Pyrethrum comes from the dried flowers of certain members of the daisy family, Chrysanthemum cinereriifolium, C. carnaeum and C. roseum. It is sometimes seen in border plantings as a flowering plant, but more often is confused with the much more frequently seen feverfew (C. parthenium).
The commercial pyrethrum industry was largely based in Kenya, but has now diversified, including plantations in northern Tasmania.
The plant has been in use of a long time, an was originally used as a treatment for body lice by the Persians and possibly by ancient Chinese. It was first commercialised in Yugoslavia in the early 1800's.
The beneficial features of pyrethrum for organic growers include its fast knockdown effect, rapid breakdown in the environment and a very low level of resistance. The rapid breakdown (it may only last for several hours in hot, sunny conditions) can also be a disadvantage. It may be necessary to apply it in the cool of evening, to reapply it at short intervals or to use one of the recently developed encapsulated products, which are claimed to have an effective life of even several weeks.
Resistance to Pyrethrum was unknown until the last decade, when some Australian blowflies displayed minor resistance. Resistance is hard to develop because of minor variations in the active ingredients of Pyrethrum depending on variety and growing conditions. Pyrethrum contains four main compounds, knock as pyrethrin I, pyrethrin II, cinerin I and cinerin II.
Pyrethrum is a contact insecticide and a stomach poison. It is useful against a wide range of insects including flies, cockroaches, fleas, thrips, aphids, whiteflies, caterpillars and some beetles. Once inside the body, pryrethrins disrupt nerve centre, causing disorienting and agitated behaviour (such as insects leaving their hideaways, and eventually death. A potential problem with Pyrethrum is that a sub lethal dose will paralyse the insect pest, making it appear that the pest has been killed. Good observation is essential and follow-up sprays may be required.
Organic gardeners should use this product with caution, as it can be toxic to humans (although it has a very low mammalian toxicity, some people show an allergy) and will definitely affect beneficials such as wasps, bees and ladybirds. The commonly used synergists such as piperonyl butoxide are also not permitted in certified growing. This is a hotly debated issue, as the synergists do extend the effective life of the product and are themselves very safe to use. Fortunately trials have shown that sesame oil (and possibly sassafras oil) are effective natural synergists which work very well.
Many synthetic pyrethroids have been produced. They are less complex than the natural extract and therefore resistance to these products is much easier to develop.
Pyrethrum sprays can be made at home from the flowers, which are produced in large numbers. Dry flowers slowly in the sun and grind them up. They can then be soaked in water, or for a more effective spray, use some kerosene in the water. Strain out the ground up flowers before spraying, dilute with clean water and add a small amount of a good biodegradable detergent or soap. Shake the product to emulsify the ingredients before applying.
Pyrethrum is best used in the evenings, when bees are inactive, but many pests are feeding, and when the breakdown will be slower than in sunlight and warmth.