Pyrethrum is economically important as a natural source of insecticide. The flowers are pulverized and the active constituents, called pyrethrins, contained in the seed cases, are extracted. Pyrethrins act on the nervous systems of all insects, and inhibit female mosquitoes from biting. When not present in amounts fatal to insects, they still appear to have a repellent effect. They are harmful to fish, but are far less toxic to mammals and birds than many synthetic insecticides and are nonpersistent, being biodegradable, and also breaking down easily on exposure to light.