Crop breeding

Where does pyrethrum come from?

If you’re a gardener, you may have heard of the common trick to plant chrysanthemums around your garden to deter pests. Marigolds work, too. This is because the flowers naturally are high in chemical compounds that insects don’t like.

A particular type of chrysanthemum, Chrysanthemum cinerariifolium, isn’t typically found in nurseries. Its flowers are very high in the natural compound, Pyrethrum. It can be used as both a natural pesticide on organic farms and as an insect repellent for humans. As such it’s been called a “green pesticide” and researchers are looking at ways to make more of it.

up-close view of clary sage plant secreting Pyrethrum through trichomes
Some plants have glands that secrete chemical compounds through trichomes. Shown: clary sage secreting sclareol through its trichomes. Credit: SV Fisk

The use of pyrethrum as an insecticide was a well-kept secret by ancient Persians, for the health of their crops, but its use is documented from around 400 B.C. Japan was the leading producer of pyrethrum before World War II, and later African countries began production.

How can planting chrysanthemums around your garden protect it? Plants have glands, called trichomes. And some plants secrete a chemical compound from these glands, in this case, pyrethrum. It doesn’t have the best smell.

In the case of Chrysanthemum cinerariifolium, the highest concentration of pyrethrum is in its flowers, which are the focus of the harvest. Growers harvest the flowers at specific stages and dry the flowers. They then grind the flowers and extract the pyrethrum. From there, different products can be made, like powders and liquids.

dog and cat cuddling on dog bed. Pyrethrum can be used in dog shampoos
Pyrethrum, a natural chemical found in chrysanthemums, can repel insects like mosquitoes and ticks. Pyrethrum can be used in dog shampoos but not cat shampoos as it’s toxic to cats, and has to be dosed just right for dogs. Credit: C. Flores

Current research about pyrethrum

In its natural state, Chrysanthemum cinerariifolium doesn’t flower until its second year of growth. Researchers at the University of Minnesota recently published a paper about the work to breed a new type of pyrethrum that can flower in its first year. This would greatly increase the yield to growers, with an early payoff.

They were also looking for varieties that grew more flowers and less of the leaves and stalks, which don’t contain as much pyrethrum. After their initial research using seeds collected from various sources, they will continue their studies and breeding in hopes to create a higher-yielding, first-year flower producer that makes more natural pyrethrum.

Confusion with names

A farmer in her field in Tanzania. Credit: Pyrethrum Company of Tanzania

Ah, if it were so easy to understand the difference in names of types of pyrethrum. Pyrethrum is the natural compound found in daisies, and Chrysanthemum cinerariifolium is the type of daisy that contains the most and is used commercially.

The compound can also be made in the lab and factories (referred to as synthetic). In this case, they are referred to as pyrethrin and are often mixed with other chemicals to enhance their effects. Permethrin is another synthetic compound based off the chemical structure of the natural pyrethrum. It is used to treat clothing, tents, and outdoor equipment, not skin.

Watch for these different spellings in the name when you go to make a purchase. As always, read label directions before using any pesticide or bug spray.

Compiled by Susan Fisk, from research published in Crop Science Journal, a publication of the Crop Science Society of America.

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

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