Norway spruce trees produce a mixture of substances as protection against insects and fungal infestation

These conifers produce various monoterpenes to defend themselves against both bark beetles and microbial pathogens.

To the point:

• Mixtures of different defense substances: Spruce trees require various monoterpenes — typical components of resin — to combat bark beetles and the fungi associated with them.
• Specific defense: The monoterpenes that are toxic to bark beetles are different from those that inhibit fungal growth.
• Synergistic effect: Mixtures of monoterpenes have proven more effective than individual monoterpenes against bark beetles and two of the three fungi tested.
• Two hypotheses confirmed: The study is the first to demonstrate both the synergistic effects of a variety of substances as well as the use of different defense compounds against different enemies as an explanation for why trees use mixtures of defense compounds rather than specific ones for defense.
   

Spruce trees make up 88 percent of the damaged trees in Thuringian forests

When traveling to the Thuringian Forest or the Harz Mountains, it is impossible to avoid coming across images that are difficult to get used to. Dead spruce trees (Picea abies) dot the landscape like skeletons. Many have already fallen or been cut down. These dead trees serve as memorials to the effects of global warming. The Norway spruce, the "bread tree" of the timber industry for decades, has been weakened by years of drought and high temperatures. In combination with monoculture cultivation, the spruce has fallen victim to a small insect: the European spruce bark beetle Ips typographus, also known as the "engraver beetle" because of the way it damages trees. However, the 4–5 mm long beetle does not carry out its destructive work alone, but rather together with associated fungi that also infest the trees. Figures confirm the enormous damage caused by bark beetles. The 2024 Forest Condition Report of the Free State of Thuringia identifies bark beetles as the primary cause of damage. It is estimated that 28 million cubic meters of spruce wood has been damaged. This corresponds to 88 percent of the total damaged wood in Thuringian forests.

Do individual plant defenses target specific pests, or does their diversity enhance their effectiveness?

The Department of Biochemistry at the Max Planck Institute for Chemical Ecology is investigating how spruce trees defend themselves against bark beetles and fungi and how these insects and their fungal allies overcome the trees' defenses. In a new study, researchers focused on why spruce trees produce such a complex mixture of defense substances in their sticky resin. Several hypotheses already existed to explain this phenomenon. One hypothesis suggests that the most effective plant defense substances against insects differ from those that inhibit the growth of microorganisms. Another hypothesis assumes that a combination of defense substances is more effective than individual substances. However, there is little evidence to support either hypothesis. Therefore, “we tested twelve natural defense substances — monoterpenes, which are the main components of spruce resin — against their most dangerous pests: bark beetles and the fungi that are carried by the beetles and that promote their attack. We wanted to find out if insects and fungi respond differently to individual substances and if mixtures of these defense substances provide better protection against bark beetles and fungi than individual substances," explains Dineshkumar Kandasamy, one of the lead authors who is currently conducting research at Lund University in Sweden.

Pest specificity and synergistic effects on spruce defense

In experiments testing the effects of different monoterpenes and their mixtures on bark beetles and three fungi, the research team showed that monoterpenes toxic to bark beetles usually do not inhibit fungal growth — and vice versa. Monoterpenes that repelled bark beetles had no effect on fungi, and fungus-inhibiting monoterpenes had no negative effect on bark beetles. The researchers were also surprised to find that the most common monoterpenes in spruce trees were only moderately effective against beetles and two of the three fungi. In contrast, the most bioactive monoterpenes were only found in very small quantities in spruce trees. "At the same time, we found that mixtures of monoterpenes were more effective against bark beetles and two of the three fungi than the individual monoterpenes were. This demonstrates a synergistic effect, whereby several chemical compounds are more effective together than individually," says Dineshkumar Kandasamy. “This is the first study to show that synergy and the fact that different chemicals are effective against different enemies can both explain why trees use mixtures of defense chemicals.”

Mixtures of chemical substances are the gold standard for plant defense

The new study helps answer one of the central questions in plant ecology: Why do plants produce mixtures of different chemical defenses? Since plants are often attacked by many different pests and pathogens simultaneously, producing such mixtures could be the most effective way for them to defend themselves against all their natural enemies. "Our findings provide one of the most conclusive explanations to date for why chemical mixtures are so common in nature," said Jonathan Gershenzon, head of the Department of Biochemistry.

Plant protection in times of climate change

These new findings could help develop natural pesticides. For instance, monoterpenes, which are toxic to bark beetles, could be applied to tree bark where the beetles land before they bore into it. For new pesticide development, it could  also be important to acknowledge that mixtures are more effective than single compounds. As plants face greater stress due to climate change, their defenses against beetles and fungi may become less effective. "Understanding how plant defenses work allows us to better predict how plants will cope with future climate scenarios and develop strategies to protect ecosystems," says Dineshkumar Kandasamy. Further studies are planned. The team wants to find out why monoterpene mixtures are more effective than individual monoterpenes. They also aim to identify the targets of plant toxins within the bodies of pests and pathogens.