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

Date:
February 21, 2023
Source:
Max Planck Institute for Chemical Ecology
Summary:
An international research team 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 study highlights the importance of chemical
communication in maintaining symbiosis between bark beetles and
their fungal partners.


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FULL STORY ==========================================================================
In a new study in the journal PLOS Biology, 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 study highlights
the importance of chemical communication in maintaining symbiosis between
bark beetles and their fungal partners.


==========================================================================
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% 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.

Fungi convert the chemical defenses of spruce into 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.

Bark beetles have olfactory sensory cells in their antennae tuned to
detect volatile 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.

* RELATED_TOPICS
o Plants_&_Animals
# Fungus # Trees # Insects_(including_Butterflies) #
Microbes_and_More
o Earth_&_Climate
# Geochemistry # Forest # Exotic_Species # Ecology
* RELATED_TERMS
o Beetle o Fungal_keratitis o Tree o Seed_predation o
Mountain_pine_beetle o Tropospheric_ozone o Aromatherapy o
Red-cockaded_Woodpecker

========================================================================== Story Source: Materials provided by
Max_Planck_Institute_for_Chemical_Ecology. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Dineshkumar Kandasamy, Rashaduz Zaman, Yoko Nakamura, Tao Zhao,
Henrik
Hartmann, Martin N. Andersson, Almuth Hammerbacher, Jonathan
Gershenzon.

Conifer-killing bark beetles locate fungal symbionts by detecting
volatile fungal metabolites of host tree resin monoterpenes. PLOS
Biology, 2023; 21 (2): e3001887 DOI: 10.1371/journal.pbio.3001887 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/02/230221144356.htm

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