Research indicates that pyrethrum based pesticides can have serious side effects to mammals. (humans) The information below, was never meant to suggest that the product  be repeatedly applied to humans by way of a mister system (for mosquito control) or applied in such a way that the product could come into contact with food meant for human consumption. Serious genetic and neurological damage can occur to humans thru repeated skin exposure. Pyrethrums/Pyrethroids are some of the least toxic pesticides, (to humans) Unfortunately, pyrethrums applied in mister systems are a very bad combination.

Pyrethrum is very effective in the control of Mosquitoes, ants, flies, crickets, fleas, and wasps.

Pyrethrum is derived from the dried flowers of Chrysanthemum Cinerariaefolium. The name given to the active insecticidal components of the dried flowers is "Pyrethrins". Kenya is the largest producer of Pyrethrum in the world, supplying 90% of the World's demand.

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The Pyrethrum flower provides a highly effective protection against mosquitoes, carriers of killer diseases such as Malaria and Yellow Fever. The physiological action of pyrethrins is to inhibit the mosquitoes from biting and causes repellency, immobilization, paralysis and death.

Pyrethrum is a complex insecticide with the following outstanding properties:

• Rapid Action: Pyrethrum is a contact insecticides attacking the nervous system of insects almost immediately and causing knockdown, soon followed by kill.

• Lack of Insect Immunity: The structure of the molecule is so complex that insect resistance to Pyrethrum is not a practical problem. A very few isolated cases have been reported, unlike synthetic insecticides. 

• Broad Spectrum of Activity: Because Pyrethrum consists of a group of related compounds, it has a wider spectrum of activity against insect species than many single insecticides. It can be used against any insect found in the house or garden: mosquitoes, flies, cockroaches, aphids, ants...

• Environment friendly - Non resistant: Pyrethrum is degraded by the combination of sunlight and air and therefore presents little of the hazards which are usually associated with other classes of persistent insecticide.

• Repellency: Pyrethrum is a powerful insect repellent which in combination with its low mammalian toxicity favors it in many applications, such as food and grain storage protection, personal protection, and will inhibit target insects from biting.

• Flushing Action: Pyrethrum has a much greater flushing effect than any other insecticide; it disturbs pests in their hiding places forcing them to get out and to get exposed to the  insecticide.

• Pyrethroids are neurotoxins: Don't be fooled into believing it is safe for human contact, because it comes from a flower. Repeated contact is dangerous.

The insecticide is natural pyrethrum. Extracted from a daisy grown in Kenya, it has been in use in the West since the early twentieth century.  At one time, pyrethrum powder was mixed with kerosene to create a sprayable liquid insecticide.  Today, pyrethrum flowers are processed into extracts to serve domestic, industrial and agricultural needs.   Insecticides using natural pyrethrum as active ingredients include Fairfield American's Pyrenone.  

Synergists Developed
Despite its power and safety, natural pyrethrum has certain limitations.  The fact that it is imported makes 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.  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.

Knockdown Effects
Because of its safety, pyrethrum has long been preferred for household and agricultural application.  Recent research is revealing new power and new uses for this old and trusted insecticide.  Combined with a synergist, natural pyrethrums are 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 it's 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.

Jamming
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 then 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 insects forget 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.

Resistance
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, pyrethriods.   But so far, science has not devised a synthetic that combines the speed, effectiveness, activation effects and biodegradability of natural pyrethrum.

More info about pyrethroids and pyrethrum:

http://www.pesticide.org/PyrethrinsPyrethrum.pdf

http://www.state.nj.us/health/eoh/rtkweb/1623.pdf