History:
Early records of pyrethrum date back to the first Century A.D. during the "Chou" Dynasty of China. Its secret was guarded for many years. In medieval times it found its way probably by the old "silk routes" through Tashkent to Persia and finally to the Dalmatian coast. Later growing spread to other parts of Europe, South America, China, South East Asia.
A few plant species that contain active insecticidal components have been commercially exploited. Pyrethrum, which is perhaps the most widely used botanical insecticide, is derived from the flowers of a plant in the genus Chrysanthemum, which belongs to the family Compositae.
Active constituents:
Pyrethrum is an insecticide made from the whole inflorescences of these plants. Its active constituents are collectively called the Pyrethrins which are organic esters formed by the combination of two carboxylic acids and three keto alcohols. The combination of these components account for the kill and knockdown properties of Pyrethrum Extract. Natural pyrethrum contains six active chemicals:

Generally referred to as pyrethrins, these chemicals are concentrated within the flowers. Little is found elsewhere in the plants, but enough that insects leave the pyrethrum-producing plants alone.
Scientific Support:
Pyrethrins are contact poisons which quickly act upon the nervous system. In small doses, insects are knocked down; the toxins excite the neurons causing convulsions. In the final stages of poisoning, the insect cannot coordinate its voluntary muscles (ataxia); the nervous system appears 'exhausted'. But insects can recover unless the dosage is sufficient to kill. (Pyrethroids, Department of Entomology, Purdue University, 2002)
Pyrethrins have low toxicity to mammals, because mammals can metabolize the chemicals. But poisoning can occur or worse, a severe allergic reaction--asthma or anaphylaxis.
Safety:
Pyrethrum is unstable; the pyrethrins breakdown in the presence of sunlight, moisture, or oxygen making the chemicals biodegradable. In the past two centuries of pyrethrum use, very few insects have developed resistance to these toxins. But at the same time it can be safely stored for years in suitable containers.
Applied fields:
Pyrethrum is also widely used in horticulture and by growers of organic crops for the control of leaf hoppers and other common pests. For pyrethrum to be fully lethal to insects, it is generally combined with a 'synergist', such as Piperonyl Butoxide to enhance the performance. Recently many "organic" products omit these synergists for reasons of purity. Instead vegetable oils or diatomaceous earth are added.