A beetle, not the wind, pollinates pandanus trees – Technologist

In Europe, Pandanus odorifer remains little known, except to a few lovers of essential oils. In a whole part of Asia, however, the plant occupies an eminent and multifaceted place. In India, it has been used since ancient times in traditional medicine and perfumery under the name “kewra.” In Japan, it enhances the culinary arts: Its large pineapple-shaped fruits are used as snacks, while its shoots replace those of bamboo. In both countries, as well as in the Melanesian islands and the Philippines, its broad leaves are also woven by craftsmen.

“People know this plant, see it and use it all the time, but never imagined that its reproduction depended on a tiny, specialized beetle,” said Atsushi Kawakita, professor of botany at the University of Tokyo. Times may well be changing. In an article published on Wednesday, March 27, in the Botanical Journal of the Linnean Society, the researcher’s team just demonstrated that the plant owes its pollination to a hoverfly of the genus Amystrops, an oval beetle barely 1 centimeter long.

It must be said that pollinating Pandanus is no easy task. Unlike most flowering plants, it is not hermaphroditic. Pollen must therefore be transported from male to female individuals. Until now, the scientific community has viewed this as the work of the wind, and there were several arguments for this. The plant is devoid of nectar, the main attraction for insects. It also lacks the large petals and bright colors that attract winged visitors. Its white inflorescence even appears discreet and downward-facing: not ideal as a landing strip.

A soft cocoon

In 2011, when a new species of Amystrops was discovered, researchers observed beetle larvae in the inflorescence of male flowers. But for these entomologists, that was not the point. The Japanese researcher and his thesis student, Toru Miyamoto, wanted to find out. In the field, they observed different insect species in the inflorescences of male plants. “The overwhelming majority were ‘Amystrops’,” said the researcher, for both larvae and adults. But it was in the female plants that they made their main discovery: individuals covered in pollen.

The scientists returned to the laboratory. There, they raised the insects under controlled conditions. They demonstrated that pollen was essential for the growth of larvae. They also discovered that the adults left the male plants and moved toward the female plants precisely when the latter was “receptive” to pollen.

The how was then established, so all that remained was to understand the why. When it came to visiting male flowers, the need for pollen explained the beetles’ interest. But what did the female flowers have to offer? It was the student’s idea to look at the plant with a thermal camera. He discovered that the inflorescence was five degrees warmer than the rest of the plant and the outside air. It was a soft cocoon for the creature.

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