Pest control professionals keep receiving calls for a reason. Wood-eating insects like beetles, termites, and carpenter ants continuously gnaw through walls or infest and degrade trees. The response has typically relied on strong insecticides; yet now some of these pests can be controlled by a particular species of fungus.
Bark beetle outbreaks are devastating to spruce trees. Eurasian spruce bark beetles (Ips typographus) feed on bark rich in phenolic compounds, organic molecules that often serve as antioxidants and antimicrobials. They protect spruce bark from pathogenic fungi—and the beetles exploit that protection. The beetles enhance the antimicrobial effect by converting those compounds into forms that are even more toxic to fungi. That appears to make the beetles resistant to fungal pathogens.
There is a way to bypass the beetles’ appropriated defenses, however. Led by biochemist Ruo Sun, a research team from the Max Planck Institute for Chemical Ecology in Jena, Germany, discovered that some strains of the fungus Beauveria bassiana can infect and kill the insects.
“Insect herbivores have long been known to accumulate plant defense metabolites from their diet as defenses against their own enemies,” she said in a study recently published in PNAS. “However, as shown here for B. bassiana, fungal pathogens are able to circumvent the toxicity of these dietary defenses and cause disease.”
First line of defense
Bark beetle populations have surged in temperate forests recently because of climate change. One host they consume is the Norway spruce (Picea abies), which produces phenolic compounds known as stilbenes and flavonoids. Stilbenes are hydrocarbons that act as secondary metabolites in plants, and flavonoids, which are polyphenols, are also secondary metabolites that frequently function as antioxidants. The spruce attaches both groups of compounds to sugars and relies on their antibacterial and antifungal properties.
When beetles metabolize these compounds, the sugar moieties are removed via hydrolysis, producing aglycones that are even more harmful to microscopic invaders. Despite this, some fungi can deactivate those compounds. Strains of the fungal insect pathogen B. bassiana have been recorded killing some of these beetles in the wild.
