The scent of trees turns hornets into flying seeds

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EEvery year, agarwood, a sought-after source of medicine and fragrance that grows in the rainforests of southwestern China, has a problem to solve. The fruits of the tree ripen during the hottest time of the year. As temperatures rise, the fruits split open and the seeds hang from the fruit, where they can dry out within hours.

To meet their needs for super-fast seed dispersal, the trees tricked a species of hornets (Vespa velutin) to become seed couriers, suggests a new study. The book, published today (June 30) in Current biology, describes how the fruits of agarwood mimic the smells given off when insects begin to feast on the leaves of agarwood. The hornets are attracted to these scents to prey on the insects but encounter a seed instead.

Vespa velutin on agarwood fruits

Zhu Ren Bin

Plants exploit animals’ senses and behavior for a variety of reasons, says Simcha Lev-Yadun, an entomologist at the University of Haifa in Tel Aviv who was not involved in the study: “They do it for defence, for pollination, for seed dispersal”. But the hornets are thought to be “generally used for pollination,” he says. “Every day you learn something that shows you that [plants] are more sophisticated than you thought,” he explains, saying it was a “joy to see a new mechanism,” in the new study that he says will also be found in other plant species.

In a series of field experiments performed on two agarwood trees (Aquilaria sinensis) plantations in Yunnan Province, China, researchers from the Chinese Academy of Sciences have tracked the fate of a total of 600 seeds from 420 barely opened fruits on four trees. They observed when and how often the hornets (genus Vespa) visited the fruit. The hornets descended and attacked the plant’s suspended, protected seeds (called diaspores) as if they were prey only 13 minutes after seed emergence, on average. As three species of hornets visited the trees, V. velutin aided in the dispersal of 84 percent of the seeds.

Once the hornets plucked out the diaspores, they carried them to other branches or to their nests, which they usually built on large, well-shaded branches near the tree trunk. The hornets only consume the fleshy outer part of the seed, called the elastioma, leaving the rest intact and viable. Overall, the hornets helped disperse 96% of the seeds. In separate lab experiments, the researchers also found that the seeds germinated in the type of shady environments favored by the hornets, suggesting that the remains of the hornets may survive as the next generation of timber trees. agar.

Researchers have tried to figure out what attracts hornets to fruit. They knew plants gave off odors as a form of self-defense after being damaged by leaf-eating insects. After identifying the volatile compounds emitted by ripe fruit using mass spectrometry, they measured the electrical activity on the hornets’ antennae in response to the presence of each chemical and found 17 unique odors based on of carbon that caused electrical activity.

Hornet on a fruit in a tree

Vespa velutin on agarwood fruits

Zhu Ren Bin

Suspecting that the fruit might mimic the compounds released when the leaves are damaged by leaf-eating insects (which happen to be hornets’ prey), the researchers placed caterpillars (Heortia velocityoids) on A. sinensis sowing. They analyzed the composition of volatile molecules released by damaged leaves, as well as the electrophysiological activity of the hornets’ antennae in response to these chemicals. They found that damaged leaves emitted 14 of 17 chemicals emitted by ripe fruit, and eight of them caused high electrical activity in the hornets’ antennae. The researchers concluded that the fruit must send out the same volatiles that the leaves unfurl when eaten by a hornet’s food.

In an email to The scientist, Study co-author Gang Wang, an ecologist at the Xishuangbanna Tropical Botanical Gardens of the Chinese Academy of Sciences, writes that he and his colleagues are convinced that “all agarwood plants (genus Aquilaria spp. ) have this phenomenon”, as they share “similar morphological fruit traits”. , short-lived seeds, common herbivore, overlap with hornet distribution, etc. While it’s unclear how common this phenomenon is in the plant kingdom, he says, “we’re the first . . . [to examine] this topic.” He suspects that other trees with short-lived seeds may use odors to attract animals, he says: “We believe that the signal from volatiles should be more attractive for rapid dispersal because it is more effective than visual signals, which are blocked by vegetation.”

Agarwood trees are heavily over-harvested by humans for the resinous agarwood they produce in response to fungal infection or damage, often called “liquid gold” and used for incense, perfumes , medicines and small sculptures throughout Southeast Asia. Hornet larvae are also farmed for human consumption by locals, says Gang, and their conservation will be important for the health of the entire ecosystem, as they are part of an “ecological network.” He adds that humans could take advantage of the hornets’ helpful behavior to help grow more agarwood by introducing the insects near agarwood populations.

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