Orchid: Alarm fragrance as Hornet attractant

Plant produces alarm pheromone from honey bees to attract pollinators

Dendrobium sinense with pollinator Vespa bicolor © University of Ulm
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A Chinese orchid species produces a fragrance that matches the alert signal of honey bees. It attracts hornets whose prey is normally bees and can thus be pollinated by the hornets. Researchers have now introduced this new and unusual plant strategy in the journal "Current Biology".

One third of all orchid species are Täuschblumen, which means they do not reward their pollinator with nectar or pollen. To attract pollinators, these orchids have many highly sophisticated tricks in store. Most Täuschblumen mimic for pollinator attract either feed-rewarding flowering plants or mating partners of their pollinators. Professor Manfred Ayasse and Jennifer Brodmann from the University of Ulm now have another example of deception in the only occurring on the southern Chinese island of Hainan orchid species Dendrobium sinense. This type of orchid attracts its pollinators, hornets of the species Vespa bicolor, by imitating compounds that have an importance as alarm pheromone in honey bees.

Hunting behavior instead of landing approach

The team of researchers knew from earlier research from their Chinese counterparts that the hornet Vespa bicolor is the most frequent visitor to orchid flowers. Observations also showed that the hornets do not land on the flowers, but only briefly but violently bump their heads against the red center of the flower, as if they were chasing a prey. Hornets feed their brood, as do all other wasps with meat. To do this, they catch other insects, including honeybees, either directly at the entrance of honey bees or on flowers.

Plant releases bee alert pheromone

Using electroanalytical techniques combined with chemical analyzes, the research team was able to show that the flowers produce several substances, including Z-11 eicosenol. This compound also occurs in honeybee alarm pheromone. It is known that the beewolf, a parasitic wasp species, uses the Z-11 eicosenol to detect their prey, honeybees based on their alarming fragrance. Otherwise, this compound is rather rare in nature. In plants, it has not been described yet.

Imitation of prey scent as an attractant

In behavioral experiments, Brodmann now showed that the hornets are attracted by the scent of these compounds as well as by the scent of the orchids themselves. With this work, the researchers from Ulm were able to substantiate the results of previous investigations, according to which pollinated by wasps orchids often fragrance signals in the Related to the prey, mimic. This also includes the species occurring in Germany, which are pollinated by wasps. However, the mechanism of pollinator attraction found in Chinese orchid D. sinense is new and is another fascinating example of orchid biology

Approach to biological control

Hornets are often a problem in China and also in the Asian region. Beekeeping is particularly affected economically, as bee hives are regularly cleaned out by hornets. However, people are often attacked and stung by the hornets. The use of scented chickens to keep hornets away from humans and beekeeping could help overcome the hornet problem. More information Soothing hornets are urgently needed for biological hornet control.

(University Ulm, 07.08.2009 - NPO)