Chemical signals from fungi tell bark beetles which trees to infect, scientists say

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Fungi can help some wood-killing beetles turn the tree’s natural defense system against itself.

Eurasian spruce bark beetle ( Ips typographus ) destroyed millions of conifers in forests across Europe. Now research shows that fungi associated with these bark beetles are key players in the insect’s hostile takeover. These fungi distort the chemical defenses of host trees to create a scent that attracts the beetles to the burrow, researchers report Feb. 21 in PLoS Biology .

These perfumes, created by fungi, may explain why bark beetles tend to swarm on the same tree. As climate change makes European forests more vulnerable to insect invasions, understanding this link could help scientists develop new countermeasures to prevent beetle attacks.

Bark beetles are a type of insect distributed throughout the world that feed and reproduce inside trees. In recent years, several species of bark beetles have aggressively attacked forests from North America to Australia, leaving ominous strands of dead trees in their wake.

But trees are not defenseless. Conifers, which include pines and firs, are real factories for the production of chemical weapons. The evergreen scent of Christmas trees and alpine forests comes from airborne varieties of these chemicals. But while they may smell great, the primary purpose of these chemicals is to trap and poison invaders.

Or at least that’s what they’re supposed to do.

“Conifers are full of resin and other substances that can do terrible things to insects,” says Jonathan Gerschenzon, a chemical ecologist at the Max Planck Institute for Chemical Ecology in Jena, Germany. “But the bark beetles don’t seem to mind at all.”

This ability of bark beetles to overcome the powerful defense system of conifers has led some scientists to wonder if fungi can help. Fungi break down compounds in the environment for food and protection. And some types of mushrooms, including some species from the genus Grosmannia are always found in association with Eurasian spruce bark beetles.

Eurasian spruce bark beetles (larvae seen here) have destroyed millions of trees in their quest to feed and breed in the conifers of Europe.DINESHKUMAR KANDASAMY ( CC BY 4.0 )

Gershenzon and his colleagues compared the chemicals released by infected spruce bark Grosmannia and other fungi, with the chemical profile of uninfected trees. The team found that the presence of fungi dramatically changed the chemical profile of the spruce trees. More than half of the airborne chemicals — produced by fungi that break down monoterpenes and other chemicals that are likely part of the trees’ defense system — were unique to the infected trees after 12 days.

That’s surprising because researchers had previously assumed that the fungal invasion barely changed the chemical profile of the trees, says Jonathan Cale, a fungal ecologist at the University of Northern British Columbia in Prince George, Canada, who was not involved in the study.

Later experiments showed that bark beetles could detect many of these chemicals made by fungi. The team tested this by attaching tiny electrodes to the heads of bark beetles and detecting electrical activity when chemicals were passed over their antennae. Moreover, the smell of these chemicals combined with the beetles’ pheromones induced the insects to burrow at a higher rate than the smell of the pheromones alone.

The study suggests that these chemicals created by the fungi may help the beetles determine where to feed and breed, possibly by signaling that the fungi have destroyed some of the tree’s defense mechanisms. The attractive nature of the chemicals may also explain the swarming behavior of the beetles, which results in the death of healthy mature trees.

But while the mushroom aroma can kill trees, it can also kill beetles. Beetle traps in Europe currently only use beetle pheromones to attract their prey. Combining pheromones with chemicals derived from mushrooms may be the secret to luring more beetles into traps, making them more effective.

The results represent “an exciting direction for developing new tools to control destructive bark beetle outbreaks” for other beetle species as well, Keil says. In North America, mild winters and drought put coniferous forests at greater risk of attack by the mountain pine beetle ( Dendroctonus pendersoae ). Finding and using chemicals derived from mushrooms may be one way to stave off the worst infestation of bark beetles in the coming years.

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