Ambrosia beetles breed their own food fungi

The bark beetle species Xyleborinus saxesenii has agricultural skills. © Gernot Kunz

Similar to ants and termites, one species of bark beetle apparently also has agricultural skills: Ambrosia beetles breed and care for certain food fungi in their nests and ensure that weed fungi and bacteria are less able to spread. On the one hand, the findings on this symbiosis can contribute to more targeted control of economically relevant bark beetles, which cultivate their food in a similar way. On the other hand, people may be able to draw inspiration from ragweed beetles in the future when it comes to keeping harmful fungi in check.

It is not just humans who specifically grow crops that are edible for them, individual species of insects have also developed agricultural skills. Certain species of ants and termites grow mushrooms that are tasty for them. "Although agricultural insects are biologically very different from humans, their cultivation methods are remarkably similar," writes a team led by Janina Diehl from the University of Freiburg. This includes planting desired plants or fungi and controlling pests and weeds.

Mushrooms in the bag

"While ants and termites are known for their agricultural skills, this evidence has so far been lacking for a third group of putative mushroom breeders: the wood-colonizing ambrosia beetles," explain Diehl and her colleagues. "Until now, it was only assumed based on observations of their social and hygienic behavior that they also promote food fungi selectively over others."

Diehl and her team have now provided experimental evidence that the ambrosia beetle is actually active in agriculture. The researchers focused on ragweed beetles of the species Xyleborinus saxesenii. In the wild, the species, also known as the Lesser Woodborer, occurs in native parks and gardens with mature trees, among other places. "The beetles bore burrows into the wood and carry symbiotic fungi into them, which spread along the walls of the burrows and represent the beetles' exclusive food source," say the researchers. "The beetles rely on the fungi for food and the fungi are not found outside the beetle environment, demonstrating strong coevolution between the partners."

Active care

In order to find out to what extent the beetles actually actively promote the growth of their symbiotic fungi, the researchers had mother beetles of the Lesser Woodworm set up nests with offspring in the laboratory, in which the typical fungal gardens soon formed. They then removed the caring individuals from some of the nests and left them in others. A genetic analysis of bacterial and fungal communities of the mushroom gardens after 40 days showed that the presence of the beetles had strongly altered the fungal community.

"One might have expected that there would be fewer edible fungi in the nests with beetles because they were eaten - but the opposite was the case, here the fungal composition was clearly shifted in the direction of edible fungi," says Diehl. In the nests without caring beetles, on the other hand, the proportion of weed fungi was significantly higher. The composition of the bacteria also differed.

inspiration for man

"These results prove the existence of active agriculture in ragweed beetles, even if the exact mechanisms controlling the fungal community still need to be investigated further," says Diehl's colleague Peter Biedermann. There is evidence that the beetles use special bacteria that produce antibiotic substances. These in turn could inhibit the growth of weed fungi. Social behavior also plays an important role: the entire group of beetles in the nest, including the larvae, work together to care for the fungi. This creates a close symbiosis between beetles and fungi: "Each species of ambrosia beetle has its own food fungus - neither can survive without the other."

This knowledge could also help to combat economically relevant bark beetles in a more targeted manner in the future. Species such as the bark beetle (Ips typographus), which primarily infests spruce and causes great forest damage, live in a symbiosis with their food fungi. Diehl's team is also interested in how exactly the ragweed beetles manage to suppress the growth of weed fungi. From the researchers' point of view, further insights into the beetles' methods could also provide valuable insights for human agriculture, which also struggles with harmful fungi. "It is extremely exciting for us to see how nature has been doing this for 60 million years," says Biedermann. "We humans can probably still learn something from these mechanisms."

Source: Janina Diehl (University of Freiburg) et al., Proceedings of the Royal Society B., doi: 10.1098/rspb.2022.1458

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