Bark beetles use the smell of fungus to pick the best trees to infest

Bark beetles may use receptors in their antennae to detect and feast on fungus-infected trees.

The Eurasian spruce bark beetle (Ips typographus) – found in Europe, Asia and some parts of Africa – burrows into the bark of Norway spruce (Picea abies) where it feeds and reproduces. In doing so, thousands of the pine-nut-sized insects can kill huge swaths of forest.

Researchers already knew that bark beetles preferred to attack trees weakened by certain fungi, potentially because fungi provide the insects with protective benefits from bothersome microbes and parasites, and could supplement the beetles’ diet. “They gang up and take down huge trees,” says Jonathan Gershenzon at Max Planck Institute for Chemical Ecology in Germany.

But how the beetles were sensing their infected hosts was not known. Gershenzon and Dineshkumar Kandasamy at Lund University in Sweden investigated this by identifying compounds released by the spruce trees infected with Grosmannia penicillata, a fungus almost always found in trees that have succumbed to bark beetle. They found that the fungus created two primary chemical compounds: camphor and thujanol.

Next, the researchers examined the beetles‘ anatomy and found scent-detecting neurons in their antennae that can detect camphor and thujanol. When they gave the insects their choice of healthy or G. penicillata-infected spruce bark in the lab, all of the beetles were more attracted to the fungus-damaged wood. The presence of the fungus-produced compounds not only attracted the beetles but also spurred them to tunnel into the bark.

When tree bark was infected with a type of fungus that wasn’t beneficial to the beetle – and which produced different chemical compounds – the insects weren’t attracted to the wood. “The beetles have evolved to sense the beneficial fungus… and avoid or ignore the ones that are not good for them,” says Kandasamy.

A more thorough investigation of which fungi produce chemicals that are attractive to the beetles would have strengthened the study, says Jiri Hulcr at the University of Florida. “We cannot say that… G. penicillata is particularly special. It may be a natural function of any member of the microbial soup that grows inside a dead tree.”

The researchers are optimistic that pinpointing bark beetle-attracting compounds could lead to more effective baiting efforts. Currently, researchers use a cocktail of attractive pheromones to lure the beetles into a trap en mass, but the method “hasn’t been that successful in the cycle of outbreaks that have started in the last 10 years”, says Gershenzon. He hopes adding these fungi-related compounds to traps could boost their effectiveness.

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