Pyrethrins

Pyrethrins are natural organic compounds (Pyrethrin-I, pyrethrin-II) that can be used for insect control. They are found in daisy-like Chrysanthemum (Chrysanthemum cinerariaefolium) flowers grown and harvested in Kenya, Africa and Australia. They are viscous liquids that oxidize readily to become inactivate. The insecticidal properties of pyrethrins are derived form ketoalcoholic esters and pyrethroic acids which are strongly lipophilic and rapidly penetrate many insects and paralyze the insects’ nervous system (Reigart et al., 1999).

Pyrethrins, as neurotoxins, attack the nervous systems insects. If in minimal amount not fatal to insects, they serve as insect repellants, thus, they are also applied to prolong the viability of storage grains and protect it from storage insect pests. They are far less toxic to warm-blooded animals but harmful to fish. It affects the skin touch receptors and balance organs of fish and tadpoles. Toxicity increases with higher water temperatures and acidity. Pyrethrins are also toxic to beneficial insects such as honeybees and toxic to other aquatic invertebrates. In humans, pyrethrins are known to irritate the eyes, the skin, and respiratory system. But they are less toxic compared to many synthetic insecticides. Pyrethrins have been considered to be one of the safest insecticides and may be used with safety in plants used as food, at homes and around animals. It has a long history of satisfactory use around mammals and birds. Pyrethrins interfere with the ionic conductance of nerve membranes by prolonging the sodium content and stimulate the nerves to discharge repeatedly causing hyper-excit-ability in poisoned animals (WHO, 1999).

A derivative from pyrethrins is pyrethroids. Pyrethroids are synthetic forms of pyrethrins. They are more stable and are less affected by sunlight and oxygen. They also tend to be more effective compared to natural pyrethrins and are less toxic to mammals. A common pyrethroid is allethrin. Pyrethroids are neuropoisons acting on the central nervous systems by interacting with sodium channels in mammals and/or insects. The main systems for metabolism include breakage of the ester bond by esterase action and oxidation at warious parts of the molecule. Induction of liver microsomal enzymes has also been observed (WHO, 1999)

Pyrethrins are biodegradable, hence non-persistent, and break down easily when exposed to light or oxygen. This means that there will be no excessive build-up of insecticides in the treated area. Natural pyrethrins are highly fat soluble, but are easily degraded; thus, do not accumulate in the body. Pyrethrin-I, pyrethrin-II, and allethrin have multiple sites in their structures which can be readily attacked in biological systems, it is unlikely that they will concentrate in the food chain

Many pyrethroids have also been linked to disruption of the endocrine system, which can adversely affect reproduction and sexual development; interfere with the immune system and increase chances of breast cancer. Pyrethroids contain human-made, or xenoestrogen, which can increase the amount of estrogen in the body (Garey et al., 1998). When tested, certain pyrethroids demonstrate significant estrogenecity and increase the levels of estrogen in breast cancer cells (Go et al., 1999). Because increased cell division enhances the chances for the formation of a malignant tumor in the breast, artificial hormones, like those found in pyrethroids, may increase breast cancer risk (PCBR, 1996). Some pyrethroids are classified by EPA as possible human carcinogen.

While the development of the synthetic pyrethroids was heralded with claims of selective toxicity to insects, both pyrethroids and pyrethrins are extremely toxic to aquatic organisms, including fish such as the bluegill and lake trout, with LC­­­­­­39 values less than 1.0 ppb. These levels are similar to those for mosquito, backfly and tsetse fly larvae, and the actual target of the pyrethroid application. Lobster ­­, shrimp, mayfly nymphs and zooplanktons are the most susceptible non-target aquatic organisms (Muller-Beilschmdt, 1990) The nonlethal effects of pyrethroids on fish include damage to the gills and behavioral changes.

Natural pyrethrins and synthetic pyrethroid compounds vary in their toxicity. Inhaling high levels of pyrethrum, pyrethrins or pyrethroids, may bring about asthmatic breathing, sneezing, nasal stuffiness, headache, nausea, lose of coordination, tremors, convulsions, facial flushing and swelling, and burning and itching sensations. Infants are severely affected due to their inability to efficiently break down pyrethrums. Animals exposed to toxic amounts may exhibit tongue and lip numbness, diarrhea, and nausea or even death. But rapid recovery can be observed from serious intoxication of mammals. Rats and rabbits are not affected by large dermal applications. Pyrethrum produces irritation and sensitization when touches broken skin and is further aggravated by sun exposure.

Pyrethrum absorption through the stomach and intestines and through the skin is slow. However, humans can quickly absorb pyrethrum through the lungs during respiration. Response depends on the pyrethrum compound used. Overall, pyrethrins and pyrethroids have low chronic toxicity and the most common problems in humans resulted from the allergenic properties of pyrethrum. Many of the natural and synthetic compounds can produce skin irritation, itching, pricking sensations and local burning sensations which may last for about two days. At high doses, pyrethrum can be damaging both to the central nervous system and the immune system which can worsen allergies. Animals fed large doses of pyrethrins may experience liver damage.

REFERENCES:

http://www.doyourownpestcontrol.com/pyrethrin.htm

http://en.wikipedia.org/wiki/Pyrethrin

http://npic.orst.edu/factsheets/pyrethrins.pdf

http://extoxnet.orst.edu/pips/pyrethri.htm

http://www.beyondpesticides.org/pesticides/factsheets/Synthetic%20Pyrethroids.pdf