Sticky contrivances

 By Athayde Tonhasca

Somewhere in Britain during the Victorian years, a four-spotted moth (Tyta luctuosa) landed on a pyramidal orchid (Anacamptis pyramidalis), intending to sip some nectar. The moth certainly didn’t expect to end up with its proboscis – the elongated mouthparts of butterflies and moths used for sucking – covered with blobs of pollen. But that was the least of the moth’s problems, as disaster loomed: the hapless wanderer was captured by an unknown collector and became a model for George B. Sowerby (1812-1884), the illustrator of Charles Darwin’s masterpiece about orchid fertilisation.

An illustration from Charles Darwin’s book on fertilisation of orchids depicting the head of a four-spotted moth with its proboscis laden with several pairs of pollinia from pyramidal orchids. Names of the species involved have changed since then. 

Those globules of pollen attached to the moth’s proboscis are known as pollinia (sing. pollinium). Each unit contains from five thousand to four million pollen grains, depending on the species. The grains are stuck together with pollenkitt, an adhesive material found in almost all angiosperms pollinated by animals. A stalk-like structure connects the pollinia to a gluey pad known as viscidium, and the whole assemblage is often referred to as a pollinarium.

A pollinarium: the pollinia on the toothpick are held in place by the sticky viscidium © Frederick Depuydt, Wikimedia Commons.

Pollen grains lumped together in a sticky package are not easily carried away by water or wind. As Darwin learned from his observations and experiments, this is done by animal vectors, mostly wasps and bees (although moths, beetles, flies and birds do the job for reasonable number of orchid species). Having pollen grains in a single unit reduces wastage during dispersal, but it’s a risky strategy: a lost pollinium means no pollination at all. So orchid flowers have undergone dramatic morphological transformations to assure their pollinia are picked up by the right pollinator. ‘If the Orchideæ had elaborated as much pollen as is produced by other plants, relatively to the number of seeds which they yield, they would have had to produce a most extravagant amount, and this would have caused exhaustion. Such exhaustion is avoided by pollen not being produced in any great superfluity owing to the many special contrivances for its safe transportal from plant to plant, and for placing it securely on the stigma. Thus we can understand why the Orchideæ are more highly endowed in their mechanism for cross-fertilisation, than are most other plants.’ (Darwin, 1862, Fertilisation of Orchids).

What are some of these contrivances mentioned by Darwin? Orchids’ stamens (comprising anthers and filaments, the male reproductive parts) are fused with the pistil (which are the female reproductive parts: stigma, style and ovary) to form a structure known as a column. The anther (the pollen-producing organ) is located at the distal – away from the centre – end of the column, and the stigma (the pollen-receiving organ) lies close by. Directly below the column there’s an enlarged petal named labellum or lip, which often is noticeably different from other flower parts in its colour, markings, or shape. For nectar-producing species, nectaries are located at the base of the labellum.

Parts of an orchid flower © Thomas Cizauskas, CC BY-NC-ND 2.0.

So the stage has been meticulously set. The distinct labellum is a perfect landing strip for an insect attracted by the orchid’s rewards, be they real (nectar) or not (when physical or chemical decoys are deployed). The pollinator lands on the labellum, touches the tip of the column, and goes away with pollinia securely adhered to its body by the viscidium, which works better on smooth surfaces such as the eyes and mouthparts of insects and beaks of birds. When the pollinator visits another flower, the pollinia are likely to be transferred to the stigma. Sticky pollinia and viscidium ensure secure removal of pollen, minimal wastage during transit, and a high probability of deposition on a receptive stigma. 

An orchid bee (Euglossa sp.) with pollinia attached © Eframgoldberg, Wikimedia Commons.

These morphological features have evolved independently in two plant groups: orchids (family Orchidaceae) and milkweeds (subfamily Asclepiadaceae of the family Apocynaceae). But pollinia are relatively more important for orchids; with more than 26,000 described species, they make up about 8% of all vascular plants and span a range of habitats in all continents except Antarctica; there are more orchid species in the world than mammals, birds and reptiles combined.

Merodon equestris (a hover fly) tangled with milkweed pollinia © Lloyd Davidson, Creative Commons.

Orchids’ highly specialized ‘lock and key’ pollination system reduces the chances of pollen being picked up by the wrong flower visitor or being transferred to the wrong plant species; the selective adaptations towards the right flower-pollinator association must have contributed to orchids’ enormous richness and diversity of forms. It’s amazing what a dab of glue here and there can do.

Figure 2 from the 1877 edition of Charles Darwin’s Fertilisation of Orchids. Darwin is demonstrating an early-purple orchid (Orchis mascula) pollinium adhering to a pencil inserted into the flower. Within 30 seconds, the loss of moisture makes the pollinium bend forward to become perfectly positioned to touch a receptive stigma, were the pollinium to be attached to a bee visiting another flower.

The queens of the Isle of Tiree

By Cathy Taylor

Great yellow bumblebees – Bombus distinguendus were once been found across the UK, they are now only found on machair grasslands in north and west Scotland, making it one of the UK’s rarest bumblebee species. The principal causes of the bee’s decline are the loss of flower-rich meadows and the intensification of farming and grazing practices. Great yellow bumblebees (GYBB’s) favour feeding on red clover, vetches and knapweed and require a continuous supply of flowers from May to September. 

The Isle of Tiree is home to one of the largest populations of great yellow bumblebees in the UK, making it an important refuge nationally. This is probably in part due to the vast planes and dunes of flower rich machair. In 2016 RSPB Scotland, in partnership with multiple other organisations, launched a project to enhance suitable forage for GYBBs and increased monitoring the of numbers of bumblebees. This project has now come to an end, but the RSPB continue to work on the conservation of GYBB’s on the island.

This is where I come in; I work for the RSPB, but my usual role is managing a busy urban nature reserve on the outskirts of Birmingham. I had the opportunity to undertake a sabbatical, to work on a different conservation project for a short time. This led me to the little piece of wildlife heaven that is the Isle of Tiree, to provide the RSPB Officer there with an extra person to survey for GYBB queens.

I surveyed for GYBB for 3 weeks on Tiree, June into July 2022. This coincided with the time that the great yellow queens would be emerging from hibernation. During this time, I repeatedly surveyed 9 machair sites, when the weather allowed. Each transect count involved one hour walking a random meandering route following the best patches of food providing flowers and recording all the bumblebees seen during this time. The phrase “think like a bee” rang in my head to determine the route of the transect, following the flowers.

I recorded the abundance of all 8 bumblebee species and collected associated data of flower usage and abundance of key flowering plants. Flowering kidney vetch was heavily favoured as the food plant of choice and was pretty much the only flower I recorded great yellows using (one hopped onto red clover for a taste before returning to kidney vetch). As it was early in the season there were fewer bees to count than would be found later in the season, so this resulted in covering more ground per survey than during the peak months. I was walking at least 1.5 miles per survey and I soon noticed that the kidney vetch seemed to have denser growth on slopes than on the flat machair planes. From this time on the majority of surveys were spent clambering up and down the steep flower rich dunes, a great workout. 

After a cool, windy and wet first few days which left me wondering if great yellow bumblebees were in fact mythical creatures, I was doing the last survey of the day in very slight drizzle and there one was. The unmistakable black stripe across the thorax meant no more puzzling over carder bees, wondering if they were GYBBs. Soon I was recording multiple GYBB queens per survey on my “good” sites. With a total of 38 surveys completed 3 weeks resulting in 51 great yellow bumblebee queen sightings. This is the highest ever number of queens recorded on Tiree and reflects the successful season that GYBB were seen to have in 2021. 

The other bee highlights of the island were the abundance of moss carder bees, the hairy ginger thighs of the red shanked carder bees and the swarms of male northern colletes. All set to a backdrop of white-sand bays with seals and otters bobbing in the shallows and machair plains covered in hares, lapwing, snipe and redshank. A truly stunning place.

With many thanks to Cathy Taylor of RPSB for providing this update.

Find out more: Tiree’s Great Yellow Bumblebee Project report

Pollinators with bad PR

By Athayde Tonhasca

When member of the serial killer community ‘Buffalo Bill’ decided to leave a message to his FBI pursuers, he stuffed the pupa of a black witch moth (Ascalapha odorata) down the throat of his last victim (The Silence of the Lambs, by Thomas Harris). Buffalo Bill may have been a psychopath, but he knew his insects: a target for superstition and ignorance, the black witch is considered a harbinger of doom in many countries. But the makers of the film version of Harris’ story wanted something even more dread-inducing: they substituted the black witch for the African death’s-head hawk moth (Acherontia atropos), another species with a long tradition of spookiness. 

An African death’s-head hawk moth © Muséum de Toulouse, Wikimedia Commons.

With a bit of imagination, you can see a human skull on the moth’s thorax, a feature that has tied the harmless creature to all sorts of legends and misconceptions. Starting with its name, A. atropos, and the names of the other two death’s-head hawk moths from Asia,  A. lachesis and A. styx, which are homages to Greek myths about mortality.

Atropos and Lachesis were two of the three Moirai, the goddesses of destiny, and Styx is the river that divides Earth from the underworld. Adding to the African death’s-head hawk moth’s capacity to awe and alarm, it can chirp when disturbed. You can listen to it here (volume up!) 

The three Moirai, by Alexander Rothaug (1870-1946).

Artists and writers have been inspired by the hawk moths’ mystique, from Edgar Allan Poe, John Keats and Bram Stoker, to stories about its nefarious influence on the mental health of King George III. The University of Cambridge’s Museum of Zoology holds a specimen taken from the king’s chambers by one of the royal physicians.

Dracula, by Bram Stoker (1847-1912), 1919 edition © British Library, image in the public domain.

All these loopy tales distract us from the tangible facts about the death’s-head hawk moth. Which is a pity, because this species is remarkable in many ways. It migrates seasonally from Africa to southern Europe, venturing now and then north of the Alps and into Britain, sometimes as far as the Shetlands, from August to October. The moth travels at night for up to 4,000 km, maintaining a straight path by adjusting its flight plan according to wind conditions. How do we know that? By fitting moths with miniscule transmitters and following them in an aeroplane (Menz et al., 2022). 

The moth’s large, unmistakable caterpillars feed on potato-related plants (Solanum spp.), and can be abundant in potato fields during years of high migration. The feeding habits of adults are less well known. Most adult hawk moth species have an extended proboscis suitable for taking nectar from flowers with long corollas. But the death’s-head’s proboscis is shorter, and it cannot reach the nectar of many flowers. So it adds to its diet by feeding on rotting fruit, tree sap, and, remarkably for a moth, honey. The death’s-head sneaks into colonies of honey bees (Apis spp.) to pilfer their honey – thus the moth’s alternative epithet: ‘bee robber’. It manages to survive such a daring raid thanks to chemical camouflage: moth and bees share some of the cuticular hydrocarbons that bees use to identify nestmates. The moth’s short legs and stout body also help it wiggle in and out of the hive. But these ruses don’t work all the time: beekeepers often find moth remains in their hives.   

A death’s-head hawk moth caterpillar © Erik Streb, and imago (adult stage) © Stahre, Wikimedia Commons.

The death’s-head hawk moth is a poor pollinator candidate, even though we know little about its ecology. At a first glance the other 1,400 or so hawk moths, also known as hummingbird moths, sphinx moths or sphingids (family Sphingidae) – mostly from Africa and the Americas – don’t look promising either. They don’t have pollen-carrying apparatus, and most of them are very good at keeping their distance from flowers. They usually feed by hovering in front of a flower, probing it with their straw-like proboscis. Hovering, a trait shared with hover flies, hummingbirds and some bats, is an energy-demanding flight mode, but it allows the moth to dart from approaching enemies and stay clear of spiders, frogs and other predators lurking on the flower. 

A convolvulus hawk moth (Agrius convolvuli) © Charles J. Sharp, Wikimedia Commons.

Plants however tell a different story. Flowers visited by hawk moths have characteristics that meet their visitors’ needs: in general, they open at night, produce copious volumes of sugar-rich nectar, are of white or pale colours, have long nectar tubes, and lack unnecessary landing zones. This fine-tuning between hawk moths and plants suggests adaptations for sphingophily, or pollination by hovering moths (a development from phalaenophily, which is pollination carried by flower-alighting moths). For plants, a smaller pool of specialised nocturnal visitors has the advantage of reducing the chances of pollen transfer between the wrong species. As a result, many plants are pollinated by hawk moths, of which certainly the most famous is the Darwin’s orchid (Angraecum sesquipedale). 

Two species pollinated by hawk moths: the greater butterfly-orchid (Platanthera chlorantha) © Jörg Hempel, and jasmine tobacco (Nicotiana alata) © Carl E Lewis, Wikimedia Commons.

Hawk moths are among the strongest flying insects; some are capable of commuting for several kilometres in search of flowers, sustaining speeds of over 19 km/h. They may carry only a few pollen grains attached to their proboscis or other body parts, but this pollen can be dispersed over large areas, which is advantageous for plants’ genetic diversity. Long-distance distribution may reduce the impacts of habitat fragmentation, which is the inevitable result of human occupation of natural areas. Indeed, Skogen et al. (Annals of the Missouri Botanical Garden 104: 495-511, 2019) showed that pollen dispersal by the white-lined sphinx or hummingbird moth (Hyles lineata) promotes gene flow between populations of the endemic Colorado Springs evening primrose (Oenothera harringtonii). You can watch hawk moths at work here.

A white-lined sphinx © Larry Lamsa and Colorado Springs evening primrose © Juanita A. R. Ladyman, Wikimedia Commons.

The role of moths as pollinators is poorly understood, but this gap in our knowledge is understandable. It is not easy to study pollination in daytime: at night, data collection is downright difficult. But we are learning more and more about moths’ contribution to the pollination of wild plants and crops. Nocturnal moths comprise around 90% of the 180,000 or so known species of Lepidoptera, so there is much to be discovered about busy but secretive hawk moths. 

The Hireling Shepherd, by William Holman Hunt (1827-1910). The cheeky shepherd boy shows a death’s head hawk moth to the girl, who looks unimpressed.

Zetland Park celebrations

In 1880 the Earl of Shetland gifted land in the centre of Grangemouth for the creation of what became Zetland Park. It’s safe to say that raingardens at that time were not on the agenda. Fast forward to a glorious sunny day in 2022 and a newly created raingarden was one of the stars of the show as Grangemouth turned out to celebrate its impressively transformed public park.

Mind you changes in Zetland Park aren’t entirely new.  For the duration of  World War Two a corner of the park, previously housing a popular rose garden, was given over to allotments. And it is that very corner of the park that now boasts a striking nature-based solution in the shape of a brand new raingarden.

The Rose Garden, we should stress, lives on, but by integrating a raingarden this corner now has a versatility it perhaps lacked before.  Prone to occasional flooding, the rose garden will benefit from a highly sustainable drainage solution. It’s a piece of blue-green infrastructure well worth celebrating. The rain garden in essence means the rose garden has a better future.

Created by the Green Action Trust, who tapped into funds provided by NatureScot, National Lottery Heritage Fund and Falkirk Council, this is a slice of sustainable drainage which will demonstrate a range of benefits in an urban setting.  These include showing how we can create sympathetically designed nature based solutions which offer much needed mitigation against pressing climate change issues such as the threats of flash floods and drought.

Creating the rain garden

What’s more the raingarden is good news for biodiversity and will provide habitat for a range of species, including pollinators and other insects. One of the biggest bonuses for people will the health and wellbeing opportunities; good quality greenspaces which allow people to connect with nature are widely acknowledged to help tackle a range of health issues, from encouraging active outdoor activities to improving our mental health. With raised beds the opportunities for more people to get involved.

And what a result. The stunning rain garden at Zetland Park, set in a tranquil corner

“Working in partnership with the Green Action Trust on the development of Zetland Park Raingarden has been a really positive experience”, according to Allana Hughes, the Zetland Park Project Officer at Falkirk Council. “Using their knowledge and ability to secure funding to further support the project, we have transformed the park’s rose garden from an area which frequently flooded and was in decline into a garden which is once again loved by the community.”

The local council are justifiably proud of the regeneration of Zetland Park which boasts meadows, a naturalised pond and both young fruit trees and mature trees.  The ‘Grand Re-Opening Event’ in late August, which fell on a glorious sunny day, was designed to celebrate that progress and invite the local community to enjoy and rediscover their local park.

The event brought together a partners village where a range of nature focussed groups including The Scottish Wildlife Trust, RSPB, Froglife, and NatureScot were able to share examples of their work.  A range of activities organised by the Friends of Zetland Park, including a raucous duck-race, sports events and live music,  provided a cheerful back drop to a fantastic day. Money raised from this event will be used to continue to develop the rose/rain garden.

The Friends of Zetland Park should be delighted with the carefully designed transformation of this popular public greenspace, and the Earl of Shetland would surely have been delighted to see his legacy living on.

The ‘rain garden team’ from Green Action Trust. Left to right – Laura Shofield (Development Manager), Emilie Wadsworth (Operations Director), and Rachel Howlett (Raingardens Development Officer)

Notes: The Green Action Trust worked with a range of partners including Falkirk Council and the local Grangemouth community. Financial assistance came from bodies including NatureScot (who manage the Scottish Government’s Nature Restoration Fund), National Lottery Heritage Fund, and Falkirk Council.

Images 1 and 2 courtesy and copyright of Falkirk Council.

The humongous fruit, the tiny pollinator and the duplicitous fungus

By Athayde Tonhasca

Among the range of exotic fruits available to us on grocery stalls, we are not likely to find jackfruit (Artocarpus heterophyllus). But that could change, as the worldwide cultivation and consumption of the fruit have been increasing steadily, fuelled in part by its use as a meat and starch substitute. Jackfruit is a source of dietary fibre and it’s high in potassium and vitamin B. The pulp can be eaten fresh, dried or roasted; seeds can be boiled, roasted or ground into flour. Pulp and seeds can be turned into soups, chips, jams, juices, and ice cream. The jack tree requires little care once it’s been established, and produces high-quality, rot-resistant timber used for furniture and musical instruments. The species is cauliflorous (flowers and fruits grow from trunks and large branches), yielding 150-200 fruits per year. The jackfruit is the largest of all tree-borne fruits, reaching up to 60 cm in length and weighing up to 50 kg. You don’t want to take a nap under a jack tree.

A jack tree loaded with fruit © Crops for the Future, an opened fruit © Kinglaw, and jackfruit seeds and flesh © Aznaturalist. Wikimedia Commons.

Because of all these good points, the jackfruit is frequently cited as a potential contributor to food security throughout the tropics, especially considering the tree’s capacity to withstand high temperatures and droughts. But despite the growing interest in jackfruit, we know little about its reproduction. 

The jack tree is monoecious, that is, it has male and female flowers on separate inflorescences. We are not sure how pollen is transferred between flowers: wind, some beetles and flies have been proposed as possible pollinating agents. Researchers in Florida, USA, found an unexpected candidate: a hitherto unknown species of gall midge (family Cecidomyiidae), Clinodiplosis ultracrepidata (Gardner et al., 2018. International Journal of Plant Sciences 179: 350-367). These midges are attracted by the sweet scent released by the flowers. A midge may get some pollen attached to it when visiting a male flower, and a subsequent visit to a female flower could result in pollination. 

A female C. ultracrepidata and a grain of pollen adhering to the abdomen of a midge © Gardner et al., 2018. International Journal of Plant Sciences 179: 350-367.

But there’s a twist in this relationship: pollination by C. ultracrepidata may depend on the jack tree being sick.    

The fungus Rhizopus artocarpi is a common fungal disease of jackfruit flowers and fruit. It initially infects male flowers, later spreading as a greyish growth of mycelia. The fungus advances slowly until the whole inflorescence or young fruit rot and fall off. Fruit rot, as the disease is known, can cause total loss of fruit if conditions are right for fungus development.

A fruit (A) and a male inflorescence (B) infected with Rhizopus sp. © Ghosh et al., 2015. Biological Control 83: 29-36.

Fruit rot could be bad news if unchecked, but some fungus-covered inflorescences are needed by female midges to lay their eggs and by their larvae to feed and develop. Absence of fungal infestation may break the reproductive cycle of C. ultracrepidata, with unknown consequences for jack tree pollination.

This tale illustrates important facts about pollination. First, we have only the vaguest idea of the players involved; C. ultracrepidata likely originated from Asia, sneaked into America by accident, and remained unknown to science until someone bumped into it. There must be countless other anonymous species quietly pollinating crops and wild plants. Second, we have a partial, often highly speculative understanding of processes. For example, we don’t know how relevant C. ultracrepidata is for the pollination of jack tree flowers in Florida or elsewhere: the study did not rule out other insects or wind as contributors. We also don’t know the equilibrium point in this tripartite relationship, i.e., what’s the level of fungal infestation that’s tolerable for a jack tree and sufficient for midge reproduction.  

There is much to be learned in the field of pollination ecology. Considering its relevance to food security, the economy, and to our health, questions about pollination mechanisms and pollinating agents are worth pursuing.  

Jackfruit for sale in New York City. You may be needing a bigger bag © EdwinAmi, Wikimedia Commons.