Mushrooms summon bark beetles to attack

bark beetle

A bark beetle (Ips typographus) surrounded by fungal mycelium. © D. Kandasamy, V. Grabe/ Max Planck Institute for Chemical Ecology

Bark beetles have already destroyed large areas of forest in this country. They do their job particularly efficiently if the affected trees have already been weakened by special fungi. As researchers have now discovered, these fungi even "call" the beetles to reinforce them by emitting a special attractant that they produce from spruce resin. The new findings could help mitigate future bark beetle infestations and save spruce forests.

The large octagonal spruce bark beetle, also known as the "book printer", is a pest that has already destroyed several million hectares of coniferous forests in Europe. It burrows into the bark of spruce trees, where it mates and lays its eggs. The hatching larvae eat their way through the outer layers of the tree trunk, destroying the pathways important for water and nutrient transport. It has been known for a long time that the beetles mainly attack host trees that are already infected with certain fungi such as Grosmannia penicillata. The fungus probably weakens the tree's defenses to such an extent that the beetles can do their work without any problems.

From spruce resin to attractant

But the complicity of fungus and beetle in the "tree murder" is apparently even closer than expected. Researchers led by Dineshkumar Kandasamy from the Max Planck Institute for Chemical Ecology in Jena have found out in experiments how exactly beetles and fungi come together. To do this, they first let Grosmannia fungi grow on a medium containing spruce bark powder and analyzed which chemical compounds they released into the air. It turned out that the fungi use terpene compounds from the spruce resin to produce oxygen-containing derivatives such as camphor and thujanol.

These chemical compounds are very attractive to book printers, as the research team demonstrated in special test arenas. Further tests also revealed that the beetles even have special olfactory sensory cells in their antennae that are specialized in perceiving these very connections. "It was clear to us that these volatiles serve as chemical signals that maintain the symbiosis between bark beetles and their associated fungi," summarizes Kandasamy. However, the fungus partners not only attract the beetles, but also stimulate them to build tunnels with the scents produced, as behavioral observations have shown.

New pheromone traps for pest control

In nature, the symbiosis between fungus and beetle would look like this: Grosmannia fungi attack a spruce, weakening its defenses and at the same time sending out special attractants that they produce from the resin of the tree. Bark beetles perceive these scents and go to the already weakened spruce. There they burrow into the bark to lay their eggs and also send out attractants that attract more bark beetles. At some point the spruce dies from the double infestation. If it is already weakened by persistent drought and high temperatures, the beetle-fungus duo has an even easier time of it.

These new findings could help fight bark beetle outbreaks more efficiently in the future. Until now, so-called pheromone traps have mostly been used in the fight against the pests. These imitate the attractants that bark beetles send among themselves to attract other species. However, in recent outbreaks, these traps have not proven effective. The researchers are therefore now testing whether the effect can be restored if the scents from the fungal metabolism are added to the traps.

Source: Max Planck Institute for Chemical Ecology; Article: PLOS Biology, doi: 10.1371/journal.pbio.3001887

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