A hidden chemical war is unfolding inside spruce trees

To investigate this process, the team used advanced tools including mass spectrometry and nuclear magnetic resonance (NMR) to identify the defensive chemicals produced by spruce trees and track how bark beetles process them. The researchers found that beetles feeding on spruce trees absorb defensive compounds from the phloem, especially phenolic glycosides such as stilbenes and flavonoids.

Inside the beetles, these compounds are chemically altered. The insects convert them into aglycones, which no longer contain sugar molecules and have much stronger antimicrobial effects. These transformed chemicals provide the beetles with effective protection against fungal pathogens. "We did not expect the beetles to be able to convert the spruce's defenses into more toxic derivatives in such a targeted way," said lead author Ruo Sun from the Department of Biochemistry.

A Fungus That Can Disable Beetle Chemical Defenses

The scientists next examined how these beetle defenses affect the fungus Beauveria bassiana. "Although this fungus has not been effective in controlling bark beetles in the past, we found strains that had naturally infected and killed them. We therefore wanted to investigate more closely how they were able to successfully infect the beetles," Ruo Sun explains.

Laboratory analyses and enzyme tests showed that the fungus uses a two-step detoxification strategy. The first step is glycosylation, which adds a sugar back onto the toxic aglycones. The second step is methylation, which attaches a methyl group to the sugar. The final compounds, known as methylglycosides, are not harmful to Beauveria bassiana.

This chemical modification unexpectedly helps the fungus infect beetles, especially those that previously fed on spruce tissue rich in phenolic compounds. The methylglycosides are also resistant to beetle enzymes that would normally break them down and restore their toxicity through hydrolysis.

Gene Studies Confirm the Detoxification Mechanism

To confirm the importance of this pathway, the researchers disabled the genes responsible for methylglycosylation in Beauveria bassiana. Fungi without these genes were much less successful at infecting bark beetles, showing that the detoxification process is critical for overcoming the beetles' chemical defenses.

The study shows that tree defense chemicals can be repeatedly altered as they move from plants to insects and then to pathogens. These changes have major implications for the ongoing evolutionary struggle among trees, pests, and fungi. "We have demonstrated that a bark beetle can co-opt a tree's defensive compounds to make defenses against its own enemies. However, since one of the enemies, the fungus Beauveria bassiana, has developed the ability to detoxify these antimicrobial defenses, it can successfully infect the bark beetles and thus actually help the tree in its battle against bark beetles," summarizes study leader Jonathan Gershenzon.

Improving Biological Control of Bark Beetles

The findings may help scientists develop more effective biological control methods. "Now that we know which strains of the fungus tolerate the bark beetle's antimicrobial phenolic compounds, we can use these strains to combat bark beetles more efficiently," says Ruo Sun. The study also highlights the need to examine whether pests have developed resistance or detoxification strategies when biological pesticides are used.

In future research, the team plans to investigate how common the methylglycosylation detoxification pathway is in different strains of Beauveria bassiana and in other fungi that infect bark beetles. They also aim to understand how this pathway interacts with other pathogen traits that influence infection success.