Spotted Arum: Strong scent lures potential pollinators

When bees carry pollen from flower to flower, they establish sexual contacts between plants.

There is usually a reward for this service, such as nectar.

However, some flowering plants do not offer their pollinators fair compensation;

they merely fake an attractive offer.

One of these tricky plants is the spotted aroid (

), whose leaves are often unspotted.

As is usual with the arum family, the species, which is widespread in Europe, wraps its inflorescence in a bract.

At flowering time, this light green leaf, sometimes tinged with purple, opens in such a way that a club-shaped outgrowth of the inflorescence becomes visible.

The plant owes its German name "Aronstab" to him, but also the English "lords-and-ladies".

When the entity that evokes such different associations breaks down stored starch, it produces heat.

As a result, it promotes the evaporation of organic matter, which smells quite repulsive.

However, for small flies, whose larvae grow in feces and decaying organic matter, this smell is auspicious.

Scientists led by Eva Gfrerer from the University of Salzburg have confirmed that the spotted aroid produces numerous scents that promise such insects a suitable nursery.

Together with Rüdiger Wagner from the University of Kassel and Marc Gibernau from the Corsican University in Ajaccio, she studied small dung flies (

) and identified butterfly gnats (

), also known as toilet flies.

The researchers used cow dung as a source of scent, which reliably attracts these insects.

Large spectrum of different fragrances

A combination of gas chromatograph and flame ionization detector was used to identify a large number of organic substances that are contained in the scented cocktails of cow dung and spotted arum.

As Gfrerer and her colleagues report in the "Scientific Reports", they also registered which components the small flies are susceptible to.

It turned out that the olfactory receptors in the antennae of the insects react to a total of 78 scents of the spotted arum with electrical signals.

Eighteen of these substances were also found in the exhalations of the cow dung.

Thanks to a wide spectrum of different scents, the spotted arum can probably attract different types of flies, each of which shows particular preferences when it comes to placing their eggs.

From an insect perspective, however, the inflorescence of the aroid is never a good choice.

After all, it doesn't provide any food for fly larvae.

However, the plant manages to use the olfactory deception maneuver to induce sexual contact with its own kind: the club-shaped outgrowth of the inflorescence exuding a promising scent literally lures the helpful insects into the trap.

The female flies tumble into the funnel of the bract, where they pollinate the female flowers.

Provided they brought suitable pollen from another inflorescence of the arum.

Sterile flowers, whose bristles form an impenetrable enclosure, prevent the small flies from escaping immediately.

Only the following day, when the female flowers have withered and the male flowers are scattering their pollen, does the plant trellis that blocked the way outside wither.

Covered in pollen, the flies now flee.

Often enough, they soon fall for a blooming aroid again.

Various relatives of the spotted arum also lure their pollinators with false promises.

), which unfolds its inflorescences in spring on islands in the western Mediterranean

, is particularly impressive .

The bract, whose cup-shaped base encloses the flowers, measures about half a meter.

When it opens, it reveals its reddish-brown patterned interior.

At the same time, the very hairy outgrowth of the inflorescence begins to spread its special scent.

The English name “dead horse arum” reveals what it smells like.

Pervasive stench attracts flocks of bluebottles

Not only the typical smell of carrion that the plant exudes has an irresistibly attractive effect on female flies.

A freshly blossomed hornroot also heats up considerably.

Similar to a dead animal's carcass at times, and much more powerful than a Spotted Arum.

With temperatures up to 20 degrees above ambient and a penetrating stench of decay,

attracts hordes of blowflies.

The insects do not disappear into the bract's cup without a trace as the scientific species name would suggest ("muscivorus" means "fly-eating").

When the flies have been powdered with pollen, they are released again - and often promptly pounce on the next flowering hornroot.

But not all aroids ruthlessly exploit the needs of their visitors.

In South American rainforests, for example, some members of the genus Philodendron offer a comfortably warm shelter in return for the service of pollination.

Your guests, stately beetles from the cockchafer family, appreciate such comfort.

Because they have to maintain a high operating temperature, even tropical nights are often too cool for them.

Having an inflorescence to warm up in helps them conserve metabolic energy that would otherwise be required to heat up.

When their quarters wither and cool down, the beetles immediately look for a new warming blossom and feed it with the pollen they have brought with them.