Plants that changed the world: chrysanthemum
This small white flower is responsible for the widely used natural insect repellent pyrethrum, discovers Kiri Degon
Concentrated within the head of a little white flower, Chrysanthemum cinerariifolium, are six chemical compounds (esters) known as pyrethrins. When they work together, they produce the substance pyrethrum, which can poison insects.
It has been used for centuries as an insecticide in the Middle East and is believed to have been brought to Europe in the early 19th century.
How does it work?
Pyrethrum is a broad-spectrum insecticide, meaning it affects a wide range of insects in gardens and farms, including leafhoppers, spider mites, harlequin bugs, ticks, pickleworms and more. It disrupts the central nervous systems of all types of flying and crawling insects, causing their nerve impulses to fail.
In high concentration, when entering an insect’s body, the pyrethrins which make up pyrethrum attach themselves to the sodium channels lining the length of the nerve cells. As sodium is required for transmitting nerve impulses, the nerve cells stop functioning properly when the pyrethrins attach and the insect’s nervous system shuts down, eventually leading to its death.
In a low concentration, the toxicity can affect an insect’s behaviour by producing avoidance reactions, causing it to flee. For this reason, chrysanthemums are often used as companion plants to repel insects from nearby crops and ornamental plants.
As well as protecting crops and flowers, pyrethrum can induce abnormal behaviour in female mosquitoes, making them unable to bite normally – this could have wider implications for human health.
Properties of pyrethrum
- Since pyrethrins are extracted from chrysanthemum flowers, their supply is sustainable and environmentally friendly.
- High levels of UV break down the pyrethrins to a point where they are non-toxic in the environment.
- They are between 10 and 100 times less toxic than some of the synthetic pyrethroids.
- Pyrethrum exposure is relatively safe for humans and warm-blooded animals in low doses, though some symptoms can occur, such as itching, runny nose and sneezing.
Pyrethrum looks set to continue to be in high demand as an insecticide. With threats of a global food crisis, an unsustainable population and climate change, pyrethrum is one of the most sought-after insecticides in the world thanks to its benefits in agriculture.Lead image:
KENPEI/Wikipedia CC BY
- Aromatica: History of pyrethrum
- Laura Pickett Pottorff: Pesticides permitted in organic gardening
- US Centers for Disease Control: Properties of pyrethrum
- Carrots Love Tomatoes – by Louise Riotte
- McLaughlin Gormley King Co: Pyrethrum growing regions
Questions for discussion
- What are the positives and negatives to using a broad-spectrum insecticide in your garden or farm?