(Chrysanthemum cinerariaefolium) is a perennial temperate plant with small white, daisy-like flowers from which natural insecticides, the pyrethrins, are derived. Traditionally, pyrethrum was produced in many African countries where hand-labor was used to plant, harvest, and dry the crop. Political upheaval, drought, and lack of an organized development and marketing structure resulted in unreliable pyrethrum supplies for U.S. manufacturers.
In the late 1980's, however, Australia began a pyrethrum development project and is now a major producer of the crop. This has helped to stabilize the industry. Kenya, Tanzania, Rwanda and Ecuador still remain the primary suppliers of pyrethrum, however. Smaller amounts are grown in Japan, Brazil and India. The United States is the principal consumer of world supplies of pyrethrum. Demand for pyrethrum-based insecticides is on the rise because of its long safety record, very low toxicity, and rapid breakdown. Because of the selective and relatively small-scale use of pyrethrum for over 160 years, there has been relatively little development of insect resistance. As a result of these characteristics, in recent years the use of pyrethrum based products has increased dramatically on organic farms and for home insect control.
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.
For pyrethrum to be fully lethal to insects, it is generally combined with a 'synergist', a chemical that enhances the pyrethrins action on the nervous system. The synergist may be as simple as vegetable oil or diatomaceous earth, or more complex like piperonyl butoxide.
Pyrethrins have low toxicity to mammals, because mammals can metabolize the chemicals. 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.
Natural pyrethrum, despite its power and safety, has certain limitations. The fact that it is imported means it comparatively expensive. Moreover, some insects - houseflies for example - are able to detoxify modest amounts of the poison in their bodies. These tend to recover from any but the heaviest doses. In addition, natural pyrethrum tends to break down in sunlight, rapidly losing its effectiveness after outdoor use.
Researchers have dealt with the detoxification problem by combining pyrethrum extract with a liquid synergist, piperonyl butoxide, which fools the insect's metabolism so that it doesn't break down pyrethrum in the body. Mixed with this chemical, a small amount of pyrethrum can control insects effectively.
As for the tendency of the substance to degrade in sunlight, this has turned out to be a blessing in disguise.
Pyrethrum is considered
biodegradable and is sought for sensitive applications like the post-harvest treatment of fruits and vegetables. Natural pyrethrum is so safe that the U.S. Government approves its use on such insect-prone foods as tomatoes, even while they are on their way to the supermarket or processing plant. And in 1946 the city of Amsterdam added pyrethrum to the municipal water supply to kill a population of insects that were threatening 'to choke the system. The insects were destroyed, while humans continued to drink, wash and cook with the treated water without suffering any harm.
Pyrethrum has long been preferred for household and agricultural applications. But recent research is revealing new power and new uses for this old and tested insecticide. Combined with a synergist, natural pyrethrum is one of the fastest-acting insecticides known. Even before it kills, it knocks down and paralyzes insects almost immediately. When it encounters pyrethrum, the insect is thrown into a state of nervous disorder. It runs from its hiding place and scuttles around erratically, or adopts a confused flight pattern. Both responses show that the insect has lost all control of its central nervous system. This contact effect is called activation. Recent practice exploits the activation effect by adding small amounts of pyrethrum to a routine residual agricultural formulation.
Pyrethrum activates hidden insects, driving them from cover and into contact with the main insecticide. This "flushing" action has been most successful in the control of such hard-to-hit pests as the cotton bollworm and the gypsy moth.
Recently, researchers have identified a subtle effect that occurs even before activation takes place: jamming. The jamming phenomenon suggests new uses for pyrethrum in the battle against malaria. To show how jamming works, you need only a cage full of voracious female mosquitoes and some extremely brave volunteers. Those who put their bare arms in the cage can expect to get some 20 to 50 bites per minute. But if the cage is exposed to trace amounts of pyrethrum for only five minutes and the arm is reinserted, no bites are recorded, even though the insects otherwise seem completely normal. Apparently small amounts of pyrethrum can jam the "black box" of the insect's food-searching mechanism. The insect forgets to eat as it were. Because of this effect, low-level pyrethrum applications have been shown to reduce the risk of malaria carried by indoor mosquitoes.
There's more to pyrethrum's bag of tricks. The reason is not fully understood, but insects do not become resistant to natural pyrethrum. After decades of use, no insect population has ever developed significant pyrethrum resistance. Intense study of the pyrethrum, molecule has produced the related synthetic materials, pyrethroids. But so far science has not devised a synthetic that combines the speed, effectiveness, activation effects and biodegradability of natural pyrethrum.