Category: pollination (Page 1 of 2)

Different flower colour, different visitor

16 May 2024 / wvanstrien

Camouflaged fritillary still attracts pollinators

Fritillary Fritillaria delavayi has brown leaves in many places

Flowers of the Chinese fritillary Fritillaria delavayi can be as brown or grey as the stony soil. But there are still pollinators that manage to find them, Tao Huang and colleagues show.

At high altitudes in the Hengduan Mountains in southwest China, plants grow on bare, stony soil. Green leaves are very noticeable here, and to escape from herbivores such as caterpillars, many plant species have developed an unusual brown or grey leaf colour that matches the background. An example is Corydalis hemidicentra.

The fritillary Fritillaria delavayi goes one step further than other plant species: in some places not only the leaves, but also the flowers are stone-coloured. Tao Huang and colleagues wondered whether pollinators could find such camouflaged flowers.

Lung diseases

Fritillaria delavayi is a perennial bulb plant that may have regular green leaves and a bright yellow flower. It grows at an altitude of 3700 to 5600 meters. It is easy to see why this fritillary took on camouflage colours in many places. The small bulbs are used in traditional Chinese medicine, because they contain substances that are beneficial for lung diseases. There is a great demand for them. The plant is difficult to grow because it requires cold and dry air. And so, bulbs are dug out in accessible places.

In some places, Fritillaria delavayi even has brown flowers

In such places the plant developed a stone-coloured appearance. Brown or grey plants are not noticeable, especially if also the flower is brown or grey. But the flowers must be detectable by pollinators, that transfer pollen from one flower to the pistil of another. The flowers cannot fertilize themselves.

Field observations show that two species of bumblebees come to yellow flowers to collect nectar, pollinating the flowers in the process. But they cannot perceive brown or grey flowers, so they do not visit them. How can these flowers be pollinated?

Frequent visits

By other insects, it turns out. The camouflaged flowers of Fritillaria delavayi attract three species of the Anthomyiidae fly family. The flies are looking for nectar and pollen and sometimes mate within the flowers. They do not perceive brown or grey flowers any better than bumblebees do, but they are attracted to the smell. Huang shows that the camouflaged flowers are smaller than the yellow ones, an adaptation to the small bodies of the flies. The flies are less efficient pollinators than bumblebees, but this is compensated for by more frequent flower visits.

As a result, stone-coloured flowers set seed just as well and produce as many seeds as yellow ones. This means that camouflage is not at the expense of reproduction. And camouflaged flowers do indeed protect the plant from human collectors, according to previous research with slides: people clearly have more difficulty finding brown or grey flowers than yellow ones.

Willy van Strien

Photos:
Large: Fritillaria delavayi with brown leaves and yellow flower
Small: Fritillaria delavayi with brown leaves and brown flower
©Yang Niu

Sources:
Huang, T., B. Song, Z. Chen, H. Sun & Y. Niu, 2024. Pollinator shift ensures reproductive success in a camouflaged alpine plant. Annals of Botany, 9 May online. Doi: 10.1093/aob/mcae075
Niu, Y., M. Stevens & H. Sun, 2021. Commercial harvesting has driven the evolution of camouflage in an alpine plant. Current Biology 31: 446-449. Doi: 10.1016/j.cub.2020.10.078

Mutualism, no deception

14 September 2023 / wvanstrien

Smelly Gastrodia orchid provides food for fly larvae

Gastrodia foetida rewards its fly visitors for pollination

For its pollination, Gastrodia foetida, attracts female flies that normally visit mushrooms to lay their eggs on. The orchid seems deceptive, but it is not, Kenji Suetsugu discovered.

Many orchids are cheaters. Whereas most plants cooperate with insects and offer them nectar as a reward for pollination, such orchids have their flowers pollinated without offering a reward in return. They lure their pollinators with false pretences. For example, some orchids mimic female insects to abuse males who want to mate and, in their futile attempts, pick up pollen from one flower and deposit it on another.

Another type of deception is perpetrated by orchids of the genus Gastrodia. They attract fly females who want to lay eggs by mimicking the smell of material in which fly larvae grow up, such as fermenting fruits or decaying mushrooms. But the promise is false, as turns out when females visit these flowers. If they lay eggs on them, which they do occasionally, the larvae that hatch die of starvation.

Gastrodia foetida is an exception, Kenji Suetsugu discovered.

Entrapped

Gastrodia foetida is a rare plant from the forests of Japan and Taiwan. As do other Gastrodia species, the plants have no normal leaves, and the succulent flower does not look like much to us: it is inconspicuous and brown. But to females of some fly species the flower is attractive because of its musty smell; foetida means stinky. A common visitor is Drosophila bizonata, a species with larvae that develop in decaying mushrooms.

When a female fly enters the flower, the hollow lip in the flower bends up to the column that carries pistil and stamens. The female is stuck in the resulting channel between column and lip. To escape from that trap chamber, she has to crawl through a narrow opening along the stamens, and then the pollen, which is packed in two clumps, gets attached to her back (unless there had been another fly before, because then the clumps are already gone). If she then visits another flower in which she is locked up again, the pollen clumps end up on the pistil and this flower will produce many seeds. In other Gastrodia species, things go in the same way.

Decomposing flowers

But unlike those other species, the stinky orchid really is a suitable place to lay eggs on. Suetsugu frequently found eggs on flowers of Gastrodia foetida that had been visited by a female fly. And surprisingly, the eggs hatch and the larvae do not die, but grow well. Three or four days after pollination, the flowers fall off, leaving only the ovary behind. As the flowers decompose on the soil, the larvae feed on the floral tissue until they mature and pupate. Two weeks after pollination they emerge as adult flies.

Although the larvae of Drosophila bizonata are mushroom eaters, these flowers apparently meet their needs.

Mutual service

It is not clear why mushroom eating fly larvae can also grow well on these flowers. It may have to do with the fact that the orchid cannot make its own sugars through photosynthesis, like normal plants, because it does not have the green leaves necessary for this process. Instead, it steals sugars from fungi. Suetsugu suggests that, as a result, the plant tissue may have chemically similarities to that of mushrooms.

In any case, Gastrodia foetida appears to have gone from deception back to mutualism with pollinators, but with a reward other than nectar. Flies pollinate the flowers, and the succulent decomposing flowers then serve as food for their larvae. It is the first time that this form of ‘nursery pollination’ has been demonstrated.

The mutualism is indispensable for the plant, but not for the fly; it still can lay its eggs on mushrooms also.

Willy van Strien

Photos: ©Kenji Suetsugu
First: Gastrodia foetida
Second: Drosophila bizonata carrying pollen on its back in de flower; the trap chamber (the column above, the lip below) is open

Source:
Suetsugu, K., 2023. A novel nursery pollination system between a mycoheterotrophic orchid and mushroom-feeding flies. Ecology, online 23 August. Doi: 10.1002/ecy.4152

Flower opener

22 February 2022 / wvanstrien

Without flying fox no fruits on Dillenia tree

The flowers of the tree Dillenia biflora cannot open on their own. That means that the pollinator, a flying fox, has an extra job to do, Sophie Petit and colleagues write.

The relationship between a plant and its pollinators can be special, and the tree Dillenia biflora may have one of the most remarkable. Its peculiar flowers cannot open. Their petals are fused and form a globose corolla, a lid that covers the anthers and stigma.

Dozens of Dillenia species exist that are pollinated by bees that come to collect pollen; these species do not offer them nectar. So, it was a mystery what happens in Dillenia biflora with its permanently closed flowers and inaccessible anthers. There must be pollinators, because the tree produces fruits with seeds and the flowers cannot self-pollinate. Sophie Petit and colleagues wondered: who are the pollinators, and how do they do it?

Long teeth

The flowers produce scent, are pale-coloured, large and stout and last only one night. People had noticed large bats, or flying foxes, near the trees at night, and pollen was found in flying fox droppings. These findings suggested that these animals play a role in pollination. On the floor, corollas can be found with four tiny holes.

To find out what happens, the researchers placed video cameras near trees at night, when flying foxes are active. They conducted their research on Fiji’s two largest islands, Vanua Levu and Viti Levu, where Dillenia biflora grows in rainforests.

The footage was clear. Trees are visited at night by the Fijian blossom bat Notopteris macdonaldi, which roosts during the day in large groups in caves. The animals grasp the lid of a flower with their four long canines, pull it away and drop it. Hundreds of anthers and a stigma then are accessible.

Mutual dependence

A flying fox has good reason to do the job: unlike those of related species, the flowers of Dillenia biflora turn out to contain a copious amount of nectar. While the animal is drinking, its snout gets covered with pollen, part of which ends up on the stigma of the next flower it visits. That flower is then pollinated and will form seeds.

It is beneficial for a flower to remain closed. The contents then are safe from rain, from insects that feed on them, and from moths and geckos that sip nectar without pollinating the flowers.

The researchers suspect that more Dillenia species have a similar exclusive relationship with flying foxes, because more species exist with flowers that do not open. They also think there are more bat species that open the flowers and enjoy the hidden food source, such as the Pacific flying fox, Pteropus tonganus.

These trees are completely reliant on nectar-feeding bats that remove the corollas: without their visit, the flowers are aborted and don’t reproduce. Conversely, nectar is the main food source for these flying foxes. That has implications for nature conservation. Many Dillenia species are threatened, and to conserve them, it is necessary that the bats do well. In turn, the flying fox Notopteris macdonaldi is a vulnerable species, and its conservation requires that the trees do not disappear.

Willy van Strien

Photos:
Large: The Pacific flying fox Pteropus tonganus also possibly opens Dillenia flowers; it feeds on fruits, pollen and nectar. Paul Asman, Jill Lenoble (Wikimedia Commons, Creative Commons, CC BY 2.0)
Small: Labeled flower of Dillenia biflora. © Sophie Petit

Source:
Sophie Petit, S., A.T. Scanlon, A. Naikatini, T. Pukala & R. Schumann, 2022. A novel bat pollination system involving obligate flower corolla removal has implications for global Dillenia conservation. PLoS ONE 17: e0262985. Doi: 10.1371/journal.pone.0262985

Hoverflies entrapped

8 May 2020 / wvanstrien

Orchid deceives pollinators, but still offers a reward

The orchid Cypripedium subtropicum lures hoverflies by mimicking an aphid colony covered with honeydew. The hoverflies fall into a trap and while struggling out, they pollinate the flower, as Hong Jiang and colleagues write.

Pollination normally follows the principle ‘give a little, take a little’. Pollinators, such as bees, butterflies and flies, drink nectar from flowers, and through their visits they transfer pollen from one flower to another. A delicacy in exchange for pollen transport.

However, not all plants play a fair game. Some orchids, for instance, mimic a female wasp to attract male wasps. The males fruitlessly attempt to mate and while moving, they pick up or deposit pollen. Such deceptive flowers lure insects with false promises and make use of their services without paying a reward. On the contrary: a deceived male is just wasting its time.

Now, Hong Jiang and colleagues describe another form of deceit in Cypripedium subtropicum, an orchid species that grows in mountain forests in southwest China, Tibet and north Vietnam and that is pollinated by hoverflies. It promises its visitors not a mate, but food. Peculiarly, cheated insects do receive a reward from the plant – albeit an unusual one.

Aphid colony

The flowers of Cypripedium subtropicum are dark brown and have an enlarged labellum that has the shape of a pouch and is speckled with white hair tufts. In the eyes of hoverflies, the researchers think, the whole looks like an aphid colony covered with honeydew. And that’s what hoverflies are fond of. Honeydew is a sweet and sticky substance secreted by aphids because the sap they suck from plants contains an excess of sugars. Experiments showed that hoverflies don’t land on orchids that have their white hair tufts removed.

But the imitation goes further. The flowers also smell like an aphid colony: they emit an odour similar to that of alarm pheromones that aphids use to warn each other when danger is imminent.
And to finish it off, the white hair tufts are nutritious and contain a high amount of sugar – just like honeydew. Cypripedium subtropicum mimics the colour, smell and taste of an aphid colony.

Narrow way out

But when hoverflies enjoy the sweet food, it becomes clear that the orchid uses a trick to be pollinated. The labellum has an opening between the white hair tufts. At some moment while eating, a hoverfly will fall into the hole. Crawling back through the opening is impossible because of the curved margin. The animal is trapped.

The only way out is a narrow cleft at the top of the back of the pouch, through which the hoverfly can scramble out. It will first pass the flower’s pistil and then the stamens. When it squeezes along the stamens, a smear of pollen will attach on its back. And if it is trapped again during the next flower visit and tries to escape, it will deposit that pollen on the pistil. Subsequently, it picks up a new dose of pollen.

Cypripedium subtropicum forces hoverflies to pollinate it by trapping them, but at least they get an edible reward in return. The promise is not entirely false in this case.

Willy van Strien

Photo: ©Hong Jiang

Source:
Jiang, H., J-J. Kong, H-C. Chen, Z-Y. Xiang, W-P. Zhang, Z-D. Han, P-C. Liao & Y-i Lee, 2020. Cypripedium subtropicum (Orchidaceae) employs aphid colony mimicry to attract hoverfly (Syrphidae) pollinators. New Phytologist, online April 26. Doi: 10.1111/nph.16623

Pollen on wings

30 March 2020 / wvanstrien

Brush-like fireball lily is specialized for butterfly visits

On butterfly wings, the pollen of fireball lily, Scadoxus multiflorus, is transferred from one plant to another, as Hannah Butler and Steve Johnson show.

Like bees, butterflies are pollinators. They visit flowers to drink nectar, pick up pollen and deposit pollen grains on the pistil when visiting the next flower. Flowers pollinated by butterflies are often red or orange, because that colours are attractive to butterflies. In addition, the structure of the flowers is specialized for butterfly pollination, write Hannah Butler and Steve Johnson.

The ‘salver form’, in which petals form a platform on which a butterfly can settle while inserting its proboscis into a flower, was already known. In this model, pollen grains stick to the proboscis and the head of the butterfly. Now, Butler and Johnson describe another flower form with a different pollen transfer mechanism: the ‘brush model’.

Fluttering

A brush model plant is fireball lily, Scadoxus multiflorus, of Africa, also known as indoor plant. The stamens and pistils extend beyond the petals, and because the flowers are placed close together in an umbel, stamens and pistils of different flowers overlap. The plant cannot fertilize itself; pollen has to be brought from another plant for seeds to develop.

Large butterflies that visit the plant do the job. A frequent visitor is the mocker swallowtail Papilio dardanus, principally males. How does it transfer pollen from one plant to another?

The butterfly flutters along the inflorescence to inspect it, touching many stamens and pistils with the wings. When drinking nectar, it continues fluttering. The flat pollen grains from the stamens it touches stick between the scales at the ventral surface of the wings, as macro photos show. And part of the grains that a butterfly carries with it will fall on the pistils; the butterfly can pollinate several flowers during a single visit, even when it doesn’t drink nectar.

So, the brush-shaped umbel is a specialisation for butterfly-wing pollination. The fireball lily belongs to the amaryllis family. As it turns out, other red flowers of that plant family also have a brush model and deposit their pollen on butterfly wings.

Willy van Strien

Photo: Mocker swallowtail Papilio dardanus (male) on fireball lily Scadoxus multiflorus. ©Steven D. Johnson

Source:
Butler, H.C. & S.D. Johnson, 2020. Butterfly-wing pollination in Scadoxus and other South African Amaryllidaceae. Botanical Journal of the Linnean Society, online March 12. Doi: 10.1093/botlinnean/boaa016

Higher quality nectar

23 July 2019 / wvanstrien

Evening primrose responds to sound of insects’ wing beats

When a flying moth or bee is close to the evening primrose Oenothera drummondii, the flowers detect their buzz. Within minutes, they will produce nectar that is more rich in sugars, Marine Veits and colleagues discovered.

Plants have no ears and therefore they are unable hear. Yet, as it turns out, they perceive sound. The wing beats of a passing moth or bee produce sound waves that are detected by the beach evening primrose Oenothera drummondii, Marine Veits and colleagues show. Rapidly, the plant will change the quality of its nectar by increasing the sugar content. The researchers suspect that, by doing so, the plant increases its reproductive success.

The beach evening primrose, which grows on beaches in Israel, relies on insects for the pollination of its flowers, to be able to set seed. It blooms at night and attracts hawk moths. Flying from flower to flower, they pick up pollen from one flower and deliver it on the pistil of the next one. At dusk, bees visit the flowers.

Energy drink

To keep the pollinators busy, plants must maintain a supply of nectar as a reward for their services. Preferably no soft stuff, but an energy drink: nectar with a high sugar content. But it takes the plant energy to synthesize it, and there is a risk that the precious nectar will be degraded by microorganisms or robbed by ants if it is not picked up by pollinators soon enough.

So it would be nice if a plant would produce high-quality nectar only if there were pollinators nearby. But how can it know?

The researchers hypothesized that plants might be able to detect the sound waves produced by the wing beats of flying insects and respond to it. An unusual idea, but with a series of experiments they showed this to really happen.

When they played back the recorded sound of flying bees to a beach evening primrose plant, the yellow petals of the flowers started vibrating. Soon after, within three minutes, the sugar content of the nectar had increased; before the sound, the flowers produced nectar with a sugar concentration of 16 percent, after the buzz it was 20 percent. Artificial sound at frequencies similar to the sound of flying moths and bees had the same effect, but sound with much higher frequency did not. Nothing happened in silence either.

Soundproof

The nicest test perhaps was with flowers that were contained in soundproof glass jars padded with acoustically isolating foam. These flowers did not respond when the sound of a bee or moth was play backed.

An increased sugar content is an extra reward for flower visitors. They probably will stay longer or go on to visit another flower of the same species. That increases the chance that they pick up or deliver pollen, augmenting the plant’s reproductive success.

If an insect passes by, it does not necessarily make sense for a plant to rapidly increase the sugar content of its nectar. That is only useful if this insect will remain in the area for a while or if it is not alone, because in that case, pollinators will taste the sweet nectar. Video recordings in the field showed that when one insect passes by, there usually are others around. If the weather is fine, many bees or moths are active simultaneously.

Now, more fieldwork is needed to assess whether the evening primrose’s response to insect buzz actually results in more offspring.

Willy van Strien

Photo: © Lilach Hadany

Source:
Veits, M., I. Khait, U. Obolski, E. Zinger, A. Boonman, A. Goldshtein, K. Saban, R. Seltzer, U. Ben-Dor, P. Estlein, A. Kabat, D. Peretz, I. Ratzersdorfer, S. Krylov, D. Chamovitz, Y. Sapir, Y. Yovel & L. Hadany, 2019. Flowers respond to pollinator sound within minutes by increasing nectar sugar concentration. Ecology Letters, online, July 8. Doi: 10.1111/ele.13331

Unopened flower

13 December 2018 / wvanstrien

Moth larva enforces self-pollination in Canada Frostweed

The larva of the moth Mompha capella inhabits a flower bud of Canada Frostweed and prevents it from opening, as Neil Kirk Hillier and fellow researchers show. Pollinators cannot visit the flower, which has to pollinate itself instead.

Canada Frostweed (Crocanthemum canadense), a perennial plant of Northern America, is attractive to the moth Mompha capella, which lays its eggs on it. Then something unusual happens: the plant loses control over its reproduction.

The plant produces yellow flowers that normally open just after sunrise, revealing the female pistil and male stamens. Bees and flies visit the flowers, transferring the pollen from one to the next, so that the flowers are cross-pollinated. Multiple stamens lay against the five yellow petals, retracted from around the pistil to prevent self-pollination. Within a few hours, a flower’s own pollen has disappeared and the pistil is covered with pollen from other flowers. The petals fall off, the green sepals close over the pistil and protect the developing fruit with seeds within.

But when a moth has left its eggs on the plant, the larvae that hatch from these eggs crawl into a flower bud, one larva per bud. And then things are very different, Neil Kirk Hillier and colleagues discovered.

Cap

The larvae start to eat. And they don’t do it randomly, but first feed on the bases of the still folded petals. The severed petals no longer grow and don’t unfold when the flower should open, but remain folded like a cap over stamens, pistil and developing fruit, keeping the flower closed. Pollinators cannot enter. Because the stamens are compacted around the top of the pistil, the pollen is in contact with the pistil and seeds develop through self-pollination. Almost all of them will be consumed by the larva in the end.

Frostweed duped

As a consequence, the Canada Frostweed plant produces less offspring. A yellow flower produces on average about forty seeds, and a larva saves only one or two of them. Reproduction, however, is not in immediate danger. This is because the plant not only produces a small number of yellow flowers that open, unless caterpillar disturbs the process, but also a large number of flowers without yellow petals and only four or five stamens, which appear later in the year. These flowers never open and produce seeds through self-pollination. While they make less seeds than yellow, open flowers (only six or seven seeds per flower), there are much more of them. So seeds are produced anyway.

But seeds of open flowers that develop after cross-pollination are necessary for the exchange of hereditary material. In plant populations with a high infestation rate, such exchange is limited, and genetic variation is low.

Larva safer?

The researchers don’t mention what benefit a larva gains by intervening in the flowering process. If the flower opened and had been pollinated normally, seeds to be consumed would have appeared as well. Perhaps in a closed flower, the larva is safer from predators and parasites.

Willy van Strien

Photo: Homer D. House, 1918 (Wikimedia Commons)

Source:
Hillier, N.K., E. Evans & R.C. Evans, 2018. Novel insect florivory strategy initiates autogamy in unopened allogamous flowers. Scientific Reports 8: 17077. Doi:10.1038/s41598-018-35191-z

Unrewarded services

6 April 2018 / wvanstrien

Orchid utilizes fungi and fruit flies without paying

The orchid Gastrodia pubilabiata lives at the expense of other species. It steals sugars from fungi, which also attract fruit flies that provide pollination service, as Kenji Suetsugu shows, without receiving any reward in return.

While most plants produce sugars from carbon dioxide using energy from sunlight in a process called photosynthesis, the orchid Gastrodia pubilabiata leaves the job to others. The small and inconspicuous plant, which grows in Japan and Taiwan, does not have green leaves, as it lacks chloroplasts, the cell organelles that conduct photosynthesis. With its roots, it steals sugars from the underground hyphae of a number of mushroom forming fungi species; the fungi obtained these sugars from dead organic material. The fungi get nothing in return.

And while most plants produce nectar as a food resource for insects (or other animals) that pollinate the flowers in return, this orchid doesn’t. To be pollinated, it exploits fruit flies (Drosophila species) without rewarding them.

Deceived

The flies need fermenting fruit or decaying mushrooms to lay their eggs in, and their larvae will consume that stuff. Apparently, the brown-coloured flowers of Gastrodia pubilabiata smell like fermenting and decaying substrates, as the flies are sometimes deceived into laying their eggs on the flowers. Consequently, the larvae will find no suitable food and die. But the orchid has been served. While visiting a flower, the flies pick up pollinia, masses of pollen grains, which they deliver to the next flower they visit, thereby pollinating that flower.

Service

The orchid thus takes nutrients from mushrooms and is pollinated by fruit flies, and neither of these partners receives any reward for its services. Both are victims of a parasitic and deceptive plant.

Now Kenji Suetsugu shows that mushroom-forming fungi still provide another service. Old mushrooms attract fruit flies that have to lay their eggs, and upon arrival, they will also visit the orchid flowers that mimic fermenting and decaying material. Suetsugu conducted experiments in which he removed decaying Mycena mushrooms from the orchids’ proximity or added extra specimens; Mycena species are the main victims of theft by the plant. He found that the more decaying mushrooms are around, the more pollen is removed from and delivered to orchid flowers by flies that are misled, and the more seeds are produced.

So, the fungi not only function as food providers, but also as magnets that attract pollinators – without reward.

Willy van Strien

Source:
Suetsugu, K., 2018. Achlorophyllous orchid can utilize fungi not only for nutritional demands but also pollinator attraction. Ecology, online March 25. Doi: 10.1002/ecy.2170

Deceit, abuse and benefits

28 March 2018 / wvanstrien

Complex relationships between arum, blowflies and lizard

With its smell of rotting carrion, the dead-horse arum Helicodiceros muscivorus is irresistible to blowflies and a lizard. The blowflies will be abused, the lizard benefits. Ana Pérez-Cembranos and colleagues unraveled these complex relationships.

On islands in the Mediterranean Sea, a plant occurs with a very bad smell, the dead-horse arum, Helicodiceros muscivorus. Its odour contains chemical components that are also emitted by a decomposing dead animal. It irresistible to a female blowfly searching for carcasses to lay her eggs on to make sure that the carnivorous larvae will have food. The dead-horse arum takes advantage of that behaviour.

The plants release their odour on the first day of blooming. Blowflies that perceive the smell cannot ignore it. Upon approaching the source, they find a pink or red curved bract, the spathe, with the hairy end of the spadix (inflorescence), which produces the smell. When they land, the spadix turns out to be warm. To blowflies, the imitation is perfect: this is rotting carrion. Guided by the heat, they crawl into the tube that is formed by the base of the spathe around the lower part of the spadix, which bears female and male florets.

Trapped

Once inside, the blowflies don’t find what they need, which is decaying meat. But if they want to leave, they cannot. Spikes on the spadix keep the door closed. The blowflies are trapped.

Unintentionally, they provide a service to the arum during their imprisonment in the floral chamber. The female flowers at the bottom of the spadix are blooming this first day, and blowflies that had been misled by the arum before, now deliver the pollen that they picked up on that occasion. The plant has its female flowers pollinated.

The next day, the female flowers have faded and the male flowers are mature. The stench and the heat disappear, the spikes wilt and the blowflies escape, and while passing the male flowers, they are loaded with pollen. And here is the second benefit to the plant: the blowflies take the pollen with them to female flowers elsewhere – if at least they find another foul smelling arum on their way and are again misled into visiting it.

So, the blowflies are coerced to pollinate the dead-horse arum without receiving any reward such as nectar. On the contrary: they lose time that they should have spent on searching for genuine carcasses.

Basking

Now Ana Pérez-Cembranos and colleagues show that the Balearic lizard Podarcis lilfordi is also misled by the arum’s odour. The animal is omnivorous and sometimes forages on carcasses, which are also attractive as a heat source; lizards are cold-blooded and when the weather is cool, they may use a rotting carcass as a perching site for basking. In addition, they capture the blowflies that arrive at the cadaver in search for a site for oviposition.

The lizards respond to the smell of the dead-horse arum as they do to the smell of a carcass and will approach the source. If that turns out to be a dead-hors arum instead of a dead animal body, they will not find a meat meal, but they do find a basking place and blowflies, which they take from the spathe or grab from the tube. The lizards thus take away a number of pollinators, but, according to the researchers, enough are left to ensure pollination.

Fruits

So, the lizard isn’t an enemy of the arum. And after the flowering period, when fruits are ripe, a mutualism even develops between both. The lizards consume the fruits and disperse the seeds in their faeces; passage through the lizard’s intestine increases the probability of germination. On Aire Island, a the small island off the southeastern coast of Menorca, where the research was done, the dead-horse arum is a newcomer. It is estimated to have grown there for only about fifty years. In that period, it spread rapidly over the island and nowadays it locally occurs in great densities. That is because of the lizard, which has learned to eat the fruits and now is the main disperser of the seeds, the researchers think.

Willy van Strien

Sources:
Pérez-Cembranos, A., V. Pérez-Mellado & W.E. Cooper, 2018. Balearic lizards use chemical cues from a complex deceptive mimicry to capture attracted pollinators. Ethology 124: 260-268. Doi: 10.1111/eth.12728
Angioy, A-M., M. C. Stensmyr, I. Urru, M. Puliafito, I. Collu & B. S. Hansson, 2004. Function of the heater: the dead horse arum revisited. Proceedings of the Royal Society London B 271: S13-S15. Doi: 10.1098/rsbl.2003.0111
Stensmyr, M.C., I. Urru, I. Collu. M. Celander. B.S. Hansson & A-M. Angioy, 2002. Rotting smell of dead-horse arum florets. Nature 420: 625-626. Doi: 10.1038/420625a

Sweet snack

15 June 2017 / wvanstrien

Wild bees can do without flowers – for a while

When spring arrives in California, wild bees emerge before flowers appear that offer nectar, providing the animals with energy. To survive, they temporarily use sugary honeydew, as Joan Meiners and colleagues discovered.

It seems weird for bees to visit non-flowering shrubs, because they need flowers to find nectar, which contains sugars, and pollen, which contains protein; these nutrients are necessary for themselves and their larvae. Yet, in the Pinnacles National Park in California, Joan Meiners observed many wild bees of different species visiting shrubs on which no flower was to be found.

Honeydew

With a series of experiments, she and her colleagues found out what the bees were looking for at the non-flowering shrubs: the animals were accessing sugary honeydew, the sweet secretions of sap-feeding scale insects. It appeared that bees visit flowerless shrubs only early in the springtime, when they emerge while there are hardly any flowers blooming, and that all these bees belong to solitary species, not living in colonies where a stockpile of nectar is available. Apparently, in early spring honeydew is an alternative source of energy for these bees, a new discovery.

Now, the question is how the bees are able to find this alternative food source. They are specialists in detecting and distinguishing colours and scents. Flowers depend on bees for pollination, because as bees visit multiple flowers in succession, they transfer pollen from the stamens of one flower to the pistil of the next one, so that this second flower can grow seeds after fertilization. Because bees are indispensable, flowers attract them with showy scents, colours and shapes.

Still, bees manage to find the colourless, odourless honeydew as well.

Looking for food

Are they attracted by the black mold fungus that covers the honeydew? The researchers ruled out this possibility by painting a number of branches black: these branches were not visited by the bees. Do the scale insects form a clue to the honeydew? No, because if the sap-sucking insects were temporarily inactivated with a mild anti-insecticide, no bees were seen nearby; they only came when the scale insects were producing honeydew. But on the other hand, they did detect sticks on which the researchers had sprayed a sugar solution, and they did already within an hour.

The biologists propose that the bees are continuously looking for food, and if one bee locates some honeydew, other bees will notice and visit the food source as well.

Using honeydew as an extra source of energy, the bees can survive a period without nectar. But in the end they do need flowers, because the larvae cannot develop on a diet of sugars alone, but have to ingest a high amount of proteins, and therefore they need pollen. So, every female has to gather pollen for her offspring.

Once plants start flowering, bees lose their interest in honeydew-bearing shrubs and visit flowers instead. The mutual relationship between bees and flowers – where pollination is exchanged for food – is not jeopardized.

Willy van Strien

Source:
Meiners, J.M, T.L. Griswold, D.J. Harris & S.K.M. Ernest, 2017. Bees without flowers: before peak bloom, diverse native bees find insect-produced honeydew sugars. The American Naturalist, online May 30. Doi: 10.1086/692437

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