© Stuart M Bennett 2001



In general, insecticides derived from plants are low in toxicity. Pyrethrins are widely used insecticides in the home. They have a rapid "knockdown" for insects and have a low potential for producing toxicity in humans. The major toxicity of pyrethrins is allergy.

Pyrethrum is the initial plant extract of certain plant species of the genus Chrysanthemum, native to southwestern Asia, whose aromatic flower heads, when powdered, constitute the active ingredient in the insecticide called pyrethrum. Pyrethrum is a mixture of four compounds: pyrethrins I and II and cinerins I and II.. The crude pyrethrum contains about 30 - 35% pyrethrins and about 50% impurities. The plants were formerly considered a separate genus, Pyrethrum. The typical species, the perennial C. coccineum, is the florists' pyrethrum, commonly called painted lady. Large deep rose-coloured petals surrounding the yellow centre, or disk, are borne on long simple stems above the crown of finely cut leaves. Modern varieties exhibit various colours--white, lilac, and shades of red.
The powdered flower heads of C. coccineum, C. cinerariaefolium, and C. marschalli are chief sources of the insecticide. The active substances in pyrethrum are contact poisons for insects and cold-blooded vertebrates. The concentrations of pyrethrum powder used in insecticides are nontoxic to plants and higher animals; therefore, these insecticides find wide use in household and livestock sprays as well as in dusts for edible plants.

The structure pictured above is that of Allethrin or Bioallethrin or Pyrethrin, but there are a few more names that it goes by and it is the active component of the insecticide, here are some of the other names; DL-Cistrans-chrysanthemum monocarboxylic acid; Cyclopropanecarboxylic acid, 2,2-dimethyl-3-(2-methylpropenyl)-, ester with 2-allyl-4-hydroxy-3-methyl-2-cyclopenten-1-one; (.+/-.)-Allelrethonyl (.+/-.)-cis,trans-chrysanthemate; Allethrine; Allyl cinerin I; Cinerin I allyl homolog; Cyclopropanecarboxylic acid, 2,2-dimethyl-3-(2-methyl-1-propenyl)-, 2-methyl-4-oxo-3-(2-propenyl)-2-cyclopentene-1-yl ester; D-Allethrin; ENT 17510; Pynamin; Pyresyn; Cyclopropanecarboxylic acid, 2,2-dimethyl-3-(2-methyl-1-propenyl)-, 2-methyl-4-oxo-3-(2-propenyl)-2-cyclopenten-1-yl ester; (.+/-.)-3-Allyl-2-methyl-4-oxocyclopent-2-enyl 2,2-dimethyl-3-(2-methylprop-1-enyl) cyclopropylcarboxylate; (+)-trans-Bioallethrin; (+)-trans-Chrysanthemumic acid ester of (.+/-.)-allethrolone; trans-Allethrin; Allyl cinerin; Allyl homolog of cinerin I; Bioaletrina; Cyclopropanecarboxylic acid, 2,2-dimethyl-3-(2-methyl-1-propenyl)-, 2-methyl-4-oxo-3-(2-propenyl)-2-cyclopenten-1-yl ester, d-trans-; D-trans Allethrin; D,L-2-Allyl-4-hydroxy-3-methyl-2-cyclopenten-1-one-D,L-chrysanthemum monocarboxylate; ENT 16275; FDA 1446; FMC 249; Necarboxylic acid; NIA 249; Pallethrine; Pyresin; 2-Allyl-4-hydroxy-3-methyl-2-cyclopenten-1-one ester of 2,2-dimethyl-3-(2-methyl propenyl) cyclopropane carboxylic acid; 3-Allyl-2-methyl-4-oxo-2-cyclopenten-1-yl chrysanthemate; 3-Allyl-4-keto-2-methylcyclopentenyl chrysanthemum monocarboxylate; (RS)-3-Allyl-2-methyl-4-oxocyclopent-2-enyl (1R,3R)-2,2-dimethyl-3-(2-methylprop-1-enyl)cyclopropanecarboxylate; (.+-.)-Allelrethonyl (.+-.)-cis,trans-chrysanthemate.

Chemical Formula: C19H26O3

How do Pyrethrins and Pyrethroids work...? (Pyrethroids are synthetic pyrethrins).

All nerve cells and membranes have a specific electrical charge...that's why you get some bright sparks sometimes..., anyway, so like a magnet there is a polarising effect caused by ions (charged atoms) if the amount of ions change there is a de-polarising effect which causes a neurotransmitter to be released, which is how our nerves communicate with the brain. Pyrethrins work by creating multiple potentials across the membranes by delaying the closing of the ion channel. They act as contact poisons affecting the insect's nervous system but even though they are a nerve poison, they are not an anti-cholinesterase as are organophosphates and carbamates. Usually pyrethroids contain a synergist which allows the primary insecticide to be more effective by restricting the production of an enzyme the insect uses to detoxify the pyrethrins, one of the most well known is piperonyl butoxide .

LD50/LC50: Synthetic pyrethroid compounds vary in their toxicity as do the natural pyrethrins. Pyrethrum carries the signal word CAUTION. Inhaling high levels of pyrethrum may bring about asthmatic breathing, sneezing, nasal stuffiness, headache, nausea, incoordination, tremors, convulsions, facial flushing and swelling, and burning and itching sensations. The most severe poisonings have been reported in infants, who are not able to efficiently break down pyrethrum. The lowest lethal oral dose of pyrethrum is 750 mg/kg for children and 1,000 mg/kg for adults. Oral LD50 values of pyrethrins in rats range from 200 mg/kg to greater than 2,600 mg/kg. Some of this variability is due to the variety of constituents in the formulation. Mice have a pyrethrum oral LD50 of 370 mg/kg. Animals exposed to toxic amounts may experience tongue and lip numbness, nausea, and diarrhea. Symptoms may also include incoordination, tremors, convulsions, paralysis, respiratory failure, and death. Pyrethroids can cause two quite different responses at near lethal doses in rats; aggressive sparring and a sensitivity to external stimuli progressing to tremors is the one response and pawing and burrowing behavior, and salivation leading to chronic seizures is the other. Human response to these two different types of pyrethroids has not yet been evaluated. Recovery from serious poisoning in mammals is fairly rapid. Rats and rabbits are not affected by large dermal applications. On broken skin, pyrethrum produces irritation and sensitization, which is further aggravated by sun exposure.

In human beings, pyrethrins, pyrethroids, and their metabolites are not known to be stored in the body nor excreted in the milk. The urine and feces of people given oral doses of pyrethrum contain chrysanthemumic acid and other metabolites. These metabolites are less toxic to mammals than are the parent compounds. Pyrethrins I and II are excreted unchanged in the feces. Other pyrethrum components undergo rapid destruction and detoxification in the liver and gastrointestinal tract.

Ecological Effects:

Pyrethrin is extremely toxic to aquatic life, such as bluegill and lake trout while it is slightly toxic to bird species, such as mallards. Toxicity increases with higher water temperatures and acidity. Natural pyrethrins are highly fat soluble, but are easily degraded and thus do not accumulate in the body. These compounds are toxic to bees also.Because pyrethrin-I, pyrethrin-II, and allethrin have multiple sites in their structures that can be readily attacked in biological systems, it is unlikely that they will concentrate in the food chain.

Pyrethrins have little residual effect. In stored grain, 50% or more of the applied pyrethrins disappear during the first three or four months of storage. At least 80% of what remains is removed by handling, processing, and cooking. Pyrethrins alone provide limited crop protection because they are not stable. As a result, they are often combined with small amounts of antioxidants to prolong their effectiveness. Pyrethrum compounds are broken down in water to nontoxic products. Pyrethrins are inactivated and decomposed by exposure to light and air. Pyrethrins are also rapidly decomposed by mild acids and alkalis. Stored pyrethrin powders lose about 20% of their potency in one year. As the pyrethrins are purified, their stability decreases; thus, pure pyrethrin-I and pyrethrin-II are the least stable of the pyrethrins. Purified pyrethrins are very expensive and are only available for laboratory uses.

Physical Properties:


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