Pyrethrum refers to several Old World plants of the genus Chrysanthemum (e.g., C. coccineum) which are cultivated as ornamentals for their showy flower heads. It is also the name of a natural insecticide made from the dried flower heads of C. cinerariifolium and C. coccineum. Pyrethrum was used for centuries as an insecticide and as a lice remedy ("Persian Insect Powder" , "Persian Pellitory") in the Middle East. It was sold worldwide under the brand Zacherlin by Austrian industrialist J. Zacherl .
Some members of Chrysanthemum, including these two, are placed in another genus, Tanacetum, by some authorities. Both genera are members of the daisy (or aster) family, Asteraceae. They are perennial plants with a daisy-like appearance and white petals.
C. cinerariifolium is called the Dalmatian chrysanthemum, denoting its origin in that region of the Balkans (Dalmatia). It looks more like the common daisy than other pyrethrums. Its flowers, typically white with a yellow center, grow from numerous fairly rigid stems. Plants have blue-green leaves and grow to between 45 to 60 cm in height. The plant is economically important as a natural source of insecticide. The flowers are pulverized and the active components called pyrethrins, contained in the seed cases, are extracted and sold in the form of an oleoresin. This is applied as a suspension in water or oil, or as a powder. Pyrethrins attack 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 an insect repellent effect. They are harmful to fish, but are far less toxic to mammals and birds than many synthetic insecticides and are non-persistent, being biodegradable and also breaking down easily on exposure to light. They are considered to be amongst the safest insecticides for use around food. Kenya produced 90% (over 6,000 tonnes) of the world's pyrethrum in 1998, called py for short. Production in Tanzania and Ecuador is also significant.
C. coccineum, the Persian chrysanthemum, is native the Caucasus and looks somewhat like a daisy. It produces large white, pink or red flowers. The leaves resemble those of ferns, and the plant grows to between 30 and 60 cm in height. The flowering period is June to July in temperate climates (Northern hemisphere). C. coccineum also contains insecticidal pyrethrum substances but it is a relatively poor source compared to C. cinerariifolium.
Other species such as C. balsamita and C. marshalli also contain insecticidal substances, but are less effective than the two species mentioned above.
Pyrethroids are synthetic insecticides based on natural pyrethrum (pyrethrins); one common example is permethrin. A common formulation of pyrethrin is in preparations containing the synthetic chemical piperonyl butoxide: this has the effect of enhancing the toxicity to insects and speeding the effects when compared with pyrethrins used alone. These formulations are known as synergized pyrethrins.
Because of the natural insecticidal properties of the pyrethrums, they are used as companion plants, to repel pest insects from nearby crops. One might, for instance, plant them among broccoli plants in order to protect them from any of several insect pests. They are thought to repel aphids, leafhoppers, spider mites, harlequin bugs, ticks, pickleworms and imported cabbage worms, among others.
Common names for Chrysanthemum cinerariaefolium include: