed. It was the clever management of willow planting around the swale, allowing the subtle presence of overhanging soft willow branches to fringe the car park edges, which helped demarcate the swale and protect it from compaction and damage.

The good work doesn’t stop at the car park edge. Additional planting beyond the car park borders stretches up nearby banking to impressive hawthorn and a few conifers (including a young Scots pine which will become an impressive tree in due course). Across from the studio’s entry driveway is a flat area of open grass which it is intended to make into a wildflower meadow.

So the Dance Connect surroundings became a multi-functional project, capable of helping nature (the emerging bumblebees on the willow catkins being a great example), mitigating against heavy rainfall incidents, and providing a natural and pleasant arrival point for visitors. 

Co-owner Rachel has certainly been busy adding to her banquet for bees, and along with Jane Shields of Living Water, who is also an ecological designer for the Kinross-shire Raingardens Challenge, planted woodland wildflowers into the willow area, starting with foxgloves at the margins. A few red campions were growing on the site already and more have been established now in the edges of the willow woodland, with lesser celandines adding vibrant early spring colour in the wetter soil in the base of the willow swale. 

Nature rarely stands still. The original mulch covering the soil beneath the trees has long gone, and grass, nettles and docks are beginning to colonise the area. The aim now is to try and create a more varied, beneficial and interesting woodland floor.

The work to enhance the natural features around the studio will continue for some time. The diversity of the grounds certainly increases interest for visitors and passers-by alike, and if you visit you won’t be the only one enjoying the experience. As we head through spring and summer – a range of pollinating insects are sure to make their moves around Dance Connect.

Don’t stop and smell the flowers

By Athayde Tonhasca

In his endeavour to name and catalogue the world’s fauna and flora, Carl Linnaeus (1707- 1778) came across an unusual South African plant. He described its peculiar star-shaped flowers covered with long ‘hairs’ (setae) as flore pulchre fimbriato (flowers beautifully fringed). But what may have surprised Linnaeus was the flowers’ aroma, which he labelled odor hircinus aphrodisiacus lascivus.

The distinguished Swedish taxonomist must have had a strange sense of smell, because most people who sniff the flowers of Stapelia hirsuta (the name given by Linnaeus) don’t recognise goaty, aphrodisiac or lascivious odours. Instead, they describe the scent as like the rotting flesh of dead animals. Not surprisingly, the plant is known as the carrion plant – but starfish flower and hairy stapelia are nicer alternatives.

A carrion plant © S Molteno, Wikipedia Creative Commons

Stapelia hirsuta is one of the many plant species that produce scents of carrion, dung or other animal products to attract insects that feed on decaying organic matter, particularly flies and beetles. This phenomenon is known as brood site mimicry (or oviposition site mimicry) because insects are tricked into visiting those flowers in the hope of finding a dead animal or a pile of dung in which to lay their eggs. Brood site mimicry is widespread across several angiosperm families. So it must have evolved independently, and for the purpose of pollination.  

Many of these plants have big, blotchy, wrinkled flowers with brown, red or purple hues. These features don’t make them popular choices for bouquets and wreaths, but have great visual appeal to saprophagous insects (from sapro: putrid, rotten; and phagous: eating). Some plants go further: they are thermogenic – that is, they produce heat – which is another enticing feature to insects looking for freshly dead or decomposing bodies.

The thermogenic dead horse arum (Helicodiceros muscivorus) has the stench, looks and warmth of carrion: irresistible to blowflies © Arunsbhat, Wikipedia Creative Commons

To us, the whiffs of putrefying carcasses are an olfactory aggression. But to some insects, they are chemical cues functionally no different from sweet-smelling fragrances of garden favourites. In fact, the relationship between brood site plants and their pollinators is complex and fine-tuned. Plants need to produce scents with precise amounts of sulphur and other chemical compounds to attract saprophages. These insects depend on ephemeral resources – carcasses and dung don’t stick around for too long – so they do not waste time and energy going after dubious sources. Moreover, many brood site plants (possibly most) do not produce nectar, so their chemical signals must be convincing. And they are: flies sometimes are duped into laying eggs on flower surfaces. 

We may wonder why saprophagous insects carry on being taken for a ride, since there is little or nothing to gain by visiting carrion flowers. The possible reason is that those plant cheats are relatively rare. So if an insect picks up the right chemical signals, these are likely to come from a genuine source such as a ripe, rich carcass. The possible rewards outweigh the risks of a wasteful but occasional visit to a flower.

Carrion flowers are most common among arum (Araceae), birthwort (Aristolochiaceae), dogbane (Apocynaceae) and orchid (Orchidaceae) families, which are mainly from tropical, subtropical and arid regions – you may have your own carrion flower experience by paying a visit to New Reekie at Edinburgh’s Royal Botanic Garden. 

Despite their unfamiliarity to us, these flowers are reminders that pollination is not just about bees, and pollination by flies (myiophily or myophily) is not just about hoverflies (family Syrphidae), the group of flies best known for pollination services. Carrion flowers attract mostly bristle-covered flies in the families Calliphoridae (blow flies, carrion flies, bluebottle flies) Sarcophagidae (flesh flies), Sphaeroceridae (small dung flies, lesser corpse flies) and Muscidae (house flies, stable flies): none of them are normally associated with pollination.

Calliphoridae and Sarcophagidae flies, unusual pollinators © Brian Gratwicke (L) and Janet Graham, Wikipedia Creative Commons

Most of what we know about flies is related to their roles as agricultural pests and vectors of human and animal diseases. But flies are important decomposers and recyclers, and are vital for many food chains: numerous birds, bats and fish depend on them. Flies are believed to be important pollinators in alpine and subarctic ecosystems and in the forest understory, where bees are not abundant; midges and hoverflies have been proven to pollinate some crops. But we don’t know a great deal more about flies’ contribution to pollination. They are generally much less hairy than bees and lack specialized structures for pollen transport, but are some of the commonest flower visitors, including 100 or so crops. Flies are one of the most diverse groups of insects, with about 160,000 described species that occupy all types of habitats. The list of plants pollinated by flies is bound to grow as we learn more about the lives of these largely unloved insects.

Sphaeroceridae and Muscidae flies: visitors of dung, carrion and flowers © James K. Lindsey (L) and TristramBrelstaff , Wikipedia Creative Commons

Wish you were here?

Insects and other invertebrates dominate virtually every ecosystem in the world. They represent around 80% of the world’s biodiversity, and Scotland alone has at least 24,000 invertebrate species. Pollination is just one of the crucial ecological services they provide. 

With such diversity and such importance you might think we have pretty well got our insect knowledge and surveying nailed.  Yet, one of the greatest obstacles to insect conservation is the scarcity of information about population sizes, species taxonomy, and ecology. That’s where the work of groups such as the Pollinator Monitoring and Research Partnership (PMRP) is incredibly valuable. They are responsible for large-scale pollinator monitoring, under the UK Pollinator Monitoring Scheme (PoMS).

2020 was a difficult year for field surveys across many biodiversity monitoring schemes, with PoMS being no exception, and as the 2021 season gets underway, help is needed to survey the 1-km square plots across Scotland.  Whilst 10 of the 22 squares have been allocated, 11 have yet to be taken up by volunteers (see map below).

By contributing to this work, surveyors are adding valuable data to one of the UK’s raft of excellent pollinator monitoring schemes, and have the opportunity to enjoy the outdoors whilst helping nature.

If you feel that you could contribute to this survey please contact The UK Centre for Ecology & Hydrology website contains all the information you need and details about the excellent training and support that all contributors are given. 

So what of those squares?  Here’s a look at some of the unallocated survey squares that could be waiting for you.

Square 42 is centred on the cleared township of Achanlochy near Bettyhill, and fair to say this area has seen huge changes over the centuries.  Instead of a once bustling village, the square now offers a variety of riparian, agricultural, and mixed semi-natural habitats, including a large freshwater loch.  Wildflowers supporting pollinators can be found in the hay meadows, riverside, lochside, woodland edge and heathland

You will find square 47 set amidst the fabulously rugged terrain of Assynt, near Ledmore. The square is surrounded by some of the finest hills, lochs and landscapes that the NW Highlands has to offer.  The site occupies an area of rough grazing, wet heath and moorland.  Stream sides offer some of the best locations for varied wildflowers and pollinators. 

Further south lies square 51, located at the geographic centre of Scotland low down on the northern slopes of Cruban Beag just above the River Spey. The site is primarily a wet woodland pasture, consisting of improved grass and birch woodland.  Flowers are fairly typical of damper areas and feature bog asphodel, devil’s-bit scabious, thistles and harebells.  

Into the central belt now, square 54 is situated near the western end of the Pentland Hills. This site is largely wet unimproved grassland, with extensive areas of rushes. This contrasts nicely with square  85, which takes in the slopes of Tulach Hill overlooking Blair Atholl and the River Garry.  Although bracken is a feature in places, further downslope, cuckoo-flowers make a delightful place for spotting insects in the early summer. 

Square 111  may appeal to anyone working or studying in Aberdeen. It’s located in historic Deeside, close to the waters of the river Dee at Crathes.  The site comprises a mix of pastoral and arable farmland.  

If you are based in Dumfries and Galloway, then square 121 in the Keir Hills, might appeal.  Much of the square is occupied by plantation forestry, but the location makes use of rough and improved grazing land that run uphill along a right-of-way.  

Glorious Glenfeshie is home to square 128: Situated on the western slopes of this beautiful glen, sampling starts close to the banks of the River Feshie and climbs up through the pinewoods making use of forestry tracks and wide sunny ride. Heathers and blaeberry are the dominant nectar sources. 

The sound of steam trains used to reverberate around Glen Ogle, which is square 157. The study areas contours the upper boundary of woodland, and offers fantastic views towards the Ben Lawyer hills.  Although not rich in flowering plants drier areas have heathland species such as heathers, heath milkwort and tormentil. 

Near the picturesque village of Stoer you will find square 160.  The square is primarily used for rough grazing, incorporating a range of habitats including unimproved grassland, moorland, rock, mires and lochs.  In mid-to-late summer the square is rich with typical moorland flowers such as heathers, devil’s bit scabious and heath spotted orchids.  White-tailed eagles can also be seen on thermals overhead should your attention wander at any point.

And let’s round off our Scotland trip by returning north, to Sutherland, for square 161. The most northerly PoMS square is located near the village of Talmine and feels almost like a holiday destination.  The seaward views are out over the Kyle of Tongue, with its golden sands and numerous islands.  The square itself features wet heath habitats and is used primarily for rough grazing.  Heathland plants are the main source of nectar here, with bog asphodel and devil’s-bit scabious all well presented.

Go on, sign up.  You know you want to!

For further information email @

On a wing and a prayer: butterflies in action

By Athayde Tonhasca

The peacock flower (aka red bird of paradise or pride of Barbados, Caesalpinia pulcherrima), originally from tropical regions, and the flame azalea (Rhododendron calendulaceum), an American native, are favourites among British gardeners and landscapers. These plants’ species epithets explain their popularity: pulcherrima, Latin for ‘prettiest’, and calendulaceum, ‘of the colour of calendula’, refer to their flamboyant inflorescences of yellow, red, and orange tones. 

Peacock flower (L) and flame azalea © Rennboot (L) and Daderot, Wikipedia Creative Commons

These plants are not related, but if you examine their flowers, you will notice they share one feature: their anthers (male parts) and stigma (female parts) are elongated and at a considerable distance from one another. Botanists call this spatial separation between male and female structures herkogamy. This feature, just like dichogamy (when male and female reproductive organs mature at different times), reduce the interactions between pollen and stigma of the same plant, therefore reducing the chances of inbreeding. 

Peacock (L) and flame azalea flowers © BARAKAT2011 (L) and Arx Fortis, Wikipedia Creative Commons

Herkogamy would be a headache for plants relying on bees or flies for pollination. Considering the size of the flowers and the way their reproductive structures are spread out, it’s unlikely that these insects would come into contact with both anthers and stigma. The peacock flower and the flame azalea need pollinators capable of handling these awkward morphological quirks. They need butterflies.

Butterflies are familiar, prevalent and showy flower visitors, but visitation alone does not make a pollinator. Pollination requires the transport of a sufficient number of viable pollen grains to the right stigma at the right time. And, generally speaking, butterflies are not good at it. They are not particularly ‘hairy’ and lack specialized pollen-collecting structures. Most species don’t feed on pollen, and are quite skilful in avoiding it by slurping nectar while perched on their long legs. So psychophily (pollination by butterflies) has been demonstrated here and there, such as for some orchids and in the cotton fields of the American Gulf Coast. But cases are relatively rare, and mostly in the tropics. Many an uncharitable ecologist has categorised butterflies as ‘parasitic nectar thieves’ because they deplete flowers of a metabolically expensive product without contributing to their pollination.

To the peacock flower and flame azalea, butterflies are anything but parasitic. They deal with structures that stick out like bristles on an old brush by fluttering over the flower, inspecting it before landing to sip its nectar. By doing so, they touch the anthers, and pollen grains get attached to the ventral surface of their wings. When they repeat these manoeuvres on another plant, some of the pollen will fall on or be brushed off by the pistils, enabling cross-pollination.

A blue Mormon butterfly (Papilio polymnestor) nectaring on a peacock flower in India © Project Noah

Size is important in this pollen-gathering mechanism, as reaching both anthers and stigma requires wings with large areas. So big swallowtails (family Papilionidae) are the main pollinators of these herkogamic flowers. And the more they flap their wings, the better. In the case of the flame azalea in its native Appalachian Mountains, the great spangled fritillary (Speyeria cybele) gathers pollen in its wings, but not nearly as much as the eastern tiger swallowtail (Papilio glaucus). The former stays still once it lands on the flower, while the latter keeps fanning its wings, thus increasing the chance of contact with anther and stigma. The outcome of wing pollination has been measured experimentally: when butterflies were excluded from flame azaleas, their fruit set was almost nil. 

The great spangled fritillary (L) is a flame azalea’s pollinator, but the eastern tiger swallowtail is even better © Mdf (L) and Ryan Hodnett, Wikipedia Creative Commons

An unusual flower shape may be a hint of adaptation to wing pollination. Another garden favourite, the South African blood lily – also aptly known as ball lily, fireball lily, and powderpuff lily (Scadoxus multiflorus) – seems to have been designed to make life difficult for pollinators. But butterflies are not deterred by its porcupine look. They flap their wings and get their nectar, picking up some pollen while they’re at it. 

A citrus swallowtail (Papilio demodocus) (L) and a mocker swallowtail (P. dardanus) nectaring on blood lilies © HC Butler (L) and Steven D. Johnson. Source: Butler, H.C. 2020. Scadoxus multiflorus and its butterfly pollinators. PlantLife SA

These herkogamic species do their bit to facilitate butterfly pollination. Their yellow/red colours are particularly attractive to these insects, so flowers are easily located. And their pollen is sticky and relatively flattened, so it adheres to the scales of a butterfly wing.

Blood lily pollen attached to the wing of a butterfly © Butler, H.C. 2020. Scadoxus multiflorus and its butterfly pollinators. PlantLife SA

In his ‘butterfly effect’ theory, meteorologist Edward Lorenz (1917-2008) asked whether the flap of a butterfly’s wings in Brazil could set off a tornado in Texas. This metaphor suggests that small events can lead to a ripple effect with a momentous result over time. Studies with herkogamic flowers reveal a new angle of the butterfly effect: by flapping their wings, butterflies may have much to contribute to the maintenance of biodiversity.