Pyrethrum, also known as "Persian powder" and "Persian pellitory," is a natural insecticide made out of chrysanthemum flowers. Pyrethrum was used around the world for centuries as a highly effective means for controlling lice and other pests, yet today it is little-known outside of the organic farming movement.
The process for making pyrethrum is stupidly simple: just dry the chrysanthemum petals and grind them into a powder, which is then mixed with water and voilà, you have an effective, natural pesticide. Pyrethrum is also very safe to use; the LD50 of pyrethrum in mice and rabbits is very high - these animals would have to ingest about 1% of their body weight in chrysanthemum powder before experiencing significant mortality, so human beings would have to ingest massive amounts of the powder before experiencing any ill effects.
As late as the 1930s, pyrethrum was still the gold standard for insecticides in the United States and around the world. However, fully 90 percent of the world's supply was grown on farms in Japan. With the outbreak of the Pacific War in 1941, all trade between Japan and the rest of the world was cut off, and moreover Japan converted its pyrethrum farms to growing other crops more necessary for the war effort.
At this time, pyrethrum was still the standard delousing agent used by the US military. With its supply of pyrethrum cut off, the military was desperate to find a new delousing agent. In this context, it seemed fortunate that Swiss researchers had recently discovered the chemical insecticide DDT. To replace natural pyrethrum, the United States ramped up production of DDT on a massive scale and sprayed it all over the United States and wherever the US military set foot for the next three decades, unknowingly causing massive degradation to ecosystems worldwide.
The cruelest irony was that when the US Occupation ran Japan from 1947 to 1952, they insisted that the Japanese spray cheap, US-manufactured DDT everywhere for reasons of public health, and the native Japanese pyrethrum industry was never restarted.
Pyrethin compounds are found in a number of chrysanthemum varieties in varying amounts, but in recent times pyrethrum has been obtained primarily from Chrysanthemum cinerariifolium, which contains the highest concentrations. Historically, pyrethrum's insecticidal properties were originally discovered by the Persians (hence the name, "Persian powder"), who obtained it from the Persian chrysanthemum, C. coccineum, but this species has a lower concentration of pyrethins so the powder from many more flowers are needed to achieve the same effects.
Today, pyrethrum is produced in small amounts around the world for sale to organic farmers and for use in home gardens. Another option is to simply plant actual chrysanthemum (C. cinerariifolium) plants alongside vulnerable crops or plants, a practice which has proven somewhat effective at warding off a wide variety of insects and other pets.