Symbiotic fungi produce attractants for bark beetles

When metabolizing spruce bark, the insect’s fungal partners release volatile compounds that bark beetles  recognize through specialized olfactory sensory neurons

February 21, 2023

In a new study, an international research team led by the researchers from the Max Planck Institute for Chemical Ecology demonstrates that the European spruce bark beetle Ips typographus uses volatile fungal metabolites of plant defense substances as important chemical signals in their attack on spruce trees. The researchers also show that the insects have olfactory sensory neurons specialized for detecting these volatile compounds. The fungal metabolites likely provide important clues to the beetles about the presence of beneficial fungi, the defense status of the trees, and the population density of their conspecifics.

The mass outbreaks of bark beetles observed in recent years have caused shocking amounts of forest damage throughout Germany. As reported by the Federal Statistical Office in July 2022, more than 80 percent of the trees that had to be felled in the previous year were damaged by insects. The damaged timber felled due to insect damage amounted to more than 40 million cubic meters. One of the main pests is the European spruce beetle Ips typographus. In the Thuringian Forest and the Harz Mountains, for example, the beetle, which is only a few millimeters long, encountered spruce monocultures that had already been weakened by high temperatures and extended periods of drought, which facilitated the spread of the pest and led to the death of huge forest stands within a short period of time.

Researchers have already known that chemical communication plays an important role in bark beetle mass attacks. Beetles first choose a suitable tree and then emit so-called aggregation pheromones. These pheromones attract conspecifics in the vicinity to join a mass attack that overcomes the tree’s defenses. Spruce trees whose defenses are already weakened by stresses are more readily overcome.

Bark beetles like the odor of their symbiotic fungi

Spruce bark beetles need fungal allies to successfully reproduce in the trees. The fungi are ectosymbionts, symbiotic partners that live outside the beetles. Each new generation of beetles must find their symbiotic fungi and carry them to a new host tree.

In a new study, an international research team led by Dineshkumar Kandasamy (now at Lund University, Sweden) and Jonathan Gershenzon of the Max Planck Institute for Chemical Ecology in Jena, Germany, reports that the European spruce bark beetle can find its fungal partners based on the volatile chemical compounds the fungi release when they degrade spruce resin components. “We had already been able to show that bark beetles are attracted to their fungal associates when these are cultured on standard fungal growth medium. Now we wanted to know what would happen if we grew fungi on a more natural medium with spruce bark powder added. Would beetles be attracted to fungi now? If so, which chemical compounds would be responsible for the attraction and what is the origin of these chemicals?” says first author Dineshkumar Kandasamy, explaining the study's initial questions.

Attractants for the beetles

European spruce bark beetles are associated with fungal partners of different genera. The fungus Grosmannia penicillata grew particularly well on the spruce bark medium and produced more volatile compounds than most of the other fungi tested. Therefore the researchers focused their investigations on this fungus. The researchers set up special experimental arenas where they could test whether the beetles were attracted to volatile compounds emitted by the fungi.

“We first found that European spruce bark beetles are attracted to the volatiles emitted by their associated fungi when fungi were growing on medium with spruce bark powder. However, we also showed that fungi can transform terpene compounds from spruce resin into their oxygenated derivatives and that some of these metabolites produced by fungi are particularly attractive to bark beetles. The overall conclusion is that these volatiles serve as chemical signals that keep the symbiosis between bark beetles and their associated fungi going," says Dineshkumar Kandasamy. 

The researchers found that pathogenic fungi, which are harmful to the beetles, can also metabolize spruce resin compounds. However, unlike the metabolites of the symbiotic fungi, the resulting derivatives are not attractive to bark beetles. Bark beetles can therefore use their sense of smell to distinguish whether the fungi present in the tree are good or bad for them. The scientists were particularly surprised when the behavioral observations revealed that fungal partners not only attracted the beetles but also stimulated them to tunnel. 

Sensory cells detect compounds of fungal metabolism

Further evidence that fungal metabolites make spruce trees already infested by fungi even more attractive to bark beetles was provided by electrophysiological studies of the beetles' perception of these odors. This involved testing the response of individual olfactory sensilla on the beetle antennae to different odors. The researchers were able to show that the bark beetles possess certain olfactory sensory neurons housed in sensilla that are specialized in detecting oxygenated monoterpenes emitted by the fungi.

“By enhancing bark beetle attraction to particular trees, volatiles from the fungus could increase the intensity and success of mass attacks. Fungi may help kill the host tree, overcome its defenses, provide beetles with nutrients or protect them from pathogens. The ability of the fungus to metabolize resin components that are originally produced by the tree as a defense could indicate which fungi are virulent and could serve as good partners for the beetle," says Jonathan Gershenzon. 

The results of this new study may help improve the control of bark beetle outbreaks. One of the most widely used strategy in the fight against these pests are pheromone traps, but these have not been effective in preventing recent outbreaks. Therefore, the researchers are now testing whether these odor traps can be optimized by adding oxygenated monoterpenes from fungal metabolism. An important goal for the research team is to learn more about the metabolism of the spruce resin compounds in the fungi and to find out whether this can be a detoxification reaction for the fungus or for the beetle.

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