Bark beetles can destroy spruce forests by converting the trees' defences into even more toxic substances, scientists recently discovered.
But a special fungus can help the trees circumvent this insidious and deadly toxic cycle, according to a team led by Ruo Sun and Jonathan Gershenzon from Germany's Max Planck Institute for Chemical Ecology.
The team's analysis shows that spruce trees produce compounds called phenolic glycosides to defend themselves.However, bark beetles convert these compounds into more potent antimicrobial substances that protect the beetles from pathogens.
The protective compounds are concentrated in the layer of bark colonised by bark beetles, in a layer that primarily transports sugar from the tree's leaves to its roots.
One of the best-known bark beetles is the European spruce bark beetle, known as Ips typographus in Latin, which mostly infests spruce trees and causes immense damage.
The beetle absorbs the spruce's defence substances and converts them into even stronger pesticides against pathogens, says the team writing in Proceedings Of The National Academy of Sciences (PNAS).
"We did not expect that the beetles would be able to convert the spruce's defences into even more toxic derivatives in such a targeted way," says lead author Ruo Sun.
Most trials unsuccessful
The team probed the effect of the beetle's defences on the fungus Beauveria bassiana, which attacks beetles and other insects.
The fungus is often used as a biological control agent and alternative to conventional insecticides. It has also been investigated for its potential to combat bark beetle infestations. But most field trials have been unsuccessful.
The team isolated two fungi that killed beetles in the feeding tunnels of infested trees and in labs and found both are strains of Beauveria bassiana.
"Although this fungus has not been very effective in controlling bark beetles in the past, we found strains of this fungus that had naturally infected and killed bark beetles. So we wanted to investigate more closely how they were able to successfully infect and kill the beetles," says Sun.
The fungal strains were able to convert the beetles' defence mechanism in two steps, thereby detoxifying it. The resulting products – methyl glucoside derivatives – are harmless to the fungus.
"The successful infection of bark beetles with Beauveria bassiana is due to a highly specific detoxification pathway of the fungus," Sun says.
"We have shown how a bark beetle can use a tree's defence substances to defend itself against its own enemies. However, because one of the enemies, the ascomycete fungus Beauveria bassiana, has developed the ability to detoxify these antimicrobial defences, it can successfully infect the bark beetle and thus help the tree in fighting bark beetles," says study leader Gershenzon.
"Now that we know which strains of the fungus tolerate the bark beetle's antimicrobial phenolic compounds, we can use these strains to better combat bark beetles," says Sun.
Investigations of B. bassiana strains showed that they often vary in their effectiveness against bark beetles, which could be due to strain-specific differences in the rate of toxin destruction, the study found.
The team suggests searching for further strains. "Our results provide a biochemical basis for the use of B. bassiana to combat bark beetle infestation in spruce and other conifers," says the team, adding further research is still needed. – dpa
