Through research conducted on young Scots pine trees and Diprion pini, a sawfly common to conifer forests in the Northern Hemisphere, scientists have shown, for the first time, how trees take steps to protect themselves against insect infestation even before eggs are laid on them.

Plants can detect insect sex pheromones and, upon doing so, act to protect themselves against infestation. That is the main conclusion of an international study involving CREAF researcher Ander Achotegui-Castells and published in the scientific journal PNAS (Proceedings of the National Academy of Sciences). The finding was made by looking at young Scots pine trees and the sawfly Diprion pini, both of which are common in the Northern Hemisphere.

“When it senses a potential imminent attack, the tree primes its defences, which requires little energy. Doing so enables it to react more forcefully if the attack takes place, and prevents any great energy loss if there is no attack in the end"

The study shows that the aforementioned ability allows some trees to take steps to protect themselves against infestation at a very early stage and react more effectively if the threat materializes. If a Scots pine detects insect pheromones but no eggs are then actually laid on it, its reaction is minimal. If it does identify the presence of insect eggs, however, it activates a robust response aimed at killing them off. “When it senses a potential imminent attack, the tree primes its defences, which requires little energy”, says Achotegui-Castells. “Doing so enables it to react more forcefully if the attack takes place, and prevents any great energy loss if there is no attack in the end”, he explains.

Known as the pine sawfly, Diprion pini is one of a number of insects that perforate pine needles to lay their eggs. Upon hatching, the larvae of such insects begin to eat the tree’s needles, threatening its survival. “The females lay their eggs in pine needles so that their recently hatched larvae will have tender food to eat, and that can be extremely harmful to the tree”, states Achotegui-Castell. Sawflies get their name from their saw-like ovipositor, the organ with which the females lay eggs.

Sacrificing needles

One of the ways the Scots pine defends itself against possible infestation, after identifying Diprion pini sex pheromones, consists of increasing the level of hydrogen peroxide in the needles that contain eggs, which causes the formation of necrotic tissue and damages the eggs. Its other defence mechanisms include releasing volatile compounds that attract egg parasites and increasing the expression of defence-related genes.

The survival rate of the eggs laid on the pines that had been exposed to the pheromones was just 40%, whereas it was 60% in the case of the control trees.

In the study, couples of Diprion pini laid eggs on pines that had been exposed to synthetic pheromones. One control group of pines was exposed to the pheromone solvent hexane and another was left with no treatment whatsoever. The survival rate of the eggs laid on the pines that had been exposed to the pheromones was just 40%, whereas it was 60% in the case of the control trees.

Until now, scientists had got no further than proving that a specific plant (Solidago altissima) responds to certain compounds (not pheromones) released by an insect that feeds on its leaves. The study that Achotegui-Castells carried out, alongside researchers from Freie Universität Berlin (Germany) and Lund University (Sweden), is thus a landmark in research on plants’ olfactory capabilities.