Pyrethrum’s mimicry gives flowers a double defence

Gardeners have long thought chrysanthemums help repel aphids from flower beds. New research now shows how that might work.

You can use pesticides to keep aphids under control in your flower beds, but wouldn’t it be better if they weren’t there in the first place. A common trick is to plant chrysanthemums as companion plants. They repel aphids, and so protect themselves and their neighbours. But how?

A study of pyrethrum flowers by Jinjin Li and colleagues, recently published in New Phytologist, suggests the plants mimic aphids.

Pyrethrum. Photo: Canva.

Any gardener that’s seen an aphid-infested plant will know that aphids have no trouble with each others company. At least not usually. But they do when the chemical (E) – β -farnesene (EβF) is around.

EβF is a pheromone, a chemical released to alter the behaviour of others. For aphids, EβF is an alarm pheromone. It’s a signal that an aphid is having a Very Bad Time and used to signal to other aphids that they might want to avoid that location. What Li and colleagues have found is that pyrethrum uses EβF too, when it grows flowers.

Flowers are crucial to flowering plants, as they’re critical for reproduction. They’re also often a lot of effort for a plant to produce, so protecting them makes sense. The scientists found that the gene that produces EβF is strongly expressed in inner cortex tissue surrounding the vascular system of the flower stalk. When the flower is growing, pyrethrum emits EβF to dissuade aphids from landing and damaging the plant.

In addition, the stalk has a big store of EβF in its cells. When an aphid probes the plant to get to the sap, it hits this EβF store. The chemical passes through the aphid and is released in its honeydew. The emission itself helps get the EβF out of the plant and into the air, amplifying the alarm signal.

If you’re wondering what could alarm an aphid, a ladybird can eat up to fifty of them a day.

A ladybird is likely to be on the lookout for a meal. And the colourful beetles are also smelling for a meal. When aphids warn others about predators, they also let predators know that there’s an aphid colony in the area.

As well as finding EβF repels aphids, Li and colleagues found it attracted ladybirds. Experiments showed it was the flowers that emitted most EβF that the ladybirds found most attractive. They also found that damaged plants were more attractive and young plants. What mattered for the ladybirds were the highest and purest EβF emissions.

The team say a next step would be to compare pyrethrum plants with gene-edited versions, where EβF production is switched off. This way, they can see how effective the flower defences are, and more of the mechanics of how they work.

In the meantime, if you like chrysanthemums, then you have the perfect excuse to plant a few more this weekend.

Alun Salt

Alun (he/him) is the Producer for Botany One. It's his job to keep the server running. He's not a botanist, but started running into them on a regular basis while working on writing modules for an Interdisciplinary Science course and, later, helping teach mathematics to Biologists. His degrees are in archaeology and ancient history.

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