Pyrethrum was a genus of several Old World plants now classified as Chrysanthemum or Tanacetum which are cultivated as ornamentals for their showy flower heads. Pyrethrum continues to be used as a common name for plants formerly included in the genus Pyrethrum. Pyrethrum is also the name of a natural insecticide made from the dried flower heads of Chrysanthemum cinerariifolium and Chrysanthemum coccineum. The insecticidal compounds present in these species are pyrethrins.
Description
Some members of the genus Chrysanthemum, such as the following two, are placed in the genus Tanacetum instead by some botanists. Both genera are members of the daisy (or aster) family, Asteraceae. They are all perennial plants with a daisy-like appearance and white petals.
Tanacetum cinerariifoliumTanacetum 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 do. Its flowers, typically white with yellow centers, grow from numerous fairly rigid stems. Plants have blue-green leaves and grow to 45 to 100 cm (18 to 39 in) in height. The plant is economically important as a natural source of pyrethrin insecticides.
Tanacetum coccineumC. coccineum, the Persian chrysanthemum, is a perennial plant native to 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 (12 and 24 in) in height. The flowering period is June to July in temperate climates (Northern Hemisphere). C. coccineum also contains insecticidal pyrethrins, but it is a 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.
Tanacetum cinerariifolium
Insecticides
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 present in amounts less than those 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 not persistent, being biodegradable and also decompose 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. Currently the world's major producer is Tasmania, Australia.
Sprays
Pyrethrum has been used for centuries as an insecticide,[1] and as a lice remedy in the Middle East (Persian powder, also known as "Persian pellitory"). It was sold worldwide under the brand Zacherlin by Austrian industrialist J. Zacherl.[2] It is one of the most commonly used non-synthetic insecticides allowed in certified organic agriculture.[3]
The flowers should be dried and then crushed and mixed with water.
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.
Companion planting
Because pyrethrum contains pyrethrins, they are used as companion plants to repel pest insects from nearby crops and ornamental plants.[4] They are thought to repel aphids, bed bugs (Cimex lectularius), leafhoppers, spider mites, harlequin cabbage bugs, ticks, pickleworms, and imported cabbage worms, among others, in gardens and farms. For example, they are planted among broccoli plants for protection from several common insect pests. The alarm pheromone both attracts ladybug beetles and repels aphids.[5]
Toxicity
Mammals
Rat and rabbit LD50 levels for pyrethrum are high, with doses in some cases of about 1% of the animal's body weight required to cause significant mortality. This is similar to fatal levels in synthetic pyrethroids.[6] Nevertheless, pyrethrum should be handled with the same caution as synthetic insecticides:[7] safety equipment should be worn, and mixing with other chemicals should be avoided.[8]
Pyrethrum, specifically the pyrethrin within, is highly toxic to insects including useful pollinators like bees. The risk is partially reduced as the compound has a rapid breakdown and has a slight insect-repellant activity.[10]
Common names
Common names for Chrysanthemum cinerariifolium include:
^"Using Pyrethrum". Gardening Australia. Australian Broadcasting Corporation. 25 May 2013. Retrieved 3 November 2018. "I've also marked my pyrethrum bottle 'for pesticides only'. I have another one for herbicides. It doesn't matter whether you're organic or not, accidents can happen. It's best to keep these chemicals apart." As a precaution, safety gear should be worn when using pyrethrum.
