Pyrethrin and Synthetic Pyrethroid Insecticides
Pyrethrins are derived from a natural compound that is found in Chrysanthemum species, flowers such as daisies, called pyrethrum. The various natural pest-repellent pyrethrin compounds are extracted with the most common being pyrethrin I and pyrethrin II. Pyrethrins are most effective against flying insect pests.
Pyrethrins are thick, sticky, brown plant extracts. They are generally insoluble in water, but will dissolve in other chemicals such as alcohols, oil or organic solvents. Pyrethrins are usually mixed with synergists, chemicals that do not have any insecticidal properties but help them work. A common one is piperonyl butoxide which increases the toxicity of pyrethrin to pests.
Pyrethrins are not stable in light and air, and, therefore, are not persistent in the environment. Pyrethrins act on contact, quickly affecting the nervous system to "knockdown" the pest. A few minutes after application the pest cannot move or fly away. Pyrethrin I is highly lethal while Pyrethrin II is not. Pyrethrins are normally inhaled or ingested by the pest after contact and absorbed through the lining of the stomach or respiratory tract. They affect the nervous system by attaching to a protein found on the surface of nerves called the sodium channel. This channel opens to stimulate the nerve and closes to end the signal when functioning normally. The Pyrethrins bind to the sodium channel and prevent it from closing, thus over stimulating the nerve and causing pests to lose control of their nervous system.
Pyrethrins are much more toxic to pests than mammals. Mammals are able to breakdown pyrethrins into less toxic chemicals which are readily excreted. If humans are exposed to large amounts of pyrethrins, they will show symptoms of sneezing, runny nose, sore throat and breathing difficulties. Ingestion will cause nausea and possibly vomiting.
Pyrethrins are already a green chemistry approach to pesticides since they are natural plant derived pesticides. They are suitable alternatives that can be used to reduce the risk of exposure to synthetic pesticide residues.
Synthetic pyrethroids are synthetic versions of naturally occurring pyrethrins. Pyrethroids exhibit greater stability in the environment, and, therefore, are more persistent. They are, however, designed to target specific host species. More than 100 pyrethroids have been synthesized with more insecticidal effectiveness than naturally occurring pyrethrins. One major drawback to the use of these chemicals is that insects become resistant to pyrethroids if they are overused.
Synthetic pyrethroids are insoluble in water, immobile in soil and have high absorption ability with particles such as wood. They are broken down by sunlight and microorganisms such as bacteria and, therefore, are generally not very persistent in the environment, although they are more persistent than pyrethrins.
Synthetic pyrethroids were designed to metabolize quickly in mammals and thus reduce the risk of poisoning, while still remaining toxic to insects. Synthetic pyrethroids have the same mode of action as pyrethrins, affecting the sodium channels in nerve cells and over stimulating the nervous system. Synthetic pyrethroids are less toxic to mammals than carbamates and organophosphates. Synthetic pyrethroids are not easily dissolved in water but do adhere strongly to substances. Therefore, there is a risk associated with food being consumed that has been exposed to them. Synthetic pyrethroids are quite toxic to fish and other aquatic organisms, so application near bodies of water should be done with care.
Exposure to synthetic pyrethroids can result in symptoms such as dizziness, headache, nausea, vomiting, and eye irritation. Extremely high exposure could lead to fatigue, muscular twitching and knock you unconscious. Chronic exposure includes brain and nervous system disorders and immune system failures.
Commonly used pyrethrin and synthetic pyrethroid insecticides to use for review of the labels and MSDS sheets, to fill in the product profile worksheet at the end of the unit and to discuss with the students include Astro (Permethrin), Cynoff (Cypermethrin), ExciteR (Pyrethrin), Talstar (Bifenthrin) and Warrior (Lambda-cyhalothrin). Students should contrast and compare the different insecticides and determine the use limitations of each of the chemicals. Their Excel sheet should be filled out for each of these chemicals with their half-life in the environment, water solubility, partition coefficient (K O C), and calculated GUS.
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