Eurasian spruce bark beetles (Ips typographus) burrow into the bark of Norway spruce trees (Picea abies), where they mate and afterwards lay their eggs. Recently, this phenomenon led to major outbreaks in Europe that have decimated millions of hectares of conifer forests.
According to a new study published in the journal PLoS Biology, these beetles usually attack trees that are already infected with symbiotic fungi, such as Grosmannia penicillate, which weaken the host trees, breaking down their chemical defenses and thus allowing the beetles to successfully grow and reproduce in the bark.
In order to understand what chemical signals the beetles use to identify host trees infected with the fungi, the scientists conducted several laboratory experiments on captive bark beetles and samples of Norway spruce bark. The analyses revealed that the fungi break down chemicals in tree bark resin (known as monoterpenes) into new compounds, such as camphor and thujanol, which 12 days after infection dominate the chemical mixture emitted by bark samples. Through single cell recordings of sensory neurons in the beetles’ antennae, the experts discovered that they can detect camphor and thujanol and are more attracted to bark containing these fungi-produced compounds.
According to the researchers, these tree-resin derived compounds produced by the fungi allow bark beetles to assess the presence of their symbiont and thus help them identify suitable trees for feeding and breeding. “The bark beetles currently killing millions of spruce trees every year in Europe are supported in their attacks by fungal associates. We discovered that these fungi convert volatile compounds from spruce resin to products, which may serve as cues for bark beetles to find them,” explained study senior author Jonathan Gershenzon, the director of the Max Planck Institute for Chemical Ecology.
“These compounds may also be useful in integrated pest management strategies as attractants or repellents of bark beetles perhaps in combination with pheromones. In this way, oxygenated monoterpenes and other microbial volatiles represent a rich source of untapped insect semiochemicals that can be exploited for protecting forests from devastating pest species such as I. typographus,” the authors concluded.
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