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Trees’ smart defence “smells” incoming attacks

Plants can trigger their own defences against noxious insects earlier than previously thought. In a new study, researchers found for the first time that sexual pheromones act as a warning signal to plants even before the insects have laid their eggs.
A larvae on a scots pine.
Saw fly larvae (Diprion pini). Photo: Wikimedia commons

Plants can “smell” an approaching attack by noxious insects through the odours that a female releases to attract males when she is to lay eggs. The odours, sexual pheromones, also act as an effective warning signal for the plants, whose defence systems are activated and kill a considerable percentage of the noxious insect eggs.

The findings are in an international study published in the journal PNAS.

Far fewer eggs survived

Under laboratory conditions, the researchers exposed Scots pine plants to sexual pheromones from one of the most common noxious insects, the sawfly. Female sawflies then laid eggs on the exposed Scots pine plants, as well as on plants in a control group that was not exposed to sexual pheromones. Two weeks later, the researchers examined the plants to find out the percentage of eggs that had survived.

“The eggs that survived decreased by one-third among the plants that we exposed to pheromones the day before the eggs were laid – from 60 per cent to 40 per cent”, says Olle Anderbrant, professor at the Department of Biology in Lund.

A collage of three photos of sawflies: mating, laying eggs, feeding.
Sawflies cause huge damage to scots pine. This collage shows two sawflies mating, the female laying eggs, and a larvae feeding. Photo: Benjamin Fuchs and Jona Höfflin

Olle Anderbrant carried out the study with researchers at Freie Universität in Berlin and the Global Ecology Unit CREAF in Barcelona. In the study, they show the likely process the pines use to kill the eggs. The pheromones increase the concentration of hydrogen peroxide in the pine needles when the eggs are laid. Hydrogen peroxide strengthens the plants’ defensive capacity and diminishes the survival prospects of the insect eggs.

More to find out

The researchers will now focus on trying to find out exactly how the plants sense the noxious insects’ chemical signals.

“When we know that and just how specific chemical signals have to be to trigger the plants’ defences, it may be possible to expose plants to these odours as a way to strengthen their defences”, concludes Olle Anderbrant.

The results were published in an article in the journal Proceedings of the National Academy of Sciences of the USA, PNAS.

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