Beetles and fungi, strategic allies against spruce forests

The bark beetle, a small beetle that feeds on spruce wood, has strategic allies that allow it to coordinate real mass attacks on trees: mushrooms. Under the supervision of Dineshkumar Kandasamy, a team of researchers from the Max-Planck Institute for Chemical Ecology in Jena (Germany) studied the relationship between the typographer and certain fungi. The results were published on February 21 in the journal PLOS Biology.

Since 2018, a proliferation of bark beetles has killed 60,000 hectares of spruce trees in France. On a European scale, the damage amounts to millions of hectares. However, the spruce has a formidable weapon to defend itself: resin. It is very effective thanks to both mechanical and chemical action, sticking and poisoning the insects. The bark beetle’s strategy to overcome this consists of attacking a spruce tree in large numbers, to the point of overcoming its defense capacities, which are limited as the resin is costly in terms of energy and resources.

To locate a spruce in the middle of a forest, a scout bark beetle will detect certain volatile compounds that it emits. As soon as a target is found, it will release pheromones to attract its congeners. But that’s not all, because certain fungi associated with these insects also play a major role. The bark beetle has a small cavity in its cuticle to carry spores, allowing it to seed mushrooms tree after tree. In return, the insect benefits from the weakening of the defenses of the spruce by the fungus. “The relationship between certain bark beetles and certain fungi is very intimate and ancient in evolution”, recalls Hervé Jactel, director of research in forest entomology at Inrae. The work of Dineshkumar Kandasamy brings a new dimension to the complexity of this mutual aid.

Pheromone traps

The researchers compared in the laboratory the behavior of bark beetles faced with substrates, agars, based on spruce bark, some of which were seeded with fungi. They then observed that the insects were more strongly attracted by the inoculated agars. The researchers identified the molecules having this attractive effect and determined that they were derived from the alteration by fungi of certain volatile compounds in the resin.

To complete their conclusions, the researchers highlighted the presence of cells specialized in detecting these molecules on the bark beetle’s antennae. The behaviors of bark beetles in the presence of these altered volatile compounds are multiple, between increasing the effect of pheromones and encouraging them to dig tunnels, highlighting a real chemical communication system that allows insects to assess the interest of a tree and coordinate their attacks.

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