Pyrethrum

• The term “pyrethrum” refers to the powder made from the dried flowers of Tanacetum cinerariifolium.

• The term “pyrethrins” refers to the 6 insecticidal components which are found in the powder. All 6 are organic esters formed from one of two acids (chrysanthemic and pyrethric acid) and one of three keto-alcohols (pyrethrolone, cinerolone, jasmolone). Pyrethrins are unstable molecules which degrade within a few days in light and warmth. They can be stored in a refrigerator at –4°C.

• The term “pyrethroids” refers to the semi-synthetic derivatives of pyrethrins. These products have the advantage that they are much more stable than pyrethrins.

• The plants with the highest concentrations of active constituents grow well in sunny, semi-arid regions at 1600-2000 metres above sea level. Kenya, Tanzania and Ecuador are important producers. They are best harvested during a warm, dry period when the flowers are fully developed. The plants are dried after harvesting, and finely ground. Extraction may be with kerosene, paraffin or alcohol.

• Often a synergist is added, such as piperonal butoxide or various oils.

• Pyrethroids have low toxicity for mammals including humans but are highly toxic, e.g. to fish. They are sometimes used for illegal “chemical” fishing.

Category: Medicine Notes