Nature gives rise to fascinating adaptations, which can sometimes lead to organisms living in some strange situations.

The relationship between bark beetles, spruce trees, and a type of fungus is a case in point. According to a new study published in Proceedings of the National Academy of Sciences, the bark beetles protect themselves from fungal infections by leeching off the trees’ stored antibiotic compounds. But the fungus still manages to keep the beetles in check by infecting and killing some of the beetle populations.

Examining the ways each organism has adapted to its neighbors could help scientists create organic treatments for parasites, the researchers said.

No friends here

Spruce bark beetles (Ips typographus) are a common pest that burrows deep into a tree, siphoning away nutrients. Although bark beetles are rarely aggressive enough to actually kill a tree, their presence can threaten those weakened by natural or human-made disasters. What’s more, “little can be done to control most bark beetles once trees have been attacked,” according to a report by California’s Integrated Pest Management Program.

Then there is the fungus, Beauveria bassiana. This fungus is neither the tree’s nor the beetle’s friend; as a pathogenic species, it can infect both organisms. The researchers behind the new study wondered whether the beetles could be exploiting the spruce tree’s rich defensive resources to protect themselves from the fungus.

To find out, the team performed detailed chemical analyses of the antibiotic chemicals produced by spruce trees to fight off fungal infections. Their investigation suggested that the beetles used the compounds for their own defense. Surprisingly, that involved a sophisticated process in which the insects converted these chemicals into even more toxic substances, with greater protective ability against fungal attacks.

Grasshoppers infected and killed by the pathogenic fungus Beauveria bassiana, Credit: Agricultural Research Service / U.S. Department of Agriculture

An odd trio

“We did not expect the beetles to be able to convert the spruce’s defenses into more toxic derivatives in such a targeted way,” Ruo Sun, study lead author and a biochemist at the Max Planck Institute for Chemical Ecology Jena in Germany, said in a statement.

The researchers also found that the fungi were able to adapt in turn to the beetles’ new defenses.  Overall, the tree’s chemical defenses experienced “multiple transformations and retransformations throughout the food chain—with far-reaching consequences for the evolutionary arms race between hosts, pests, and pathogens,” the researchers said.

It’s quite unlikely that any organism involved in this system knowingly keeps the others at bay, but it’s certainly a naturally derived solution to pest control, according to the paper. For instance, scientists could take inspiration from the fungus to create more effective pesticides. Some fungal strains “had naturally infected and killed [the beetles],” Sun said. As such, identifying these strains became the central goal of their research, she added.