As an organic gardener, you’ve probably seen a substance called pyrethrum mentioned as an organic pesticide ingredient. You may even have used it yourself. Pyrethrum (sometimes called “py” for short) is a low-toxicity insecticide, often applied as a powder, which traces its organic pedigree to the fact that it’s derived from the dried flower heads of two species of chrysanthemum. These are the Dalmatian and Persian chrysanthemums, C. cinerariflorum and C. coccineum, respectively. “Pyrethrum” was originally a collective name applied to these two chrysanthemums, as well as several other varieties.
Pyrethrum has been in use for more than 200 years for both agricultural and hygienic reasons. The active ingredients of pyrethrum are the compounds known as pyrethrins, which are extracted from the seed cases as an oleoresin. The compounds act as a deadly nerve toxin to many insects, including thrips, scale, aphids, and spider mites, and will inhibit the biting reflex in female mosquitoes. Even when not present in quantities toxic to insects, pyrethrum still serves as an effective repellent. You’ll need to be very careful when using pyrethrum if you maintain ornamental fish ponds or if your property is located immediately adjacent to a stream or body of water, as it’s also quite toxic to fish.
Photo courtesy of ConspiracyofHappiness at Flickr.com.
Pyrethrum can be toxic to birds and mammals, but it’s much less so than synthetic insecticides currently in use. Even better, it’s non-persistent; unlike most insecticides, it biodegrades when exposed to light, converting to organically-safe components in as little as 12 hours. This instability makes it one of the safest pesticides for use when growing produce; ironically, however, it’s the same reason why pyrethrum is very rarely used in commercial agriculture. It doesn’t last long enough to suit large-scale farmers.
Pyrethrum should not be confused with either permethrin or pyrethroids, which are must nastier and more persistent synthetic derivatives of pyrethrum.
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